American Journal of Plant Sciences, 2011, 2, 569-577
doi:10.4236/ajps.2011.24068 Published Online October 2011 (
Copyright © 2011 SciRes. AJPS
Morphological and Physiological Responses of
Weedy Red Rice (Oryza sativa L.) and Cultivated
Rice (O. sativa) to N Supply
Marites A. Sales1, Nilda R. Burgos2, Vinod K. Shivrain2, Brad Murphy4, Edward E. Gbur, Jr.5
1Department of Plant Pathology, University of Arkansas, Fayetteville, USA; 2Department of Crop, Soil and Environmental Sciences,
University of Arkansas, Fayetteville, USA; 3Syngenta Crop Protection, Vero Beach, FL, USA; 4Department of Horticulture, Univer-
sity of Arkansas, Fayetteville, USA; 5Agricultural Statistics, University of Arkansas, Fayetteville, USA.
Received April 13th, 2011; revised May 28th, 2011; accepted August 25th, 2011.
Red rice (Oryza sativa L.), a noxious weed in rice production, competes with cultivated rice for nutrients. Accumulation
of more N in red rice than in cultivated rice may be due to a mechanism different from that of cultivated rice. To test
this assumption, red rice and cultivated rice were grown in nutrient solution to compare their growth and physiological
responses to N supply. Experimental design was a split-plot, where main plot factor was rice type (Stf-3, ‘Wells’);
split-plot factor was N treatment [T1 (complete nutrient solution); T2 (–NH4NO3); T3 (+NH4NO3 for 24-h post-N defi-
ciency); and T4 (+NH4NO3 for 48-h post-N deficiency)]. Nitrogen deficiency was defined as N sufficiency index (NSI) <
95%. Height, tiller number, biomass, and root morphology were monitored to determine morphological responses. Stf-3
red rice had significantly greater growth measurements than Wells in terms of shoot and root characteristics. At T4, Stf-
3 showed higher increment in root length and surface area than Wells. Shoot tissue concentrations of N and total sug-
ars were measured to determine physiological response in N-deficient and N-supplemented plants. Stf-3 had greater N
and sucrose tissue concentrations at N-deficient conditions compared with Wells, implying a stress-adaptive molecular
mechanism regulated by N and sucrose availability.
Keywords: Hydroponics, Nitrogen Concentration, N Uptake, Rice (Oryza Sativa L.), Root Morphology, Sucrose
Concentra tion, Sugars
1. Introduction
Rice is a staple food for more than half of the world’s
population. The United States produces less than 2% of
the volume of world rice production, but is a major rice
exporter, providing 12% - 14% of the annual volume of
the global rice trade [1]. Arkansas is the largest rice-
growing state, containing over 45% of U.S. rice acreage,
according to the USDA National Agricultural Statistics
Service. A major challenge facing rice producers in the
southern U.S. is weed competition. Red rice, a weedy
rice relative belonging to the same genus and species as
the cultivated rice (Oryza sativa) is one of the most dif-
ficult weed species to control because of its similarity to
the crop [2]. About 60% of the rice fields in Arkansas are
infested by red rice [3]. Red rice has a competitive ad-
vantage over cultivated rice because it grows taller and
faster, and tillers profusely, thus depriving cultivated rice
of necessary nutrients, light and space owing to its height
and massive root system. Under non-competitive condi-
tions, red rice produces almost double the grain yield of
commercial cultivars [4]. When competing with culti-
vated rice, one red rice plant·m–2 reduced yield of ‘New-
bonnet’ rice, a tall cultivar by 219 kg·ha–1 [5]. Red rice
caused an estimated loss of $ 275 ha–1 in 2006 alone [3].
These economic losses include damaging effects of plant
lodging and price docking of rice grains contaminated
with red rice kernels. In addition, red rice uptake of even
half of the optimum fertilizer N requirement for rice cul-
tivars, estimated at 200 kg·N·ha–1 in the southern U.S. [6],
is enough to drastically reduce rice yields and the eco-
nomic benefits of N fertilization. Red rice accumulated
more fertilizer N and produces more biomass than
‘Drew’ rice under field conditions, suggesting that it
could have higher yields even in low N supply [7]. The
Morphological and Physiological Responses of Weedy Red Rice (Oryza sativa L.)
and Cultivated Rice (O. sativa) to N Supply
implied tolerance to N-deficient conditions in weedy red
rice is a trait that would be of agronomic importance in
cultivated rice.
Nitrogen is the most important inorganic macronutri-
ent and is a limiting factor in crop productivity. It is a
major constituent of proteins, cofactors, and secondary
metabolites [8], and thus affects all levels of plant func-
tion [8-10]. Plants contain 1% - 6% N by weight and
absorb N as both nitrate (3
) and ammonium (),
depending on plant age and type, environment, and other
factors [11]. Before can be used in the plant, it
must be reduced to or ammonia (NH3). The NH3
produced is assimilated into amino acids that are subse-
quently combined into proteins and nucleic acids. Nitro-
gen is also an integral part of chlorophyll needed for
photosynthesis [12], so high photosynthetic activity, vig-
orous vegetative growth, and a dark green color are indi-
cators of adequate N supply. Plants regulate photosyn-
thesis to balance the flow of C through an optimized dis-
tribution of its N resources [13]. The profound effect of
N supply on overall plant growth and development is
modulated by C status [14], and most likely, cross-talk
with other factors, such as hormones, cytokinins and ab-
scissic acid [15]. Nitrogen deficiency, therefore, affects
other metabolic pathways.
In recent years, elucidating plant response to stress has
been facilitated by investigations at the cellular level.
One morphological adaptation to nutrient deficiency is
alteration of root architecture, such as increased number
and length of root hairs to reach a wider area of the en-
vironment and, consequently, increase nutrient acquisi-
tion [8]. Molecular analyses have also revealed other
phenotypic expressions of nutrient stress adaptation, such
as increased densities of transport molecules to enhance
nutrient utilization [16], release of plant compounds to
increase bioavailability of soil nutrients [17,18], and en-
hanced nutrient uptake capacities regulated at the level of
membrane transport [19-21]. General response systems
to nutrient stress involve use of stored polysaccharides or
recycling of cellular components to prevent severe defi-
ciencies in respiratory substrates and maintain important
biochemical pathways [22-24]. A degradative process
known as vacuolar autophagy was induced by starch
starvation in maize [25,26] and rice [27] and would be a
likely process in any stress response pathway which uses
starch as a precursor. Alteration in carbohydrate metabo-
lism in response to N also indicates changes in the flux of
soluble sugars in the plant.
Removing weeds from the paddy field increases the
amount of N in the rice plant [28]. Plant density is an
important factor in competition because it is inversely
related to resources available to the plant [29]. When
cultivated rice was planted with red rice at varying densi-
ties, only rice cultivars with comparatively high tillering
capacity, leaf area and dry stem weight could compete
very well with red rice [30,31]. Since red rice has mor-
phological and physiological features that suggest com-
petitive advantage over cultivated rice in adapting to N-
poor conditions, it is expected to accumulate more N and
produce more biomass compared with cultivated rice at
low N conditions. Comparing morphological and physic-
ological responses of weedy and cultivated rice types
under N stress conditions is the first step towards eluci-
dating adaptive mechanisms in red rice that are either
absent or less efficient in the cultivated rice, hence, this
2. Materials and Methods
2.1. Plant Material
Rice types compared were the tall, awnless, medium-
grain red rice accession Stf-3 and ‘Wells’. Accession
Stf-3 is a strawhull red rice collected from St. Francis
County, Arkansas, USA. A strawhull red rice was se-
lected because it is the most prevalent red rice type based
on hull color, and is most similar to Wells rice in height
at maturity [32]. Wells is a long-grain rice cultivar,
which matures approximately 124 d after planting. Be-
cause of its high milled rice yield, stable head rice yield,
and tolerance to rice blast and sheath blight [33], Wells
was planted in 31% of rice production areas in Arkansas
by 2006, making it the rice cultivar of choice [34]. The
second most popular cultivar was planted in only 13% of
rice area.
Seeds were surface-sterilized with 10% H2O2 for 10
min followed by 70% ethanol for 5 min, then washed
thoroughly in sterile deionized water and germinated at
30˚C for 48 h in Petri dishes lined with moist filter paper.
Uniformly germinated seeds were transferred into 6 cm
diameter wells in black plastic trays (27 cm × 53 cm)
(Pro-Tray, Hummert, MO, USA) fitted into 35-L plastic
tubs (36 cm × 62 cm × 31 cm) (Multi-Reservoir, Ameri-
can Agritech, AZ 85283, USA) containing aerated, de-
ionized water until a week after germination when it was
replaced with half-strength nutrient solution [35].
2.2. Nutrient Solution
The nutrient solution was composed of NH4NO3 (40
ppm), NaH2PO42H2O (10 ppm), K2SO4 (40 ppm), CaCl2
(40 ppm), Mg2SO47H2O (40 ppm), MnCl24H2O (0.5
ppm), (NH4)6Mo7O244H2O (0.05 ppm), H3BO3 (0.2
ppm), ZnSO47H2O (0.01 ppm), CuSO45H2O (0.01 ppm),
FeCl36H2O + citric acid (monohydrate) (2 ppm), and
Na2SiO35H2O (0.1 mM). Nutrient solution pH was
Copyright © 2011 SciRes. AJPS
Morphological and Physiological Responses of Weedy Red Rice (Oryza sativa L.)
and Cultivated Rice (O. sativa) to N Supply
Copyright © 2011 SciRes. AJPS
2.4. N Treatments maintained at 5.0 (SympHony® pH/conductivity meter,
VWR International, Arlington Heights, IL 60004, USA);
pH was adjusted every other day for the first week, then
daily. Water that evaporated from the system was re-
placed by deionized water daily. Nutrient solution was
replaced weekly, using half-strength solution for 2 weeks;
full-strength nutrient solution was used thereafter.
To simulate N-deficient conditions, defined as N suffi-
ciency index (NSI) < 95% [11], plants were subjected to
four treatments at R0 stage: T1 (complete nutrient solu-
tion; control); T2Nutrient solution without NH4NO3
until NSI < 95%; T324 h supply of complete nutrient
solution post-N deficiency; and T448 h supply of com-
plete nutrient solution post-N deficiency (Figure 1).
2.3. Hydroponics Culture Conditions
To assess both early and late molecular responses for
subsequent microarray experiments, 24 h and 48 h time
points for N supplementation, respectively, were selected.
At R0, T1 plants were transferred into tubs with fresh
nutrient solution, while T2-4 plants were transferred to
fresh nutrient solution without NH4NO 3 and grown until
NSI < 95%. In both years, it took 3 - 5 d without
NH4NO3 to drop NSI below 95%. Following published
procedures [37], NSI was monitored daily at mid-morn-
ing using a chlorophyll meter (SPAD-502, Konica Mi-
nolta Sensing, Inc., USA); NSI was calculated for each
rice type using the Formula (a) where average reading
was calculated from all plants under similar growth
stages, with three readings per plant. Readings were
taken from the same spot in the mid-region of the
youngest fully expanded leaf on the main culm of each
plant [37]. T1 and T2 plants were harvested when the lat-
ter reached NSI < 95%; T3 and T4 plants were transferred
to fresh nutrient solution containing NH4NO3 and har-
vested after 24 h and 48 h, respectively.
The trays described previously had 57 mm deep wells
with five drain holes. Each seedling was placed on a
plastic 3 mm mesh. Each tray contained 12 plants; each
rice type was grown in four trays under greenhouse con-
ditions from August to September (day temperature:
22˚C - 39˚C; night temperature: 21˚C - 30˚C) and from
April to May the following year (day temperature: 21˚C -
27˚C; night temperature: 19˚C - 27˚C). The greenhouse
was set to a day:night length of 14:10 h using supple-
mented lighting from 400 W metal halide lamps (Philips
34415-0, Philips Electronics, NY 10020, USA). Tem-
perature and relative humidity were monitored (HOBO®
Temperature Data Logger H01-001-01, Onset Computer
Corp., MA 02532, USA). Plant growth stages were des-
ignated using a growth staging system as a guide [36].
Since the rate of development for rice grown in the
greenhouse has not been documented, four extra plants
per tray served as control for destructive sampling to
check for the “green ring” inside the shoot meristem,
which marks the R0 stage [36] when weedy red rice and
cultivated rice demonstrated differential accumulation of
fertilizer N in field experiments [7]. In both plant types,
R0 was at V8 (eight leaves with visible collar on main
2.5. Experimental Design
A split-plot design was employed, in which whole plot
factor was rice type (Stf-3, Wells) and split-plot factor
was N treatment (Full, N-starvation, 24 h and 48 h N-
Figure 1. Schematic diagram of the N treatments.
Average reading of plants in N-starved tub (TT)
SI, %100
Average reading of plants (T )
 (a)
Current Distortion Evaluation in Traction 4Q Constant Switching Frequency Converters
readdition). There were four replications, with three
plants per replication per N treatment. Randomization
was constrained by the following: each rice type was
placed at both sides of the greenhouse, on two benches;
each N treatment was randomly assigned to a row of
three plants within each tub.
2.6. Data Collection and Statistical Analyses
To determine inherent morphological and growth rate
differences between red rice and Wells rice, height and
tiller number were measured weekly. After imposition of
N treatments, biomass and root characteristics were re-
corded at harvest. To determine biomass production,
plant samples were separated into roots and shoots and
oven-dried to a constant weight at 60˚C. Prior to oven-
drying, root samples were gently washed with deionized
water, blotted dry with paper towels, stained with me-
thylene blue in 10% ethanol, and stored at 4˚C until
scanned (Epson Twain Pro, Seiko Epson Corp., Japan)
for length, surface area, average diameter and number of
root tips. Scanned images were analyzed using Win-
RHIZO 5.0 (Regent Ltd., Canada).
To determine physiological responses at N stress,
shoot tissue concentrations of total N and sugars were
quantified. Concentrations of glucose, fructose, and su-
crose in leaf tissue were analyzed because these would
indicate changes in carbohydrate metabolism in response
to imposed nutrient stress. To measure total N concentra-
tion, shoots from one plant per replication were oven-
dried as described for biomass determination, and their
dry weights recorded prior to grinding in a rice mill
(3383 L-10, Thomas Scientific, USA). Ground shoot
tissues were analyzed for total N by the Dumas combus-
tion method at the Agriculture Diagnostic Laboratory of
the University of Arkansas, Fayetteville.
To determine concentration of total sugars, youngest
fully expanded leaves from one plant per replication
were freeze-dried to a constant weight at –70˚C in a ly-
ophilizer (Freezemobile 25SL, Virtis, USA) before
grinding. Three 100 mg samples of ground tissue from
each plant sample were then extracted for total sugars
following a modified procedure [38]. Sugar extracts (1
mL) were analyzed for fructose, glucose and sucrose
concentrations ([Fruc], [Glu] and [Suc]) by high per-
formance liquid chromatography (Alliance 2690 Separa-
tion Module, Waters, USA) using acetonitrile: 2-propa-
nol:water (825:35:140) as solvent and passed through
250 mm × 2.0 mm columns (Phenosphere 5 μ NH2 col-
umns, Phenomenex, CA 90501, USA) at a flow rate of
0.6 ml min–1 at 40˚C. Sugar concentrations were calcu-
lated using the Formula (b) where 500 = factor for a 1
mL extraction volume. Data were subjected to analysis of
variance using SAS® (v8.2, SAS Institute, Inc., Cary, NC,
USA). When F-tests were significant, means were sepa-
rated using Fisher’s protected LSD at a significance level
of 0.05.
3. Results and Discussion
3.1. Developmental Differences between Rice
The two rice types reached R0 within 2 d of each other.
There was no difference in Year 1, but the time lag ex-
tended to 2 d in Year 2. This year difference may be at-
tributed to greenhouse temperatures, as experiments were
established at different periods of the year. In general,
indica varieties require higher minimum temperatures
than japonica varieties [39-41]; red rice is an indica,
while Wells is a japonica. Optimum germination of ja-
ponica rice seeds is at a 20˚C day temperature, and at
30˚C - 35˚C day temperatures for an indica [42]. The
optimum temperature range for photosynthesis in indica
rice varieties was reported to be 25˚C - 35˚C, higher than
that of japonica (18˚C - 33˚C) [43]. During vegetative
growth, indica varieties were more sensitive to lower
temperatures than japonica varieties when partial regres-
sion of days to heading on mean temperatures was done
[44-46]. Air temperature was the most important factor
which affected yields of indica varieties, followed by day
length [47]. Red rice ecotypes also differ in maturation
period [32].
3.2. Overview of Data Analysis Results
There were significant differences in plant responses to
N treatments between years, thus data were analyzed
separately (Table 1). In Year 1, only shoot tissue [N]
showed significant interaction effect of rice type and N
levels. In Year 2, shoot tissue [N] and [Suc] as well as
root length and surface area showed a strong evidence of
rice type and N level interaction (Table 1).
3.3.Morphological Differences
Aboveground traits. In both years, Stf-3 grew taller and
produced more tillers than Wells under full N supply
(Table 2). Rice type effect on shoot biomass production
was evident only in Year 2, with Stf-3 producing more
than Wells (Table 3). Aboveground morphological dif-
ferences in the greenhouse reflected those in field condi-
tions, where Stf-3 can grow up to 130 cm at flowering [4]
hile Wells can be as tall as 100 cm at maturity [33]. w
1Total amount in a 2 μl injection500, μg
Sugar concentration (μgg )Total weight of sample, g
 (b)
Copyright © 2011 SciRes. AJPS
Morphological and Physiological Responses of Weedy Red Rice (Oryza sativa L.)
and Cultivated Rice (O. sativa) to N Supply
Copyright © 2011 SciRes. AJPS
Table 1.Table of p-values of ANOVA f-tests. Bold values followed by * are significant at α = 0.05.
Source of variation
Year 1 Year 2
Response variables
Rice type (R) N level (N) R × N Rice type (R) N level (N) R × N
Plant height 0.0133* 0.6716 0.1801 0.0010* 0.0001* 0.1750
Number of tillers 0.0034* 0.9508 0.1484 0.0017* 0.0148* 0.1125
Root length 0.0687 0.6556 0.7216 0.0005* 0.0034* 0.0206*
Root surface area 0.1116 0.7715 0.6817 0.0008* < 0.0001* 0.0009*
Average root diameter 0.0039* 0.0848 0.2078 0.0006* < 0.0001* 0.4216
Number of root tips 0.0634 0.2932 0.7969 0.0014* 0.0979 0.1961
Shoot dry weight 0.1169 0.4765 0.0843 0.0074* 0.3724 0.9356
Root dry weight 0.0706 0.4019 0.1825 0.0042* 0.0261* 0.9346
Total dry weight 0.1044 0.5876 0.0895 0.0065* 0.3030 0.9763
Shoot tissue total N 0.0069* < 0.0001* 0.0275* 0.0110* < 0.0001* 0.0100*
Shoot tissue total sugars
Fructose 0.0754 0.0698 0.5310 0.1358 0.0021* 0.0858
Glucose 0.3030 0.0841 0.5385 0.2030 0.0034* 0.0545
Sucrose 0.0075* 0.0285* 0.1456 0.3801
< 0.0001* 0.0162*
Table 2. Growth characteristics affected by rice type, grown
in complete nutrient solution (T1).
Variable Year Stf-3a Wellsb LSDc
1 90.31 68.66 10.35
Height (cm) 2 70.05 50.28 6.60
1 7 3 1
Tiller number 2 8 3 2
1 0.361 0.446 0.022
Ave. root diameter (mm) 2 0.365 0.453 0.025
aWeedy red rice, n = 4. bCultivated rice, n = 3. cMeans were separated using
Fisher’s protected LSD at α = 0.05.
Table 3. Growth characteristics as affected by rice type,
grown grown in complete nutrient solution (T1), Year 2.
Rice type No. of root tips
(× 103) Shoot DWc (g) Root DW (g) Total DW (g)
Stf-3a 56.124 3.73 1.09 4.82
Wellsb 10.836 1.41 0.41 1.82
LSDd 16.906 1.54 0.37 1.90
aWeedy red rice, n = 4. bCultivated rice, n = 4. cDW = dry weight dMeans
were separated using Fisher’s protected LSD at α = 0.05.
Changing N supply resulted in detectable differences in
whole-plant aboveground characteristics in Year 2 (Ta-
ble 1). The findings that Wells generally has lower re-
sponse to N compared with red rice agree with findings
in field conditions, with respect to biomass accumulation
Belowground traits. Average root diameter in both
years also differed between rice types (Table 2). In Year
2, red rice had 6 times more root tips and 3 times more
shoot and root biomass than Wells (Table 3). Differences
in root length and surface area due to the interaction of
rice type and N treatment was also evident, particularly
in Stf-3 (Table 4). T2 plants had visible, but not sig- ni-
ficant, retardation in root growth and expansion of root
surface area in Stf-3 relative to plants grown in T1, but
the change in Wells was imperceptible (Table 4). At T4,
Stf-3 significantly increased root length and root surface
area, but not Wells. N treatment effect on root biomass
was also evident in Year 2, with the greatest root dry
weight observed at T1 (Table 5). Thus, red rice response
to restoration of full N supply after starvation was evi-
dent in root morphology within 48 hr, but not in Wells
Table 4. Growth characteristics affected by the interaction
of rice type and N treatme nt, Year 2a.
Root length (m) Root surface area (m2)
N treatment Stf-3 Wells Stf-3 Wells
T1 (complete) 88.17 14.54 1006 203
T2 (–NH4NO3) 62.90 22.70 609 298
T3 (24 h complete
post-N deficiency) 85.63 23.37 1010 346
T4 (48 h complete
post-N deficiency) 131.43 29.22 1700 446
bLSD1 54.28 570
LSD2 27.97 319
aRice types were weedy red rice (Stf-3) and cultivated rice (Wells). Means
were separated using Fisher’s protected LSD at α = 0.05 (n = 4). bLSD1
separates means within same rice type; LSD2 separates means for different
rice types.
Morphological and Physiological Responses of Weedy Red Rice (Oryza sativa L.)
and Cultivated Rice (O. sativa) to N Supply
Table 5. Growth characteristics and shoot nutrient concen-
trations affected by N treatment, averaged over rice types,
Year 2a.
N treatment Root DW (g) Fructose (µg·g–1) Glucose (µg·g–1)
T1 (complete) 0.52 38.61 62.73
T2 (–NH4NO3) 0.89 51.43 87.63
T3 (24 h complete
post-N deficiency) 0.78 32.90 61.71
T4 (48 h complete
post-N deficiency) 0.81 29.98 58.34
LSDb 0.24 10.39 15.56
aRice types were weedy red rice (Stf-3) and cultivated rice (Wells), n = 8.
bMeans were separated using Fisher’s protected LSD at α = 0.05.
Root characteristics are correlated with nutrient access
to and uptake from the rhizosphere and are significant
factors in underground competition. Changes in root
architecture are typical responses in plants during
nutrient stress as an adaptive mechanism to increase
nutrient access [8]. The effect of N supply on root growth
of Stf-3 observed in Year 2 confirmed similar findings in
cultivated rice [49] which showed that 3
root elongation and growth of root hairs. On the other
hand, N supply effect on root morphology was not
evident in other studies [50] as was observed with Wells
in this current research. Since differences in root
morphology between Stf-3 and Wells had been consistent
regardless of N supply, genotypic effect was strongly
evident. Stf-3 responded more to N supplementation than
Wells, producing longer roots and greater root surface
area after some recovery period, which equate to greater
N uptake capacity than that of Wells. While Stf-3 had
visibly longer and finer root hairs, Wells had consistently
thicker roots compared with Stf-3. Larger roots offer
stronger plant support, but have smaller surface areas and
fewer root tips for nutrient absorption. The number of
root tips is indicative of the ability of plants to absorb
nutrients [51]. Therefore, more root tips and greater root
surface area in Stf-3 than in Wells must have contributed
to greater leaf tissue [N] in Stf-3 than in Wells. Root
characteristics of Stf-3 indicate that, at the whole-plant
level, an extensive root system and a faster root growth
response to N supplementation contribute greatly to the
nutrient uptake advantage of weedy rice over cultivated
3.4. Physiological Differences
Shoot tissue [N]. Differences in shoot tissue [N] as af-
fected by the interaction of N treatment and rice type
were significant in both years (Ta bl e 6). Stf-3 had higher
[N] in its shoot tissue than Wells when grown under
Table 6. Shoot tissue N concentrations (mg·kg–1) affected by
the interaction of rice type and N treatment.
Year 1 Year 2
N treatment Stf-3a Wellsb Stf-3 Wells
T1 (complete) 42.15 31.63 53.90 46.28
T2 (–NH4NO3) 28.48 22.20 33.20 29.05
T3 (24 h complete
post-N deficiency)32.53 26.47 43.28 36.90
T4 (48 h complete
post-N deficiency)36.30 26.43 46.50 50.33
cLSD1 5.02 9.81
LSD2 3.10 4.77
aWeedy red rice, n = 4. bCultivated rice, n = 3. cMeans were separated using
Fisher's protected LSD at α = 0.05. LSD1 separates means within same rice
type; LSD2 separates means for different rice types.
complete nutrient solution. The [N] in Stf-3 and Wells
declined by 32% and 30%, respectively, in Year 1 and
38% and 37%, respectively in Year 2 at NSI < 95% (T2),
relative to plants grown in T1. At T4, Stf-3 showed a sig-
nificant increase in shoot tissue [N] in Year 1. This was
observed even earlier (T3) in Year 2. Although Wells did
not show a significant increase in [N] even at T4 in Year
1, it showed full recovery of shoot tissue [N] under the
same N conditions in Year 2. Within 48 h of post-N defi-
ciency (T4), both rice types had lesser shoot tissue [N]
than plants grown in T1 in Year 1, but in Year 2 both rice
types recovered faster from N stress than in Year 1,
showing similar shoot tissue [N] as those grown in T1.
Differences in shoot tissue [N] as affected by
interaction of N treatment and rice type confirmed that
exogenously applied N at varying levels was absorbed at
different amounts by Wells and Stf-3, and that accumula-
tion in shoot tissue also varied according to N supply.
Generally, shoot tissue [N] was greater in Stf-3 than in
Wells at control and treated conditions, except in Year 2
when both plants had similar concentrations at T2 and T4
(Table 6). This corroborated reports of higher N uptake
capacity of Stf-3 as indicated by its inherently more
extensive root system and its apparent root growth
response to added N compared to Wells. There is
evidence, therefore, supporting our hypothesis that red
rice is able to accumulate N better than cultivated rice,
considering its biomass production and shoot [N]. Both
plants attained [N] similar to unstressed plants (T1) after
48 h of N supply post-N deficiency (T4).
Total sugars. [Fruc] and [Glu] were affected by N
treatments in Year 2 (Table 5), where the greatest
concentration was observed at T2. Differences were most
detectable in [Suc], considering that in higher plants,
sucrose is the major sugar for transport throughout the
Copyright © 2011 SciRes. AJPS
Morphological and Physiological Responses of Weedy Red Rice (Oryza sativa L.) 575
and Cultivated Rice (O. sativa) to N Supply
plant. Sucrose concentrations differed by rice type
(LSD0.05 = 0.036), with Stf-3 having greater [Suc] (0.194
mg· g–1) than Wells (0.114 mg·g–1). The effect of N
treatment on [Suc] was also evident in Year 1, when the
lowest [Suc] was observed at T1, averaged over rice type
(Table 7). In Year 2, the interaction effect of rice type
and N treatment on [Suc] was evident, when [Suc] in Stf-
3 was greatest at T2 and declined with duration of N
supply post-N deficiency (Table 8). A similar trend was
observed in Wells, except that the change in [Suc] from
one treatment to another was not significant.
Varying shoot tissue [Suc] indicate that Stf-3 res-
ponded to N treatments to a greater extent than Wells in
Year 2, since Wells [Suc] at optimum N concentrations
was not different from that at 0 N (Table 8). Moreover,
[Suc] in Stf-3 declined quickly with time of recovery,
approaching its baseline level at full N, whereas [Suc] in
Wells hardly changed regardless of N treatment. In-
creased [Suc] in red rice under N deficiency corroborates
evidence for the involvement of soluble sugars in stress
response and their role as nutrient and metabolite
signaling molecules [52]. Thus, under N deficiency,
increased [Suc] in both Stf-3 and Wells, albeit compara-
tively lower in the latter, may be a stress signaling
mechanism for the plant to stimulate N uptake [53-55].
In this case, the signaling mechanism of Stf-3 may be
more efficient than that of Wells. For example, plants are
Table 7. Shoot sucrose concentrations as affected by N
treatment, averaged over rice types, Year 1a.
N treatment Sucrose (mg·g–1)
T1 (complete) 0.079
T2 (–NH4NO3) 0.198
T3 (24 h complete post-N deficiency)0.163
T4 (48 h complete post-N deficiency)0.200
LSDb 0.041
an = 7. bMeans were separated using Fisher’s protected LSD at α = 0.05.
Table 8. Shoot sucrose concentrations affected by the inter-
action of rice type and N treatment, Year 2a.
Sucrose (mg·g–1)
N treatment Stf-3 Wells
T1 (complete) 0.292 0.386
T2 (–NH4NO3) 0.626 0.432
T3 (24 h complete post-N deficiency)0.443 0.274
T4 (48 h complete post-N deficiency)0.181 0.237
bLSD1 0.304
LSD2 0.242
aRice types were weedy red rice (Stf-3) and cultivated rice (Wells). Means were
separated using Fisher’s protected LSD at α = 0.05 (n = 4). bLSD1 separates
means within the same rice type; LSD2 separates means for dif- ferent rice types.
able to adapt to cold stress by accumulating sugars [56].
However, much remains to be done in characterizing the
many signaling pathways of sugar-induced responses to
stress, considering that most investigations have been
limited to sugar-induced stress responses in relation
tohormones and growth regulators [57].
4. Implications and Recommendations
Our findings corroborate earlier reports on red rice ac-
cumulating more N than cultivated rice. Differences in
response to N treatments between Stf-3 and Wells rice
suggest different adaptive mechanisms within the N me-
tabolic pathway, as well as the role of sucrose as a stress
signaling molecule. For instance, the stimulatory effect
of N supply, particularly 3, on root elongation, has
been demonstrated to regulate the transcription of many
genes in rice, including those involved in signal trans-
duction, transcription regulation, auxin transport and
ethylene synthesis. Genomic analysis to identify genes
involved in these pathways in response to N stress condi-
tions would help answer these questions.
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