14 ff3 fs6 fc0 sc0 ls0 ws1f">continents. In the tropical and sub-tropical countries,
where cassava is produced, total human calories intake
from cassava products exceeds 200 billion kcal/day [6].
Because cassava roots are very low in protein content
(values range among cultivars from 5 to 19 g/kg dry matter,
based on an average conservative Kjeldahl nitrogen-
to-protein conversion factor of 2.49 - 3.67 [7]), human
requirement for protein and other essential nutrients are
commonly fulfilled by other food sources. Cassava leaves
are also consumed and constitute an excellent source for
protein supplement (leaf crude protein contents on a dry
basis range among cultivars from 21% to 39%; [8]),
minerals and vitamins for the human diet in many Afri-
can and Asian countries, as well as in certain regions of
Brazil [9-11]. Nevertheless, cassava roots and leaves are
deficient in sulfur-containing amino acids (e.g. methion-
ine and cysteine) [11].
In countries where cassava is traditionally used di-
rectly for human consumption (about 70% of total cas-
sava production), particularly in Africa and Latin Ame-
rica, cultivars low in cyanogens (commonly called sweet
cultivars) are preferably used to avoid health hazards.
When using cultivars high in cyanogens (commonly called
bitter cultivars), much of the hydrocyanic acid (HCN) is
normally removed from cassava roots and leaves by us-
ing a mix of complex traditional methods and modern
technologies during food processing and preparation [12].
Its often poorly-processed food products contain some
anti-nutrient elements such as free HCN, phytates and
polyphenols, and particularly acetone cyanohydrin, which
is commonly associated with an upper motor neuron dis-
ease known as “konzo syndrome in some African coun-
tries [13-15]. This occurs mainly with large intake of in-
adequately processed bitter-cassava products in areas hit
by long drought and with shortages of balanced diets.
Also, cassava leaves have value as a protein supple-
ment in animal nutrition either in feed formulation for
mono-gastric animals or as a fresh forage to supplement
low-quality roughages in ruminant feeds [16]. All parts
of cassava plants (i.e. storage roots and shoots) are valu-
able sources for animal feed that could be either fed or
grazed fresh in case of sweet cultivars, or dried and ensi-
laged in bitter cultivars [17,18]. For decades, Thailand
was the largest exporter of cassava dried root chips,
mainly to European countries, where it were used as a
cheap component in the industry of animal feed concen-
trates. A significant portion of storage roots is used
worldwide for starch extraction, glucose manufacturing,
alcohol, and recently for biofuel.
The cassava crop is propagated vegetative by using
short woody stem cuttings (from 6-month old plants or
older) planted horizontally, vertically, or inclined on flat
or ridged lands at population densities from 5000 to
20,000 cuttings per hectare depending on the cropping
systems and purpose of production [19,20]. Lower popu-
lation densities are practiced in intercropping systems,
commonly with grain legumes and cereals such as maize
and sorghum. When grown in mono-cropping systems,
higher densities at 10,000 plants per hectare, or greater,
are used. Sexual seeds are used mainly in breeding pro-
grams, though its use in commercial cassava production
is a promising option to obviate constraints, particularly
diseases, associated with vegetative propagation [21].
Storage roots are generally harvested 7 - 24 months after
planting, depending on cultivar, purpose of use and
growing conditions. Due to root perishability and rapid
deterioration after harvest (within 2 - 3 days), fresh roots
have to be used immediately after harvesting, either eaten
on the farm, marketed for consumption, processed for
starch extraction, dried for flour production, roasted for
food products and/or used for animal feed. However, pre-
harvest pruning in the three weeks before harvest de-
creases root deterioration because of increases in the total
sugar/starch ratio in the roots [22]. Cassava processing
near production fields, makes it an ideal vehicle for rural
development through creating employment opportunities
in the areas where it is grown. Some of the processed
food products are commonly known as farinha da man-
dioca in Brazil and bordering countries, gablek in Indo-
nesia, and gari and foufou in West Africa [4]. Also, in
the Amazon region, local people prepare drinks such as
Mingao (by dissolving fermented starch in boiling water
and simmering) and Mani cuer a (a boiled sweet cassava
drink in northwest Amazonia) [18]. Combining fresh
cassava markets with those of its processed products
should increase marketing exibility and crop protabi-
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
164
lity, hence reducing the many risks often encountered by
the producers.
1.3. National and International Research
Support
The successes of the so-called “Green Revolution” of the
1960’s in obviating eminent famines in highly populated
developing countries across continents stemmed from the
development of high yielding semi-dwarf wheat cultivars
at CIMMYT, the International Maize and Wheat Im-
provement Center established in 1963, Mexico, and the
semi-dwarf rice cultivars at IRRI, the International Rice
Research Institute established in the Philippines in 1960.
These new highly productive cereal cultivars, under high
fertilization application supplemented with irrigation,
stimulated the formation in 1971 of the CGIAR, the Con-
sultative Group on International Agricultural Research.
More international research centers were established to
expand activities on other staple food crops, cropping
systems and natural resources management covering the
most important agro-ecological zones in various deve-
loping countries [23,24]. In the humid and sub-humid
tropics of Africa and Latin America, two new research
centers concerned with cassava research were established
in late 1960’s: IITA, the International Institute of Tro-
pical Agriculture, located in Nigeria, and CIAT, Centro
Internacional de Agricultura Tropical, located in Colom-
bia.
Given the necessary financial support, international
multidisciplinary teams of scientists were able, for the
first time, to conduct extensive research on cassava. They
collaborated with the few, already existing, national re-
search programs to improve germplasm collection and
characterization, breeding, agronomy, cropping systems
management, pest-and-disease control, and crop use.
These activities were based on increased understanding
of the physiological processes involved. Various resear-
chers reviewed results on many aspects of cassava re-
search in Africa, Asia, and Latin America over the last 3
decades [25-27].
In the following sections we review and highlight
some of the eco-physiological research conducted at
CIAT, particularly under relevant field conditions where
most cassava is grown, in relation to breeding improved
cultivars for both favorable and stressful environments
(i.e. climatic and edaphic factors). The research had laid
the foundations for cassava breeding and selection of
adaptable cultivars under both environments.
2. Cassava Research Strategy at CIAT
At first, breeding objectives were directed towards de-
veloping high-yielding cultivars for favorable conditions
where biotic and abiotic stresses were absent [28,29].
This strategy focused on selecting for high yield per unit
land area and comparing with traditional vigorous culti-
vars and/or landraces suitable for intercropping. Another
trait selected for was high dry matter content (i.e. high
starch content) in storage roots. Harvest indexes (HI),
(where HI = root yield/total plant biomass) were selected
to be higher than those (<0.5) of common landraces and
traditional cultivars [30].
However, most cassava production occurs in environ-
ments with varying degrees of stresses and with little, or
no, production inputs from resource-poor farmers. Hence,
later breeding strategy goals centered on selecting and
developing cultivars with adequate and stable yields, and
able to adapt to a wide range of biotic and abiotic stresses
[26,31,32]. This strategy was stimulated by cassava’s in-
herent capacity to tolerate adverse environments, par-
ticularly those where other major staple food crops such
as cereals and grain legumes would fail to produce. The
strategy also aimed to avoid and/or reduce the negative
consequences on the environment caused when high-
input (agrochemicals) production systems are adopted
[4].
In light of this environmentally sound breeding strat-
egy, research on cassava physiology has focused on both
basic and applied aspects of the crop under prevailing en-
vironments. The goal was to better understand and elu-
cidate the characteristics and mechanisms underlying
productivity and tolerance of stresses [4,33-35]. It was
also suggested that molecular biology tools would cer-
tainly help in achieving this goal, as would a deeper un-
derstanding of the agricultural systems and biology of
tropical crops (including cassava plant physiology) [36].
“Reference [36] pointed out that temperate-zone research
laboratories in OECD countries are currently not invest-
ing in such knowledge”.
Objectives included 1) characterizing materials from a
core collection of cassava germplasm held at CIAT for
tolerance of extended water shortages, either natural or
imposed, and of low-fertility soils; 2) studying leaf pho-
tosynthetic potential in relation to productivity under
various edaphic/climatic conditions; and 3) identifying
plant traits that may be useful in breeding programs. The
multidisciplinary and inter-institutional research app-
roach adopted was pivotal in achieving these objectives.
3. Potential Productivity in Near Optimum
Environment and Adaptability to Climate
Changes
3.1. Potential Storage Root Yield and Leaf
Photosynthetic Capacity
Building on the knowledge and insight gained about the
physiological mechanisms underlying patterns of dry ma-
tter partitioning into shoot and storage roots as related to
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 165
leaf canopy development in cassava, [29] developed a
computer-based simulation model to determine the ideal
plant type for maximum yield under favorable growth
conditions, both edaphic and climatic. The resulting
“simulated ideal plant type” required the following char-
acteristics: late branching at 6 - 9 months from planting
with no vegetative suckers, maximum leaf size near 500
cm2 per leaf blade at 4 months from planting, long leaf
life of ca. 100 days, LAI (leaf area index = m2 of one sur-
face leaf area/m2 of surface land area) between 2.5 and
3.5 during most of the growth cycle, a harvest index (HI)
greater than 0.5, nine or more storage roots per plant at a
population density of 10,000 plants/ha, and each plant
having two vegetative shoots originating from the origi-
nal cuttings. If this simulated ideal plant ever existed in
cassava germplasm or has been genetically bred for, then
it should yield in a year, according to the model predic-
tions, about 90 t/ha of fresh roots (about 30 t/ha dry mat-
ter), provided that the growing environment is optimal
(no stress).
Confirmation of the predicted potential cassava pro-
ductivity came from a maximum experimental yield of
90 t/ha fresh roots, which was equivalent to 27 t/ha of
oven-dried matter [37]. This remarkable productivity oc-
curred in a large field trial involving several (16 acces-
sions) improved cassava clones and breeding lines grown
for 308 days in the Patia Valley, Cauca, Colombia (alti-
tude 600 m, 2˚09N, 77˚04W) with annual precipitation
of 900 - 1000 mm, 60% of which occurred in the first
three months of crop establishment, and with a pro-
nounced dry period of three months before harvest. The
climate at Patia Valley is characterized by high solar ra-
diation (about 22 MJ·m–2·day–1), a high mean day tem-
perature (28˚C), and high atmospheric humidity (70%).
These climatic factors appear to be near-optimal for high
cassava productivity. Such productivity suggests that
cassava has high yield potential when grown under near-
optimum conditions. Similar productivity levels, were
obtained under irrigation in India [38]. Moreover, grow-
ing cassava in the seasonally dry environments of the
Limpopo river basin in South Africa that experiences se-
veral months of terminal drought and winter low mid-sea-
son temperatures resulted in fresh yields in some cultivars
as high as 54 and 66 t/ha at 6 and 12 months after plant-
ing, respectively [39]. Underlying this productivity is the
high photosynthetic capacity of cassava with maximum
net leaf photosynthetic rates (PN) between 40 and 50
µmol·CO2·m–2·s–1 under saturating solar radiation
(>1800 µmol·m–2·s–1 in the range of photosynthetic ac-
tive radia- tion, PAR), wet soils and high atmospheric
humidity [40]. These maximum PN are comparable with
rates observed in tropical C4 crops, such as sugarcane,
maize, sorghum, and millet [41]. Cassava is considered a
C3 - C4 intermediate species based on several physio-
logical, anatomical and biochemical leaf traits [42].
Oven-dried storage root yield across 127 accessions
screened in Patia Valley, Cauca, Colombia, was signifi-
cantly correlated with seasonal average upper canopy
leaf PN [37]. It was also positively correlated with pho-
tosynthetic nitrogen use efficiency (PNUE) (Figure 1),
attesting to the importance of internal leaf mesophyll
characteristics such as leaf anatomy and biochemical
traits.
Table 1 summarizes correlation coefficients of dry
root yield of several of these accessions, where there
were positive significant associations between yield and
leaf PN, PNUE, mesohphyll conductance to CO2 diffu-
sion, as well as activity of the C4 PEPC enzyme (phos-
phoenolpyruvate carboxylase). Cassava leaves possess
elevated PEPC activity that reaches 15% - 25% of those
in C4 tropical crops such as maize and sorghum, and
much greater than activities observed in typical C3 spe-
cies such as common beans [42].
These findings have important implications for cas-
sava capacity to fix carbon, as PEPC has higher activity
and more affinity to CO2 than the C3 Rubisco (Ribulose-1,
5-bisphosphatecarboxylase oxygenase), particularly at
higher temperatures and soil water stress (Table 2).
Thus, selections and breeding for high PN and higher
activities of both the C4 PEPC and the C3 Rubisco are of
paramount importance for yield improvement. There are
Figure 1. Relationship between oven-dried root yield (har-
vested 10 month after planting) and photosynthetic leaf
nitrogen use efficiency in field-grown cassava at the season-
ally dry Patia Valley, Cauca, Colombia. Leaf nitrogen use
efficiency values were calculated from leaf CO2 exchange
measurements of fully expanded upper canopy leaves dur-
ing dry periods of 5 - 8 month old plants using portable
infrared gas analyzers, and total leaf nitrogen content. In
these accessions LAI was near optimum through much of
the growing period [35].
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
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166
Table 1. Correlation coefficients and regression equations for various plant trait combinations in 18 cultivars selected from
the preliminary-screened 127 in Patia, Cauca, Colombia, 1987-1988. Leaf photosynthetic characteristics were determined in
upper canopy leaves from 5 - 8 month-old-plants. Leaf nitrogen content and PEPC activity were determined in upper canopy
leaves from independent leaf samples from 5-month-old-plants. Measurements were made during dry period. n = 18 [43].
Trait combination
x y Correlation coefficient (r) Regression equation (y = a + bx)
PN Yield 0.500* Yield = 0.178 + 0.047 PN
PNUE Yield 0.481* Yield = 0.605 + 0.062 PNUE
PEPC Yield 0.547* Yield = 0.804 + 0.057 PEPC
gm Yield 0.479* Yield = –0.066 + 0.014 gm
PEPC PN 0.597** PN = 18.43 + 0.69 PEPC
PEPC gm 0.532* gm = 83.5 + 2.0 PEPC
PEPC PNUE 0.698** PNUE = 6.42 + 0.58 PEPC
*, **indicate level of significance at P = 0.05 and 0.01, respectively; PN = net leaf photosynthetic rate (μmol·CO2·m–2·s–1); PNUE = photosynthetic nitro-
gen-use efficiency [mmol·CO2·kg–1·(total leaf nitrogen)·s–1]; PEPC = phosphoenolpyruvate carboxylase activity (μmol·kg–1·FM·s–1); gm = mesophyll conduc-
tance to CO2 diffusion (mmol·m–2·s–1); Yield = dry root yield (kg·m–2); Values of cultivars (means), and ranges: PN (25.1), 21 - 30.6; PNUE (12.1), 9.4 - 16.2;
PEPC activity (9.7), 6.3 - 14.0; gm (103), 93 - 126; Yield (1.36), 1.00 - 1.83; NOTE: The significant correlations between PEPC activity and photosynthetic
characteristics and yield of cassava point to the importance of the enzyme as a desirable selectable trait for cultivar improvement, particularly under stressful
environments. In these trials, the average PEPC activity (9.7) in cassava was 17% of activity in the C4 grain sorghum grown on the same plot [43].
Table 2. Activities of some photosynthetic enzymes in leaf extracts of field-grown cassava as affected by 8 weeks of water
stress commencing at 92 days after planting at Santander de Quilichao, 1993. Values are means ± S.D. Activities in µmol/mg
chl/min [44].
Unstressed Stressed
Clone PEPC Rubisco PEPC/Rubisco PEPC Rubisco PEPC/Rubisco
CM 4013-1 0.86 ± 0.12 0.28 ± 0.10 3.10 1.18 ± 0.17 0.30 ± 0.01 3.9
CM 4063-6 0.89 ± 0.05 2.30 ± 0.03 0.39 1.42 ± 0.26 0.62 ± 0.02 2.3
SG 536-1 1.46 ± 0.42 0.44 ± 0.12 3.30 1.33 ± 0.22 0.25 ± 0.08 5.3
MCol 1505 1.09 ± 0.10 0.57 ± 0.13 1.90 0.96 ± 0.16 0.89 ± 0.14 1.1
Avg. 1.08 0.90 2.2 1.22 0.52 3.2
% Avg. changes due to stress +13 –42 +45
NOTE: The enhancement of PEPC activity in stressed plants and the reduction in Rubisco activity which led to greater PEPC/Rubisco ratio. This finding indi-
cates the importance of selecting for higher activity of PEPC in cassava, particularly in dry hot environments.
large variations in the activities and in the kinetic proper-
ties of these enzymes in cultivated cassava as well as in
wild Manihot species, such as M. rubricaulis and M.
grahami. Leaves of these wild species possess very high
PN (>50 µmol·CO2·m–2·s–1), high PEPC activity in leaf
extracts (ranged from 1.5 to 5.5 µmol per mg chlorophyll
per minute, compared to 6 - 12 in sorghum, a C4 species,
and 0.2 - 0.4 in common beans, a C3 species [35]. Their
leaves also have a second, but short, palisade layer on
their lower surface coupled with numerous stomata on
both upper and lower surface (amphistomatous leaves),
traits that positively enhance CO2 uptake, as compared to
the mostly hypostomatous leaves of cultivated cassava.
3.2. Crop Adaptability to Climate Change and
Responses of Leaf Photosynthesis to
Temperature and CO2
In the face of climate change/global warming trends that
are predicted to adversely affect production of most food
crops, such as cereals and grain legumes, in the tropics
and sub-tropics, cassava role as a food, feed and biofuel-
crop, will be further enhanced because of its tolerance to
low-fertility soils, heat and drought stresses [4], and
(Figures 2 and 3).
The remarkable predicted suitability of cassava to pos-
sible increases in average surface Earth’s temperatures
caused by expected rises in atmospheric CO2 (and perhaps
other greenhouse gases) in the year 2030 and beyond (of
at least 1.5˚C, although some projections are higher, de-
pending on the Global Circulation Models used) is sub-
stantiated by the experimental data on the responses of
cassava photosynthesis to temperature and CO2. Re-
search on cassava physiology at CIAT had shown that
maximum cassava growth and productivity requires high
temperature (>25˚C), high solar radiation, high air hu-
midity and sufficient rainfall during most of the growth
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 167
Figure 2. Predicted changes in cassava suitability in the
year 2030 as average of 24 GCMs (Global Circulation Mod-
els) in sub-Saharan countries where cassava is a common
crop [45].
Figure 3. Predicted suitability changes in the year 2030 for
maize, sorghum, millet, common beans, potato and banana,
as average of 24 GCMs (Global Circulation Models), in
North Africa, and sub-Saharan region [45]. NOTE: The
contrasting suitability changes for these 6 food crops with
that of cassava in Figure 2.
period [40,42]. Figure 4 illustrates some results on the
responses of leaf PN to gas exchange-measuring tempera-
ture in normal air (containing about 335 µmol CO2/mol)
and at near-saturation photosynthetic active radiation
(PAR) for leaves that developed under cool climate
(mean daily temperatures were < 20˚C), for cool-climate
leaves that were acclimated for 7 days at warmer climate
(mean daily temperature around 25˚C), and for leaves
that were developed on the same plants in warmer cli-
mate. Also, representative responses of these sets of leaves
to measuring PAR are shown in one cultivar, M Col 2059.
In these trials, 8 cultivars representing cassava ecosys-
tems, that is: hot humid low-land, hot-dry low-land, hu-
mid high altitude, and sub-tropic cool eco-zones, were
tested and all had shown the same responses, indicating
cassava resilient response to varying climatic conditions.
Leaf photosynthesis (Figures 4(a), (b)) was lowest in
cool climate leaves , and after one week of acclimation in
warmer climate photosynthetic rates partially increased
with an apparent upward shift in optimum temperature,
particularly in the cool-humid habitat cv M Col 2059.
Rates of leaves developed in warm climate were the
highest, showing also an apparent upward shift in opti-
mum temperature. Rates in all sets of leaves were greater
in the hot-humid cultivar from Brazil, M Bra 12. These
findings attest to the adaptability of cassava to warmer
climate, and hence to its predicted suitability to future
climate changes as shown in Figure 2. The adaptation of
cassava photosynthetic capacity to warmer temperature is
also illustrated by the lack of light saturation in warm-
climate leaves (Figure 4(c)), compared to responses ob-
served in cool-climate leaves and in cool-climate leaves
acclimated for one week in warm climate.
Under field conditions at the university of Illinois,
Urbana, US, using the sophisticated “Free Air Carbon
Dioxide Enrichment (FACE)” method, increasing [CO2]
to 585 ppm within canopy for 30 days (though crop
growth stage was not reported) enhanced cassava leaf
photosynthesis, as measured at elevated [CO2, 585 ppm],
for both plants growing at ambient [CO2, 385 ppm] and
elevated [CO2, 585 ppm], with the former showing greater
response[46] (Figure 5). However, when leaf photosyn-
thesis was measured at [CO2] greater than 600 ppm,
plants grown at elevated CO2 showed, over the tested
external CO2 range, consistent and slightly higher rates
than plants grown at ambient CO2. Such data indicate
that acclimation of photosynthesis (i.e. lower maximum
carboxylation capacity of Rubisco), if it occurs due to
long exposure to higher than ambient CO2, may not re-
sult in reduction in cassava growth and productivity. Si-
milar findings were reported from Venezuela using open-
top chambers where cassava photosynthesis, growth and
root yield of field-grown plants exposed during its entire
growth period to double-ambient CO2 concentrations,
Copyright © 2012 SciRes. OJSS
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
Copyright © 2012 SciRes. OJSS
168
Figure 4. Response in terms of net photosynthetic rate (PN) of cassava to leaf temperature. (a) Cultivar M Col 2059 in a cool
habitat; (b) cv. M Bra 12 in a hot humid habitat; (c) response in terms of net photosynthetic rate (PN) to PAR irradiance in cv.
M Col 2059. refers to leaves developed in a cool climate; to leaves developed in a cool climate and then acclimated for 1
week in a warm climate; to newly developed leaves in a warm climate. Note that 1) an apparent upward shift in optimal
temperature is observed from cool to warm-acclimated and warm-climate leaves; 2) the lack of light saturation in warm-
climate leaves, compared with cool-and-warm-acclimated leaves; and 3) the higher maximum photosynthetic rates in all sets
of leaves of cv. M Bra 12 from the hot-humid habitat, compared with the cool-climate cv. M Col 2059 [42].
Figure 5. The response of cassava photosynthesis to [CO2] when grown for 30 days at ambient (385 ppm) and elevated (585
ppm) [CO2] in the field using FACE method. (Left panel) the response of photosynthesis to internal [CO2] (Ci). The dashed
and solid straight lines intersect the x-axis at the growth [CO2] of the plants used to measure these curves. (Right panel) in-
stantaneous photosynthesis for ambient-and-elevated CO2 grown plants measured at their respective growth [CO2], [46].
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 169
were significantly greater than those in ambient CO2-
grown plants [47]. Also, greenhouse-grown cassava un-
der elevated CO2 at USDA-ARS labs in Maryland, US,
had greeter photosynthesis, biomass and yield, compared
to ambient CO2 level-grown cassava [48]. These findings
contradict reports from Australia on potted indoor-grown
cassava, where leaf photosynthesis, plant growth and
storage roots were reduced in plants grown in elevated
CO2, compared to plants grown under ambient CO2 [49].
Cassava is a shrub that requires large volume of soils for
storage root development and filling, and therefore, in
the Australian experiments there were apparently feed-
back inhibition of leaf photosynthesis due to restricted
root-sinks. Moreover, cassava plant is resilient in nature
with plasticity in its growth habits forming several bran-
ches on main stems associated with reproductive organs
(i.e. flowers, fruits and seeds) in most cultivars, thus,
providing alternative sinks (in addition to its starchy sto-
rage roots) for extra photo-assimilates [29,33,35,50-52].
This type of growth and phenology behavior with multi-
ple and larger sink demands for photo-assimilates should
enhance leaf photosynthesis under elevated CO2 and,
hence, could lead to greater total biomass and yield [53,
54]. Using the EPIC crop model to assess the impact of
climate change on cassava adaptability and productivity
in marginal lands of northeastern Thailand, Sangpenchan
[55] reported that cassava grown in water-limited areas
would benefit from the so-called “CO2 fertilization” con-
tribution when combined with improved production
technologies. Moreover, the crop would likely respond to
rising CO2 by decreasing its evapotranspiration rate be-
cause of its tight stomatal control mechanism [42] and,
hence, increasing the efficiency with which it used lim-
ited water supply predicted with climate changes.
4. Response of Cassava to Air Humidity and
Prolonged Soil Water Shortage
4.1. Response to Air Humidity under Controlled
Laboratory Conditions and in Field
When cassava leaves were exposed to dry air under labo-
ratory conditions, their stomata closed in both well-wa-
tered and stressed plants without changes in leaf water
potential (Figure 6(a)). Transpiration initially increased
with rising leaf-to-air vapor pressure deficit (VPD) up to
2 kPa and then declined with further increases (Figure
6(b)). Such response contrasts with transpiration in
maize leaves where transpiration increased with rising
VPD. The closure of cassava stomata reduced also PN
beyond 1.5 kPa [56]. At canopy level in wet soils, raising
air humidity via fine misting enhanced PN (Figure 6(c))
that led to increases in storage roots [57,58]. The striking
response to humidity is a “stress avoidance mechanism”
that underlies cassava conservative water use, particularly
0
2
4
6
8
1.0 1.8 2.6 3.4 4.2
S tomatal conductance (mm· s
-1
)
Va
p
or
p
ressure deficit
(
kPa
)
Unwatered
Well watered
(a)
0
1
2
3
4
0 1 2 3 4
Transpiration (mmol ·H
2
O·m
-2
·s
-1
)
Vapor pressure deficit (kPa)
(b) Maize
Cassava
5
10
15
20
25
30
20 30 40 50 60 70 80
Photos ynth es i s , P N(µmol·CO2·m-2·s-1)
Relative humidity (%)
Misted plants
Non-misted plants
Y = 0.32x + 10
r² = 0.69
(
c
)
Figure 6. Response of cassava to atmospheric humidity. (a)
Effect of leaf-to-air vapor pressure decit on stomatal con-
ductance in well-watered and water-stressed cassava. Note
the rapid decline in stomatal conductance, as an indication
of stomatal closure, on exposure to low humidity irrespec-
tive of soil water conditions. Leaf water potential remained
unchanged in both well-watered plants (ca. 0.8 MPa) and
the water-stressed plants (ca. 1.2 MPa) during exposure to
low humidity; (b) Effect of leaf-to-air vapor pressure decit
on transpiration of cassava and maize leaves. Note the sharp
decline in transpiration rates of cassava at large vapor pre-
ssure decit as compared with rates of maize; (c) Effect of
air humidity on leaf photosynthesis of eld-grown cassava,
with and without misting. The soils were wet in both crops.
Note the strong correlation in PN vs. RH. Sources: (a), (b),
[58]; (c), [57].
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
170
under soil water deficits, an advantage as compared to
less sensitive species such as maize.
4.2. Responses of Field-Grown Cassava to
Prolonged Water Shortages at Different
Growth Stages and Implications for Nutrient
and Crop Water Use
Two-year field trials were conducted under prolonged
early water stress (occurring 2 - 6 months after planting);
mid-season stress (4 - 8 months after planting), and ter-
minal (i.e., end-of-season) stress (6 - 12 months after
planting). Table 3 presents data on root yield, shoot and
total biomass at 12 months, and nutrient use efficiency in
terms of root production. On one hand, water stress re-
duced shoot biomass in all stages but reduction was sig-
nificant only in early stress. On the other hand, final root
yield across clones was not significantly reduced at any
water shortage treatment. However, at the end of early
and mid-season stress both shoot and storage root were
significantly reduced, but recovered after release from
stress. This response demonstrates the resilience of cas-
sava and its adaptability to prolonged water stress.
Water stress enhanced nutrient use efficiency in terms
of storage root production in both early and mid-season
stress because of the great reduction in shoot biomass,
relative to roots, and hence, lesser total nutrient uptake
[59]. This response is beneficial for soil management
where cassava producers rarely apply purchased agroche-
micals.
Another trait that underlies cassava tolerance to ex-
tended water stress, is its ability to extract water from
deeper wet layers of soils (Figure 7). Combined with
stomatal sensitivity to atmospheric humidity, deeper
rooting system increases tolerance to drought and result
in higher crop water use efficiency, compared to some
other crops (Table 4).
The findings on the effect of water stress on nutrient
use efficiency due to less nutrient uptake by lesser shoot
biomass had led to the search for genotypes that differ in
their architecture in relation to nutrient use efficiency. In
a two-year field trials, a group of 15 clones that differed
in their height, top biomass, and LAI were grown in ab-
sence of soil water shortage and with adequate fertilizer
[42,60]. In this group, short-stemmed cassava showed
root yields that approached the tall cassava with tendency
to early storage root filling. Total nutrient uptake was
less in short-stemmed cassava resulting in higher nutrient
use efficiency in terms of root production (Table 5).
Short cassava possessed lower LAI, lower shoot biomass
and higher PN, as compared with tall cassava. The LAI in
short-stemmed cassava was lower than the optimum re-
quired for efficient light interception, and hence, less
productivity when planted at 10,000 plant per hectare. At
higher population densities, short cassava may out-yield
tall ones. In low fertility soils, where cassava normally is
grown without fertilization, short cassava is beneficial
Table 3. Effect of water stress on 12-month yield, biomass and nutrient use efficiency (NUE) for storage root production (kg
dry roots/kg total nutrient uptake). Fallen leaves were excluded. Data are means of four clones in two years, [59].
Water regime Dry total biomass (t/ha) Dry roots (t/ha)Dry shoots (t/ha)N P K Ca Mg
Control 18.9 12.9 6.0 120 660 110 300 440
Early stress 14.5 11.2 3.3 150 940 150 360 650
Mid-season stress 18.0 12.9 5.1 120 840 140 330 530
Terminal stress 16.9 11.7 5.2 110 690 120 300 520
LSD 5% 2.3 NS* 1.4 12 70 9 50 65
*NS = Not significant at 5%; Control: crop was well watered; Early stress: crop was deprived of water between 2 months to 6 months after planting; Mid-season
stress: crop was deprived of water between 4 months to 8 months after planting; Terminal stress: crop was deprived of water from 6 months after planting until
harvest at 12 months.
Table 4. Comparative water use efficiency (WUE) of cassava, grain sorghum and common bean [58].
WUE
single-leaf gas exchangefield-grown crops economic yield Species
μmol CO2·mmol–1 H2O g dry weight·kg–1 H2O g dry weight·kg–1 H2O
Cassava 5.3 2.9 1.7 (HI 60%)
Sorghum 6.2 3.1 1.2 (HI 40%)
Common bean 3.5 1.7 0.7 (HI 40%)
Cassava/sorghum (%) 85 94 140
Cassava/common bean (%) 150 170 240
HI: harvest index = (dry grain or dry storage root)/(total dry weight) × 100.
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 171
Table 5. Nutrient use efficiency (NUE) for root production at 10 months after planting [kg (dry root)·kg–1 (total nutrient up-
take)] for groups of tall and short cassava cultivars [42,60]. Data are averages of several cultivars in two years within each
plant type group.
Plant Type N P K Ca Mg
Tall 110 715 132 347 589
Short 131 885 161 430 669
LSD 5% 17 85 22 77 91
Stress at 6 months after planting
106 days stress
57 days stress
40 days stress
Soil depth (cm)
Stress at 2 months after planting
90 days stress
60 days stress
30 days s
t
ress
150
180
120
90
60
30
0
Stress at 4 months after planting
113 days stress
73 days stress
46 days stress
abc
0.00 0.03 0.06 0.00 0.03 0.06 0.00 0.03 0.06
Water uptake (m3·m–3)
Figure 7. Patterns of water uptake as average of four cassava cultivars during extended periods of water deficits of different
lengths, Santander de Quilichao, department of Cauca, Colombia. Note the greater water extraction from deeper soil layers
that increased as water stress progressed over time, particularly in terminally stressed crops (C) [42].
for soil fertility management when grown at high popula-
tion density to increase canopy light interception, and
hence, productivity. The higher nutrient uptake in tall
cassava was the reason for its lower nutrient use effi-
ciency. In most cassava regions, farmers don’t recycle
the crop residue to the soil after harvest, thus a signifi-
cant amount of nutrients are removed. It was reported in
Kerala state, southern India, that under sufficient rainfall
(>1500 mm in 10 months), the short duration improved
cultivar, namely Sree Vijaya (6 months duration and high
HI, released in 1998), had greater yield and greater NUE
in terms of storage root production per unit of nitrogen
uptake than the traditional cultivar, namely M-4 (10
month duration and low HI) [61]. It appears that for soil
conservation and better use of nutrient in cassava crop-
ping systems, the choice of cultivar is an important ap-
proach. Therefore, breeding for short-to-medium stature
genotypes is warranted.
5. Comparative Advantage of Cassava
versus Other Food and Energy Crops
5.1. Performance of Cassava as a Source for
Food Energy and Biofuel in Comparison
with Some Important Food and Energy
Crops
In absence of production constraints under near-optimum
environments, cassava potential productivity is quite large
that might reach >80 t/ha fresh roots annually [4,37]. For
example, in the Patia Valley, Cauca Department, Colom-
bia, which is near ideal low-land tropical eco-zone for
cassava production, yields as high as 90 t/ha in 308 days
(equivalent to 27 t/ha oven-dried dry matter) were ob-
tained with a group of improved clones. Table 6 sum-
marizes energy yields for cassava and some other food
crops based on maximum observed yields. It is apparent
that cassava compares favorably in terms of energy yield
with other major staple food crops while having the
greatest potential. Experimental energy productivity in
cassava exceeds that observed in the most productive C4
warm climate cereals, such as maize and grain sorghum,
as well as the tropical C3 rice, but less than sugarcane.
Moreover, cassava inherent tolerance to abiotic stresses
(e.g. poor soils and prolonged drought coupled with high
temperatures) allows the crop to be sustainably produc-
tive in adverse environments where most other staple
food crops would fail to produce reasonable yields [4,5].
This advantage is illustrated by the superior predicted
suitability of cassava against several tropical food crops
in the face of expected climate changes in coming dec-
ades (see Figures 2 and 3) [45].
This inherent biological ability to produce very high
yields of starchy roots in near optimum environments, as
well as its remarkable tolerance to adverse conditions,
recently have made the crop a desired source for renew-
able energy in many countries where it is commonly
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
172
Table 6. Maximum recorded yield and food energy production of some important tropical food crops.
Crop species Annual yield (t/ha) Daily energy yield (kcal/ha)a Daily energy yield (kj/ha)b
Cassava (fresh storage roots) 71 250 1045
Maize(dry grain, 10% moisture) 20 200 836
Sweet potato (fresh storage roots) 65 180 752
Paddy rice (dry grain, 10% moisture) 26 176 652
Sorghum(dry grain, 10% moisture) 13 114 477
Wheat(dry grain, 10% moisture) 12 110 460
Banana(fresh fruit) 39 80 334
Estimates from: (a) [62]; (b) [35].
produced, including Thailand [63], a traditionally cas-
sava exporting country, and China [64,65] where a cur-
rently high demand for fuels exists. In the past decade,
cassava-derived bioethanol production has been increas-
ing due to its economic advantages, compared to other
bioethanol-source crops as shown in Table 7. Also, cas-
sava can be grown on marginal lands that do not support
production of food crops, particularly cereals, thus obvi-
ating competition with the need for food. The potential
yield of bioethanol is apparently higher in cassava than
for any other plant species, including the traditional bio-
ethanol source crops such as maize, sweet sorghum and
sugarcane. However, the gap between the potential ex-
perimental yields of cassava and the actual yields on
farmers’ fields is more than fivefold [4,5]. With the ex-
ception of India, current farmers’ yields as low as 6 - 8
t/ha exists in Africa and as high as 13 - 18 t/ha occurs in
some Asian and Latin American countries. These low
yields are normally attained with local, traditional varie-
ties grown on marginal soils without the application of
purchased agrochemicals. The expected higher demands
in developing countries for cassava products as food,
feed, and industrial uses in the face of climate changes
would call for the removal of the many socioeconomic
constraints on cassava production, uses and marketing.
Moreover, since the cassava plant has inherently high
leaf photosynthetic capacity in current air and at high
temperature coupled with high solar irradiances [40,42],
also (see Figure 4), and responds positively to elevated
CO2(see Figure 5), possible future expansion in cassava
cultivation may enhance atmospheric carbon sequestra-
tion, and hence helps mitigating adverse effects of glob-
ally warming climate [66]. Under the predicted CO2 rises
in this century (up to 700 ppm by some GCMs [67]),
cassava may be one of the few tropical food crops that
can adapt to this climatic changes by shifting upward its
optimum temperature for photosynthesis, growth and
production. Most crops increase their WUE in elevated
CO2 environments, particularly under water deficits [68],
due to both higher carbon uptake and lower stomatal
conductance to gas diffusion that lead to less transpira-
tion water losses. Cassava is equipped with a tight stoma-
tal control mechanism over gas exchanges, which is more
sensitive to changes in air humidity and soil water status
than other crops [4,35,42,56-69], also (see Figure 6),
making it highly efficient in water use (see Table 4).
5.2. Comparative Soil Nutrient Extraction by
Cassava and Other Food Crops
In contrast with the high-input technology used in the
Green Revolution crops (i.e. rice, wheat and maize),
most of cassava production in the tropical and subtropi-
cal agro-ecosystems is done by resource-poor small far-
mers on marginal lands, with often degraded soils, virtu-
ally without application of purchased agrochemicals [4,5,
71]. Cassava is tolerant to tropical highly leached acidic
soils low in pH, high in exchangeable aluminum and
particularly low in phosphorus (P) [50,70-72]. Mid-term
cassava responses in infertile sandy soils in northern Co-
lombia (private farm) (Figure 8), and long-term re-
sponses to acidic clayey soils low in nutrient contents at
CIAT Experimental Station, Santander de Quilichao
(Figure 9), illustrate both the level of tolerance of cas-
sava to poor soils and the positive responses to fertiliza-
tion. In the sandy soils (Figure 8), cassava kept produc-
ing > 2 t/ha oven-dried storage roots without fertilization
during several consecutive cropping cycles, with noted
differences among cultivars (the highest tolerance level
and greatest response to NPK fertilizers were in cultivar
M BRA 191). In the acidic clayey soils (Figure 9), cas-
sava kept producing during 6 years of consecutive crop-
ping reasonable yields (>15 t/ha fresh storage roots) in
absence of application of any major element, i.e. N, P,
and K. There were positive responses to applications of
these elements, with the largest responses observed with
the application of K. At 12 years of consecutive cultiva-
tion in this trial, dry root yields in absence of N applica-
tion, but with P and K, were 7.9 t/ha for M Col 1684, and
4.7 t/ha for CM 91-3. In absence of P application, but
with N and K, dry root yields at the 12th year were 6.1
and 4.7 t/ha, for M Col 1684 and CM 91-3, respectively.
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 173
Table 7. Comparative advantage of cassava as a potential biofuel crop versus other energy crops [65]. Yield estimates are
based on favorable production conditions.
Crop species Annual average yield (t/ha) Conversion rate to bioethanol (Liter/t) Annual bioethanol yield (Liter/ha)
Cassava 40 (fresh storage roots) 150 6000
Sugarcane 70 (fresh canes) 70 4900
Sweet sorghum 35 (biomass) 80 2800
Rice 5 (grain) 450 2250
Maize 5 (grain) 410 2050
Wheat 4 (grain) 390 1560
Figure 8. Mid-term response of cassava to fertilizer in san-
dy poor soil, Media Luna, Magdalena, Colombia. The fer-
tilized treatment received annually 50 kg N, 20 kg P, and 43
kg K/ha. (a) cv. M BRA 191, from hot-dry low-land Brazil;
(b) cv. M COL 1505, local variety in northern Colombia; (c)
CG 1141-1, and (d) CM 3306-4, CIAT breeding lines, which
out-yielded the local cultivar M Col 1505 [73].
Without K application until the 12th year, but with N and
P, yields were extremely low at 2.9 and 1.7 t/ha dry mat-
ter for M Col 1684 and CM 91-3, respectively. This was
due to the removal of more than 70% of absorbed K
along with the harvested starchy roots, indicating that K
is the most critical nutrient. However, in absence of an-
nual application of the three nutrients (N,P,K) during 12
years of consecutive cropping, oven-dried storage root
yields remained at 2.9 t/ha for M Col 1684 and 2.1 for
CM 91-3, attesting to cassava high tolerance to exhausted
acidic soils. Noteworthy, in absence of NPK fertilization,
production of reproductive organs (flowers, fruits and
seeds) was enhanced, and HI increased, indicating phe-
nology changes in cassava growing on infertile soils as
previously observed [50].
Without NPK applications for 12 years, average sea-
sonal PN of upper canopy leaves, as measured with pho-
tosynthetic active radiation > 1000 µmol·m–2·s–1 and in
Figure 9. Long-term response of cassava (cv. M Col 1684
and CM 91-3) to NPK fertilizer in a low-fertility acidic
clayey soil at Santander de Quilichao, Cauca, Colombia.
Note that the greatest limitation to cassava production was
K [74].
normal air having 350 ppm, was around 20 - 25 µmol
CO2 m
–2·s–1, compared to 30 - 35 µmol CO2 m
–2·s–1 in
plants receiving annually 100 kg/ha each N, P and K.
The leaf photosynthetic capacity in cassava remains re-
markably high, compared to other warm-climate legume
(e.g. common beans, C3) and cereal (e.g. maize, C4) food
crops, under extremely low soil nutritional status, which
may underlie its ability to sustain reasonable yields [34,
50,74,75]. Another plant trait that may explain the rea-
sonable carbon uptake rates in absence of NPK applica-
tion was the lower leaf area per plant as well as lower
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
174
LAI because of restricted new leaf formation with sma-
ller size and lower specific leaf area (leaf area/unit leaf
weight) [34,50], thus allowing concentrated and sustain-
able leaf NPK contents. In these soils, it appears that the
limitation to cassava storage root production, when
grown continuously for long period on the same land, are
in decreasing order: K > P > N. Very few, if any, food
crops will tolerate such poor soils and be able to produce
reasonably without fertilization, compared to cassava.
This comparative advantage in favor of cassava have led
many to erroneously believe that cassava removes high
volumes of nutrients, and hence renders the already poor
soil unsuitable for cultivation of other food crops.
Table 8 presents the average extracted quantities of
major nutrients by cassava storage roots and by harvested
yields of other food crops. It is clear from these data that
cassava removed less N and P per ton of dry root than
values in harvested products of other crops. Removal of
K was either similar or lower than some other crops. Be-
cause of the high yield in cassava, the crop removed
equal amounts of N and P per hectare as with other crops.
However, cassava removed more K per hectare than any
other crop, as >70% absorbed K is removed in storage
roots. Thus, the negative reputation concerning cassava
cultivation as a cause of soil degradation is not based on
sound scientific facts as illustrated here and in published
literature. Cassava is very resilient and highly tolerant to
abiotic stresses, an advantage over many other staple
food crops as shown by its higher predicted suitability to
climatic changes (see Figures 2 and 3).
6. Selection for Tolerance of Low Fertility
Acidic Soils
6.1. Rational
As most cassava production by smallholders occurs in
marginal lands with low levels of soil fertility [4,5], cas-
sava breeding strategy at CIAT focused on selection for
adaptation to farmer’s field conditions [4,31,34,77]. Cas-
sava soil-and-plant nutrition management section [70,71],
and later cassava physiology section [4,34,35,42,50,56,
72], oriented their research objects toward characteriza-
tion of CIAT cassava germplasm in response to infertile,
low-P, acidic soils in the South American tropics. From
1982 to 1996, more than 1800 accessions, including land
races, common varieties and elite CIAT breeding lines
have been evaluated for responses to P, and many clones
with high level of adaptation to low P (and with high
response to P application) have been identified and in-
cluded in crop improvement program [44,50,72-75,77,
78]. Later several dozens of cassava core germplasm
have also been tested for their tolerance to low-K soils,
with few clones with high level of tolerance have been
identified [79,80]. In the following subsections, data of
many tested accessions for their tolerance of low-P and
low-K soils, as well as responses to P or K fertilizer ap-
plication, are presented.
6.2. Performance of Some Cassava Clones at
Zero and 75 kgP/ha
For screening large accessions for response to low ferti-
lity soils, we adopted a simple field method to test at two
levels of P, i.e., zero and 75 kgP/ha. A calculated adapta-
tion index to low P, taking into account yields of a given
clone in relation to the overall mean of the trial at both
low and adequate levels of applied P fertilizer, indicates
the degree of tolerance. Data of a group composed of 33
clones from CIAT core germplasm, including land races,
common varieties and advanced breeding materials, that
were tested for three years are presented in Figure 10,
and in Table 9. Clones with low-P adaptation index
above the overall mean of the trial (1.0), have been iden-
tified as having a reasonable degree of tolerance. In this
group, there were 13 clones with high and moderate de-
gree of tolerance, with several CIAT breeding materials
Table 8. Representative dry yield (t/ha), and major nutrient extraction estimates from soils, as expressed in kg/ha and kg/t of
harvested yield, for cassava and some other staple tropical crops [70,71].
Crop species Dry yield (t/ha) N (kg/ha) P (kg/ha) K (kg/ha) N (kg/t) P (kg/t) K (kg/t)
Cassava (storage roots) 13.53 55 13.2 112 4.5 0.83 6.6
Sweet potatoes (storage roots) 5.05 61 13.3 97 12.0 2.63 19.2
Sugarcane (cane) 19.55 43 20.2 96 2.3 0.91 4.4
Maize(grain) 5.56 96 17.4 26 17.3 3.13 4.7
Sorghum (grain) 3.10 134 29.0 29 43.3 9.40 9.4
Paddy rice (grain) 3.97 60 7.5 13 17.1 2.40 4.1
Soybeans (grain) 0.86 60 15.3 67 69.8 17.79 77.9
Common beans (grain) 0.94 37 3.6 22 39.6 3.83 23.4
Groundnuts (pod) 1.29 105 6.5 35 81.4 5.04 27.1
Tobacco (leaves) 2.10 52 6.1 105 24.8 2.90 50.0
Copyright © 2012 SciRes. OJSS
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 175
Table 9. Dry root yield of cassava as affected by phosphorus application, and Low-P adaptation index for some cassava clones
(mean of 3 years). Source: M. A. El-Sharkawy, unpublished.
Clone Zero kg P/ha dry root yield (t/ha)75 kg P/ha dry root yield (t/ha) Low-P adaptation indexa
SG 779-9 7.3 11.4 1.8 HAb
CM 5830-4 7.3 11.2 1.7 HA
CG 333-4 7.2 10.7 1.6 HA
CG 5-79 7.5 10.1 1.6 HA
CM 4774-2 6.9 10.6 1.5 HA
CM 3555-6 6.2 10.4 1.4 MAc
M BRA 390 6.8 9.7 1.4 MA
SM 366-2 6.5 10.4 1.4 MA
CG 95-1 7.0 8.5 1.3 MA
CG 1355-2 6.1 10.4 1.3 MA
CG 996-6 5.6 10.6 1.3 MA
M BRA 589 5.7 10.4 1.3 MA
SM 380-3 5.6 11.2 1.3 MA
Mean of trial (33 clones) 5.4 8.8 1.0
LSD 5% for clones 1.1 1.7
LSD 5% for P level 0.4
a) Low-P adaptation index =


Dry root yield at zero P Dry root yield at 75 kgPha
(Mean clones dry root yield at zero P) Mean clones dry root yield at 75 kgPha; b) HA = High adaptation; c) MA = Moderate
adaptati o n .
Figure 10. Relationship between dry root yield of 33 clones
tested for three consecutive years at zero and 75 kgP/ha on
the acidic clayey soils at Santander de Quilichao, Cauca,
Colombia. Clones located within the right top quadrant
were identified and selected for their high tolerance to Low-
P as well as their response to P application. Source: M. A.
El-Sharkawy, unpublished.
are highly to moderately tolerant. Two cultivars of Bra-
zilian origin, i.e., M BRA 390 and M BRA 589, are to-
lerant to low-P soils, indicating the efficient selection
under poor soils in Brazil. In previously tested group of
accessions, another two Brazilian cultivars, M BRA 191
and M BRA383, were identified with high level of toler-
ance [35,78]. Table 10 [75,78] presents plant traits, leaf
gas exchange characteristics of upper canopy leaves,
along with correlations of these traits with Low-P adap-
tation index, determined on another 33 accessions. At
zero P, PN was significantly higher than values at 75
kg/ha P, and this coincided with increases in stomatal
conductance to water vapor and in mesophyll conduc-
tance to CO2 diffusion, suggesting that the difference
may be attributed to both stomatal behavior as well as to
mesophyll biochemical and anatomical differences. Since
LAI was significantly lower at zero P, the higher PN
could be partly due to less water stress resulting from
lower transpiration water losses by crop canopy. Alterna-
tively, the difference in PN may be also attributed to feed-
back inhibition at adequate P because of larger LAI,
which represents greater crop photosynthetic surface ca-
pacity. It is likely, therefore, that source-sink relationship
for photosynthetic products was implicated in this sort of
phenomenon. Application of P increased significantly
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
176
Table 10. Productivity, growth and physiological characteristics of cassava grown at low and adequate P levels at Santander
de Quilichao, and correlation with Low-P adaptation index [75,78]. Values are means of 33 clones.
Parameter Zero P 75 kg P/ha Correlation coefficients with
Low-P adaptation indexa
PN (µmol·CO2·m–2·s–1) 31* 27 0.51**
Stomatal conductance (H2O),
(mmol·m–2·s–11) 928 851 0.30 ns
Mesophyll conductance (CO2),
(mmol·m–2·s–1) 206 178 0.57**
Dry root yield (t/ha) 8.8* 12.5 0.99**
Shoot dry biomass(t/ha) 3.8* 5.7 0.58**
Total biomass (t/ha) 12.6* 18.2 0.96**
Number of storage root/plant 9* 12 0.67**
LAI (m2·m–2) 2.0* 3.1 0.55**
HI (dry root/total biomass) 0.81* 0.70 0.73**
Mean of the two P levels across 33 clones; ns=not significant at 5%; *= significant at 5%; **= significant at 1%.
number of storage roots per plant, shoot biomass and dry
root yield, but HI was lower, compared to zero P. Except
with stomatal conductance, Low-P adaptation index was
highly significantly correlated with all of the growth and
gas exchange traits measured, indicating the validity of
this index as a measure for identifying plant traits related
to productivity. It may be concluded that, carbon assimi-
lation rates and sources (i.e. leaf PN and canopy sea-
sonal LAI) as well as sink strength and capacity for
photo-assimilates (storage root number and capacity) are
of paramount importance as selectable traits for cassava
improvement under diverse edaphic environmental con-
ditions [34,35,50,72,81].
6.3. Performance of Some Cassava Clones at
Zero and 100 kgK/ha
Figure 9 and several reports [70,79,80,82-86] have
shown significant responses to K fertilizer in different
soils, particularly when cassava was grown continuously
for several years in the same field due to removal of large
K amount (>70% of extracted K) in harvested storage
roots. Moreover, research at CIAT [79,80,87] have indi-
cated the existence of genetic diversity in response to K
application and in K use efficiency, which warranted
further selection for tolerance to low-K soils.
Table 11 and Figure 11 present 5-year average data
on oven-dried root yield at 10 months after planting 15
clones on acidic clayey soil at CIAT Santander de Quili-
chao Experimental Station, in response to zero and 100
kg K/ha. In these trials, mean dry root yields at first year
were 11.1 and 13.2 t/ha for zero and 100 kg K/ha, re-
spectively. Response to K application was significant
(LSD 5% = 1.4 dry t/ha). At the fifth year of consecutive
cropping, average yields across all clones had dropped to
3.7 and 7.3 t/ha for zero and 100 kg K/ha, respectively.
The reduction in yield was so dramatic to the extent that
application of 100 kg K/ha had failed to sustain produc-
tivity that was obtained at the initial year of trial without
K application. The reduction in yields were even greater
in absence of K application (average yield dropped from
11.1at the 1st year to 3.7 t/ha at the 5th year), indicating
the large depletion of soil native K. Application of K at
the 5th year almost doubled the yield relative to value of
unfertilized crops (7.3 t/ha at 100 kg K as compared to
3.7t/ha without K application). Removal of K by storage
roots were 72% of total K uptake in both zero and 100kg
K/ha, which is in the range reported in previous trials
[79,80,87]. Thus, in these soils potassium becomes the
most limiting factor (see Figure 9) after several years of
continuous cropping without sufficient fertilization to
compensate for K removal in harvested roots. Because
most cassava production is done by resource-poor small-
holders [4,5] in marginal lands low in native essential
major nutrients, and mainly without application of ferti-
lizers, genetic alleviation of such critical situation be-
comes of paramount importance. Selection and breeding
for improved cultivars that tolerate low-K soils as well as
possess high K use efficiency (i.e. higher yield/nutrient
uptake) would benefit the poorest of the poor cassava
farmers in the tropics. Such desired materials with high
to moderate Low-K adaptation index were identified and
selected as shown in Table 11 and Figure 11 and in
[79,80,87]. Out of 15 accessions (all are CIAT breeding
materials), there were 5 clones with high Low-K adapta-
tion index (ranging from 1.5 to 2.2, relative to the overall
trial mean of 1.0), 5 clones with moderate adaptation
index (ranging from 1.2 to 1.4), and 5 clones with low
adaptation index (ranging from 0.4 to 1.0). To illustrate
the agronomic benefits of these selected materials, the
estimated K use efficiency values, in terms of dry root
produced per unit K uptake, were 260 and 180 kg dry
root/kg K uptake at zero and 100 kg K/ha, respectively,
Copyright © 2012 SciRes. OJSS
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement 177
Table 11. Dry root yield (Y, t/ha, average of 5 years) of cassava as affected by potassium application, and Low-K adaptation
index for some cassava clones. Source: M. A. El-Sharkawy, unpublished.
0 kg K/ha 100 kg K/ha
Clone Y Y Low-K adaptation indexa
CM 4777-2 8.7 13.5 2.2 HAb
CG 402-11* 8.7 12.6 2.0 HA
CM 4574-7 7.6 12.7 1.8 HA
CM 5286-3 7.6 10.5 1.5 HA
CG 165-7 6.3 10.8 1.5 HA
CM 4729-4 7.8 9.5 1.4 MAc
CG 1141-1 6.7 10.2 1.3 MA
CM 2777-2 6.1 11.2 1.3 MA
CM 3372-4 6.1 10.7 1.2 MA
CM 3306-4 5.8 10.4 1.2 MA
CM 3311-3 6.3 8.9 1.0LAd
SG 107-35 6.1 8.9 1.0 LA
CM 2766-5 5.8 8.6 0.9 LA
CM 2177-2 5.2 7.3 0.7 LA
CM 3299-4 3.6 5.5 0.4 LA
Mean of trial (15 clones) 6.6 10.1 1.0
LSD 5% for clones 1.1 1.2
LSD 5% for K level 0.4
*CG 402-11, is the tallest and greatest shoot biomass, compared to the rest of accessions.
aLow-K adaptation index =


Dry root yield at zero K Dry root yield at 75 kgKha
(Mean clones dry root yield at zero K) Mean clones dry root yield at 75 kgKha; bHA = High adaptation; cMA = Moderate
adaptation; dLA= Low adaptation.
Figure 11. Relationship between dry root yield of 15 clones
tested for five consecutive years at zero and 100 kg K/ha on
the acidic clayey soils at Santander de Quilichao, Cauca,
Colombia. Clones located within the right top quadrant
were identified and selected for their high tolerance to Low-
K as well as their response to K application. Source: M. A.
El-Sharkawy, unpublished.
in Clone CM 4777-2 showing the highest K adaptation
index (of 2.2). On the other hand, in clone CM 3299-4
showing the lowest K adaptation index (of 0.4), the K
use efficiency values were 193 and 140 kg dry root/kgK
uptake at zero and 100 kg K/ha, respectively.
In this group of clones, all, except CG 402-11, are
short-to-medium stature (<2.0 m) with low shoot bio-
mass and high HI. CG 402-11with the second highest
adaptation index of 2.0, has the tallest stature (>2.0 m),
largest shoot biomass (7.2 and 9.8 t/ha at zero and 100 kg
K/ha, respectively, compared with the respective trial
mean values of 2.9 and 4.1), the greatest total K uptake,
of which 45% - 47% in shoot( 68 at zero K and 91 kg
K/ha at 100 kg K, compared with the respective trial
mean values of 44 and 80 kg K/ha), and the least K use
efficiency (158 kg dry root/kg K at zero K and 128 at
100 kg K, compared to the overall respective trial mean
values of 210 and 148 kg dry root/kg K). These findings
substantiate data in Table 5, and support the strategy for
selecting short-to-medium cultivars in order to alleviate
pressure on soil fertility without sacrificing root yield [42,
60]. It may be concluded that the calculated adaption
index, as defined here, is a reliable agronomic indicator
for screening large number of accessions under field
conditions. Coupled with this adaptation index, it is rec-
ommended to determine nutrient uptake for estimating
Copyright © 2012 SciRes. OJSS
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
178
nutrient use efficiency.
7. Selection for Photosynthesis in Relation to
Productivity under Prolonged Drought
Coupled with High Temperatures in
Seasonally Dry and Semiarid
Environments
7.1. Rational
Current hydrological and GCMs models [66-88] predict,
within the next decades, the occurrence of extended
drought periods across continents, coupled with irregu-
larity in intensity and distribution of rainfall, as well as a
possible increase in land area prone to drought in tropical
and subtropical regions. This expected shortage in water
resources, combined with rises in Earth’s surface tem-
perature, will be significant enough to negatively impacts
agricultural productivity and food security for the pro-
jected >9 billion world population [67,89-93], particu-
larly in developing countries. The inherent capacity of
cassava to tolerate adverse environments, a comparative
advantage over most tropical staple food crops, enhanced
the expansion of the crop cultivation in more marginal
areas in sub-Saharan Africa, Northeastern Brazil and
other areas in Asia [4,54,94]. Moreover, in the coming
decades when experiencing globally warming climate,
cassava will play even more important role, as other less
adapted staple food crops will probably fail to produce
reasonably [45,54,95], (see Figures 2 and 3). Cassava
responds positively to elevated CO2 [46-48] (see Figure
5), and to high temperatures [42] (see Figure 4), two
crucial atmospheric characteristics of climate change.
Adaptation and mitigation measures then become essen-
tial approaches to obviate expected adverse effects of
climate change [66,96], with the development of im-
proved genetic and agronomic technologies being the
main elements. The CIAT’s cassava research took the
initiative to contribute to such approach.
7.2. Photosynthesis and Yield Performances in
Seasonally Dry and Semiarid Environments
in Northern Coast of Colombia
Field trials were conducted on private farms at two rep-
resentative dry locations in northern Colombia [97]. The
seasonally dry site (at Santo Tomas, Atlantic; elevation
14 m; latitude 10˚57N; longitude 74˚47W, and characte-
rized by: <900 mm of rainfall in 7 months; annual pan
evaporation of 1650 mm; mean temperature of 28˚C; an-
nual solar radiation of 6752 MJ·m–2; extended dry season
of 4 - 5 months; sandy infertile soils, >80% sand). The
semiarid site (at Riohacha, Guajira; elevation 4 m; lati-
tude 11˚32N; longitude 72˚56W, and characterized by:
<600 mm of rainfall in 7 months, 64% occurred in 3
months; annual pan evaporation of 2293 mm; mean tem-
perature of 28˚C; annual solar radiation of 6816 of
MJ·m–2; extended dry period >5 months; sandy infertile
soils, >80% sand). No chemical fertilizers nor irrigation
were applied in both trials. Two groups of CIAT breed-
ing lines and cultivars originating in Brazil, Colombia
and Venezuela were evaluated for their upper canopy PN,
measured by portable infrared gas analyzers on several
occasions during dry periods on 4 - 5 months after plant-
ing, and yields were determined at 11 months after plan-
ting.
Table 12 presents data on yield and leaf gas exchange
at both environments. At the seasonally dry environment,
upper canopy PN ranged from 25 to 31 µmol·CO2·m–2·s–1
among cultivars, whereas at the semiarid environment PN
ranged from 7 to 20 µmol·CO2·m–2·s–1, with cultivars
significant differences. Both stomatal conductance and
intercellular CO2 varied significantly among cultivars.
Average values of leaf gas exchange were much greater
in the seasonally dry environment, indicating the pro-
nounced effect of drought in semiarid environment where
variations among cultivars were wider. The top ranking
clones CG 1141-1 at seasonally dry location and CM
4013-1 at semiarid location, both are CIAT breeding
materials. Local varieties, i.e. M Col 1505 and M Col
2215, were also within the top ranks, indicating their
adaptability to these environments. The short-stemmed
M Col 2215 was tested earlier in the seasonally dry Patia
Valley, Cauca, and was identified for its tolerance to
drought, high dry matter content in storage root (>40%),
better leaf retention, high PN and elevated PEPC acti-
vity [37,43]. Therefore, it was introduced to Ecuador in
late 1980’s for testing in the semiarid western coast
(rainfall < 600 mm in 4 months), and was officially re-
leased in 1992 under the local name “Portoviejo 650”,
after farmers participation in its evaluation for three
years in various locations [44].
Oven-dried storage root yields varied significantly
among cultivars in the two environments, ranging from
0.65 to 0.76 kg/m2 in the seasonally dry, and from 0.04 to
0.33 kg/m2 in the semiarid, with the overall mean of the
former (0.67 kg/m2) was more than double that in the
later (0.23 kg/m2). Such productivity in these harsh en-
vironments attests to the remarkable potential of cassava
and its resilience and tolerance to extreme environmental
conditions, both edaphic and climatic.
Yield in both environments was highly significantly
correlated with seasonal average upper canopy leaf PN
(Figure 12), indicating the importance of selection for
higher photosynthetic capacity in relation to productivity
in these environments. The relation was due more to non-
stomatal effects (such as mesophyll anatomy and bio-
chemical components of carbon fixation pathway), since
yield was significantly negatively correlated with the
intercellular CO2 (Ci) (Figure 13). In other trials, at the
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
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179
Table 12. Average net photosynthetic rate, PN (µmol·(CO2)·m–2·s–1), stomatal conductance, gs (mol·(H2O)·m–2·s–1), intercellu-
lar CO2concentration, Ci (µmol/mol) and dry root yield (kg/m2) of field-grown cassava under rain-fed conditions at Santo
Tomás, Atlantic Department (seasonally dry environment) and Riohacha, Guajira (semi-arid environment) [97].
Locality and cultivar Dry root yield PN g
s c
i
Santo Tomás
CG 1141-1 0.76 31 0.38 179
SG 536-1 0.72 29 0.39 234
MCol 1505 0.60 29 0.43 215
CM 3306-4 0.74 28 0.38 195
MBra 191 0.65 27 0.38 196
CM 4013-1 0.72 26 0.38 224
MBra 12 0.70 25 0.36 196
CM 3555-6 0.58 25 0.37 243
CM 4063-6 0.62 25 0.37 207
MCol 2215 0.65 25 0.44 209
Mean of all cultivars 0.67 27 0.38 209
LSD (0.05) 0.15 3.5 0.064 35
Riohacha
CM 4013-1 0.33 20 0.46 271
MCol 2215 0.30 20 0.47 233
MCol 1505 0.23 15 0.49 295
MCol 1734 0.32 15 0.43 257
CG 1141-1 0.28 13 0.44 281
MCol 1684 0.23 13 0.47 295
SG 536-1 0.26 12 0.44 341
CM 4063-6 0.32 12 0.45 305
MCol 1468 0.18 9 0.47 310
MCol 22 0.20 8 0.50 312
MBra 12 0.12 7 0.59 330
CM 3306-4 0.22 7 0.42 310
MVen 77 0.04 7 0.37 307
Mean of all cultivars 0.23 12 0.46
295
LSD (0.05) 0.08 3.8 0.096 58
seasonally dry environment in Patia Valley, Cauca, yield
was significantly correlated with PN, C4 PEPC enzyme,
mesophyll conductance to CO2 diffusion, and PNUE (see
Table 1, and Figure 1). These characteristics are related
to biochemical/anatomical components of leaf photosyn-
thesis and point to the importance of PEPC as a selection
criteria in dry environments [35,42,43].
7.3. Breeding for Drought Tolerance in the
Semiarid Northeastern Brazil
The physiological research at CIAT have elucidated and
documented the many mechanisms underlying cassava
tolerance to abiotic stresses [4,35,42], and was pivotal in
enhancing interests for expanding cassava production in
semiarid areas in South America and Sub-Saharan Africa,
areas where other main staple tropical crops such as ce-
reals and grain legumes probably will fail to produce. In
1991, the Brazilian national (CNPMF) and state research
institutions in coordination with CIAT, had initiated a
5-year project for breeding cassava in drier tropics and
subtropics with funds from the International Fund For
Agriculture Development (IFAD) [4,98,99]. Cassava germ-
plasm (500 clones) from northeastern Brazil and the
north coast of Colombia was initially screened at four
semiarid sites in northeastern Brazil, characterized by
extended drought for several months, hot weather, and
sandy infertile soils with presence of pests and diseases
[4]. Some accessions were selected for their broad adap-
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
180
Figure 12. Relationships between root yield and leaf photo-
synthesis of several cassava clones grown in seasonally dry
(Santo Tomás, Atlantico) and semiarid (Riohacha, Guajira)
environments [80].
Figure 13. Relationship between dry root yield and inter-
cellular CO2 concentration (Ci) for two groups of cassava
cultivars grown under rain-fed conditions at Riohacha
(semi-arid) and Santo Tomás (seasonally dry). The Ci values
were calculated from leaf gas fluxes via standard Gaastra
equations: the higher photosynthetic rates, the lower Ci
values. Regression equation: yield = 1.52 – 0.004 Ci; r2 =
0.82 (P < 0.001) [97]. This relation indicates that yield dif-
ferences among cultivars are related mostly to differences in
leaf mesophyll characteristics (i.e. anatomical and bioche-
mical factors).
tation across sites to contribute in a recombination and
selection program (Table 13). Yields of selected clones
at 12 months and 18 months after planting demonstrate
the high potential of cassava in these drier areas. Also,
some progenies, via hybridization, have been developed
and sent to ITTA, Nigeria, for adaptation and selection
under drier areas in sub-Saharan Africa [100]. Farmers
had participated in evaluating and selecting adapted ma-
terials that resulted in rapid acceptance and, consequently,
in the release of several improved clones [101].
8. Conclusions and Future Research
The research reviewed here on cassava productivity, eco-
physiology, breeding, and responses to environmental
stresses was conducted in collaboration with a multidis-
ciplinary team at CIAT. Under favorable environments in
lowland and mid-altitude tropical zones with near-opti-
mal climatic and edaphic conditions for the crop to rea-
lize its inherent potential, cassava is highly productive in
terms of root yield and total biological biomass, confer-
ring a competitive advantage over other tropical energy
crops. Under stressful environments with extended
drought of several months and low-fertility soils, where
major cereal and legume crops might fail, cassava pro-
duces reasonably well. This inherent capacity to tolerate
complex stresses is supported by several morphological,
physiological and biochemical mechanisms and traits,
such as long leaf life, tight stomatal control over gas ex-
change, high photosynthetic potential and extensive fine
root systems. Core germplasm was characterized and
several clones tolerant to water stress and low-fertility
soils were identified to breed for drier areas in Africa and
Latin America. Selection for nutrient use efficient short-
to-medium cassava was found to be advantageous for
soil fertility conservation while retaining yield potential.
Modeling predicts the suitability of cassava in globally
warming climate versus other food crops, confirming its
high level of tolerance [45]. Cassava has high optimum
temperature for photosynthesis and growth, and responds
positively to elevated CO2 that point to its potential as
food, feed and energy crop in tropical and subtropical
zones adversely affected by climate changes. Because of
its high costs using sophisticated methodology, current-
climate research is still confined within developed tem-
perate zones [46,66,68,102]. Yet, there is an urgent need
to conduct climate research in representative tropical
ecosystems where GCMss cenarios predict the worst
consequences for agricultural productivity and food se-
curity [66,67,92,93]. The use of the Free Air CO2 En-
richment (FACE) technology in combination with rain
shelters which facilitates evaluating interactions of CO2
with soil water status [68,103], may further enhance de-
veloping improved cassava cultivars adapted to increas-
ing atmospheric CO2. Moreover, temperature influences
may be studied under field conditions using the “Tem-
perature Free-Air Controlled Enhancement (T-FACE)”
technique, as was recently described and used at U.S.
Arid-Land Agricultural Research Center, USDA, Mari-
copa, Arizona [104]. Developed countries, who are the
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Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
Copyright © 2012 SciRes. OJSS
181
Table 13. Clones [(harvested at 12 and 18 months (M)] with good level of adaptation at the semiarid screening site in Quixadá,
Ceará, northeastern Brazil, 1996.Source: (CIAT/CNPMF breeding database, 1996, unpublished).
Brazil accession code Fresh root yield (t·ha–1) Dry root yield (t·ha–1) Dry matter content in roots (%)
12 M 18 M 12 M 18 M 12 M 18 M
BGM 649 14.9 32.0 4.7 11.5 31.8 35.7
BGM 651 10.7 30.2 3.2 10.0 29.9 33.2
BGM 814 10.2 36.1 2.5 13.7 24.1 37.8
BGM 834 23.8 39.8 5.7 12.7 24.2 31.8
BGM 867 11.8 32.2 2.8 11.5 23.4 36.0
BGM 876 14.6 38.0 3.6 13.7 24.5 36.1
BGM 924 13.5 37.0 2.3 12.5 17.3 33.7
Mean of selections 14.2 35.0 3.5 12.2 25.0 34.9
Trial mean 12.7 26.6 3.2 9.2 25.2 34.5
Check varieties 7.1 27.8 1.7 9.8 23.8 35.3
Note: Farmers participated in the evaluation process and selected three clones with high yield and high dry matter content, and multiplied their planting materi-
als even before being officially released. Farmers’ participation was crucial in enhancing the selection process and in speeding up adoption and diffusion of
improved technology. The large increases in yield and in dry matter content at 18 months were attributed to the rainfall received in the last six months. At
Quixadá, long-term mean annual rainfall is less than 700 mm, 80% falling mostly in 3 - 4 months and the rest of the year is dry coupled with hot climate that
renders effective rainfall for growth to be less than 500 mm (total annual radiation = 8130 MJ·m–2, mean annual temperature = 27˚C, potential annual
evapotranspiration = 2369 mm) [4]. The soil at this site is sandy (> 80% sand) with low water holding capacity. Clones such as BGM 649, BGM 651, BGM 834
and BGM 876 had yields ranging between 3.2 to 5.7 dry t·ha–1 at 12 months, compared to 1.7 t·ha–1 of local checks. In semiarid environments such improved
germplasm is crucial for food security where staple grain crops will probably fail to produce reasonable yields.
main polluters of the atmosphere, via excessive carbon
emissions, must shoulder the costs of climate research in
tropical and subtropical regions. Oil-rich Arabian/Persian
Gulf States should take the lead in supporting developing
countries to cope with-and-adapt-to consequences of
warming climate.
Molecular biology technology is also needed with fo-
cus on applications into crop improvement. For example,
this technology is useful for genetically transferring sim-
ple qualitative traits controlled by one or two genes, as
already had been demonstrated by the successful produc-
tion and use of insect-resistant transgenic commercial
crop cultivars containing the soil bacterium, Bacillus
thuringiensis (Bt) genes that produce the toxic Cry pro-
teins. In contrast, quantitative multigenic traits such as
tolerance/resistance to compound abiotic-stresses are un-
likely to be easily amenable to genetic engineering via
inserting few exotic genes. These traits when expressed
at the whole organism level are mostly attributed to a
range of morphological, anatomical, physiological and
biochemical characteristics and mechanisms. So far,
modest advances at the experimental levels were recently
reported for the use of genetic molecular markers in cas-
sava selection and breeding efforts for developing
drought-tolerant cultivars [105]. Transgenic approaches
for improving water stress tolerance and leaf retention
were also considered [106,107]. Genetic modifications
through the use of the modern recombinant DNA tech-
nology may play an important role in improvement of the
crop only when it complements and integrates with other
fields of science and not done in isolation [53,108].
Moreover, to be cost-effective, the technology outputs
must be tested and evaluated in whole-plant and within
relevant cropping systems under prevailing environments
as well as under predicted climatic changes in the 21st
century [66-68,109].
Connor et al., in their recently revised Crop Ecology
text book [109, see page 269 of the book], have rightly
commented on the current research efforts targeted to-
wards genetically transferring the quite complex C4 pho-
tosynthetic syndrome to C3 rice, where significant finan-
cial support was recently devoted to, by saying: “What a
pity that cassava does not share the world-food limeligh t
with rice. This species has the most, and best studied,
intermediate photosynthetic types, and beneficial growth
and yield responses have been demonstrated in them
(Section 10.1.2). The pathway to success ought to be
shorter for this crop. It would be exciting to see progress
in the search for this current holy grail of biotechno-
logy during the lifetime of this book.” Quote. Research
institutions and donors agencies concerned with agri-
cultural research and development must be aware of the
high potential of cassava as food, feed, and industrial
crop and its role in the face of global climate change.
9. Acknowledgements
The author wishes to express his gratitude to Colombian
farmers for their hospitality during conducting some of
this research. Without the collaboration of the many for-
mer field laborers, secretaries, research associates, stu-
dents and colleagues, who are now dispersed across
Stress-Tolerant Cassava: The Role of Integrative Ecophysiology-Breeding Research in Crop Improvement
182
countries, the achievements highlighted here would have
never been obtained. The invaluable courtesy copies of
books documenting important research on crop ecology
by David Connor, Robert Loomis and Kenneth Cassman,
and on climate change by Mary Beth Kirkham, and Cyn-
thia Rosenzweig, article reprints from Andy Jarvis, Julian
Ramirez-Villegas and David Rosenthal were appreciated.
I am grateful for the waiving of the required page-
charges by the Editors of OJSS. Constructive comments
from anonymous reviewers were received. Thanks to
Farah El-Sharkawy Navarro for the editorial and the
WWWnet search assistance.
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