Vol.3, No.6, 835-843 (2012) Agricultural Sciences http://dx.doi.org/10.4236/as.2012.36101 Biomass production and distribution in seedlings of Coffea arabica genotypes under contrasting nursery environments in southwestern Ethiopia Taye Kufa Ethiopian Institute of Agricultural Research, Jimma Research Center, Jimma, Ethiopia; kufataye@yahoo.com Received 18 June 2012; revised 28 July 2012; accepted 8 August 2012 ABSTRACT In Ethiopia, the natural forests with the occur- rence of wild Arabica coffee gene pools are un- der constant threats, largely due to anthropo- genic activities. The study was conducted to compare the variability among the wild arabica coffee genotypes in biomass assimilation and allocation patterns under varying light and irri- gation condit ions at the Ji mma Research Ce nter, southwest Ethiopia. The treatments included ir- radiance (moderate and full sunlight), irrigation (well watered and water stressed) regimes and twelve coffee genotypes of different geographi- cal areas. One-year-old seedlings were used to impose the environmental stresses and to re- cord dry mass of leaves, main stem, primary branches and root growth. Each organ was se- paretly oven-dried and total dry matter produc- tion and allocation patterns were measured and analyzed. The results depicted highly significant differences between the contrasting irradiance and irrigation regimes as well as among coffee genotypes for most of the parameters consid- ered. Accordingly, root, leaf, stem, whole-shoot and total biomass assimilation and root parti- tioning were higher for unshaded, water stressed and Harenna genotypes from the respective treatment groups. Significantly the lowest and highest root and shoot biomass values were recorded for Berhane-Kontir and Harenna geno- types, respectively. Likewise, coffee seedlings significantly differed in root dry mass and root to shoot ratio, dry matter partitioning due to the main and combined treatment effects. Most ac- cessions had relatively lower assimilations in shade as compared to full sun light conditions. Conversely, leaf share was significantly high under moderate shade environments and for irrigated and Berkane-Kontir genotypes. The total dry matter share varied for the seedling growth parts (root = 22%, leaf = 35%, stem = 43% and whole shoot = 78%). The root growth followed the order of Harenna > Yayu > Bonga > Berhane- Kontir populations, which were grouped into three broad cluster classes. The reverse was true for the leaf and whole-shoot partitioning, demonstrating the interplay betw een above- and below-ground growth parts partly due to geno- type, environment and/or their interactions. As a whole, the study demonstrates the need to con- sider both dry matter assimilation and parti- tioning patterns in identifying desirable geno- types and optimum environm ents for future bree- ding program in Ethiopia. Keywords: Ecological Physiolo gy; Environment; Genetic Diversity; Seedling Growth Response; Wild Ethiopian Coffee Population 1. INTRODUCTION The montane rainforest areas in Ethiopia are the only known center of origin and genetic diversity for the highland arabica coffee (Coffea arabica L.). In its origi- nal natural habitats, arabica coffee occurs in the multi- strata of forest ecosystems and thus it is a shade-adapted plant. Its response to light has caused it to be tradition- ally considered a heliophobic plant requiring high, some- what dense cover in a plantation. Today, the cultivation of coffee in open sun is not uncommon in most coffee producing countries, though its sustainability is ques- tionable. It is known that coffee trees with high produc- tivity potential are capable of high yields when they are cultivated intensiv ely without shade [1,2]. Although arabica coffee is said to be a shade-loving plant with greater quantum utilization efficiency for photosynthesis, excessive shading or light interception by the upper two to three canopy strata of various tree species would decrease growth and productivity of the Copyright © 2012 SciRes. OPEN ACC ESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 836 crop, as the plant spends much of its photosynthetic ac- tivities for maintenance. On the other side, if the light intensity is too high, there will be inadequate reaction centers in the leaves of the crop to accommodate the light energy and convert it into biochemical energy. As a result, the coffee trees excessively photo-respires and eventually most of the stored carbohydrates become de- pleted. Consequently, the trees may suffer from a serious dieback mostly due to excessive fruit load via enhanced flowering in full sunlight compared to shade conditions and coffee production limitations and coffee shade inter- actions [3,4]. Besides, excessive evapo-transpiration and severe drought stress, death of actively growing shoot parts, seasonal crinkling of leaves, frost damage and subsequent yield reduction are common problems ob- served in unshaded coffee orchards [5]. It is known that photosynthesis in coffee follows the C3 or Calvin cycle pathway, as coffee plants placed in darkness after a period of light produce a burst of CO2 following photorespiration [6]. Work done in Kenya [7] revealed that about 95% of the biomass of a plant are derived from the carbohydrates manufactured in the green parts by the process of photosynthesis and the re- maining 5% come from mineral elements absorbed by the roots. Large regular yields of coffee can be obtained only when both the carbohydrates and mineral element supplies within the tree are adequa te to meet the needs of the developing fru its and the shoots and roots which will support the following year’s crop. The capacity of the plant to produce carbohydrate depends mainly on the total green (mainly leaf) area engaged in photosynthesis and the net photosynthetic rate per unit green surface area [7]. The same author showed that dry matter pro- duction in any crop depends upon leaf area index, struc- ture of the canopy, photosynthetic rate per unit of leaf area and strength of the metabolic sinks in attracting as- similates. The physiological mechanisms involved in the adapta- tion of the juvenile trees to light have remained less in- vestigated in the diverse coffee germplasm collections at the center of origin and genetic diversity. Shade plants essentially follow strategies of optimum use and conser- vation of available energy. The need to know how the coffee plant accumulates and partitions dry matter to the various plant organs under a given management cannot be overemphasized. Moreover, this information is nec- essary as a selection criterion for coffee cultiv ars as well as for determining optimum management levels [6,7]. This aspect, however, has not yet deserved attentions in the evaluation of the immense arabica coffee genetic resources grow i ng u nder wide ecologies. The growth and seed production of a plant is a result of the integrated processes of photosynthetic carbon di- oxide assimilation and subsequent partitioning and utili- zation of the assimilated carbon. This requires efficient translocation of photo-assimilates to the developing plant parts [8,9]. However, there is little information on the phenotypic plasticity and biomass partitioning of coffee trees under varying light gradients in heterogeneous field conditions. To this effect, understanding the influence of environmental stress pressures such as direct solar radia- tion on growth and physiology of coffee plant is impor- tant to examine the coping mechanisms involved in the shade-adapted Arabica coffees. The natural coffee forests with the occurrence of maximum plant diversity include- ing the wild coffee genetic resources are under continu- ous threats of erosion due to several factors and needs urgent conservation measures [10]. According to [11], nowadays, it is not uncommon to see coffee tree drying symptoms due to climate changes that have resulted in physiological disorders between vegetative and repro- ductive growths. This is associated with continuous flowering and heavy crop loads as a result of changes in weather patterns with erratic rainfalls. The situation is aggravated by the outbreak of insect pests and diseases, requiring new corrective strategies to revive Arabica gene pools from possible fast and irreversible losses. Hence, this study would provide baseline research in- formation on the use of growth traits in the selection of coffee genotypes in breeding program for sustainable management and conservation of coffee genetic diversity and its healthy environments. Therefore, this ex-situ ex- periment was carried out to compare the variability in biomass prodution and allocation patterns in seedlings of arabica coffee germplasm accessions under varying nur- sery irradiance and irrigation conditions at Jimma Re- search Center in southwest Ethiopia. 2. MATERIALS AND METHODS 2.1. The Study Site The experiment was conducted at a common nursery garden of the Jimma Research Center, southwest Ethio- pia (7˚46'N, 36˚0'E, 1750 m.a.s.l). This is a national cof- fee research coordinating center were about 5960 live coffee germplasm are found in field gene banks for re- search work. The area receives adequate amount of rain- fall with an average rainfall of 1595 mm per annum dis- tributed into 173 days. The driest season lasts between December and January. The average maximum and mini- mum air temperatures are 25.9˚C and 11.2˚C, respec- tively, the coldest month being December [12]. 2.2. Experimental Treatments and Design Ripe red coffee cherries were collected in 2004/2005 from four wild coffee populations in the montane rain- forests of southeastern and southwest Ethiopia. The Copyright © 2012 SciRes. OPEN ACC ESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 837 recommended ideal potting medium [13] was prepared from topsoil and decomposed coffee husk at the respect- tive proportion of 3:1 (v/v) and firmly filled in black plastic pots (volume = 5.8 L) perforated at the bottom. The plant plastic pots were arranged on nursery seedbed and the prepared coffee seeds were sown in each pot. The recommended post-sowing nursery operations were applied [14] and the seedlings were uniformly managed under partial shade conditions and irrigated at every four-day intervals. After one-year of ex situ estab lishment under common garden, the shade treatment was applied for three con- secutive dry months between March and May. In this case, seedlings were divided into equal halves to impose the contrasting shade and irrigation treatments. The studied environmental modification treatments included shading levels (moderate shading and full solar light in- terception), two irrigation treatments (well-watered and water deficit) and twelve coffee germplasm genotypes of different geographical origins in Ethiopia. The central five coffee seedlings were used to record destructive growth parameters for each treatment. These include dry weight of leaves, main stem, primary branches and root system. The roots were immersed and washed in clean water to remove adhering soil. Subsequently, each plant part was separately oven-dried at 105˚C for 24 h and weighed using a sensitive balance. Finally, total dry matter production and allocation patterns were deter- mined in seedlings of arabica coffee germplasm acces- sions under contrasting nursery microclimatic varables (Table 1) monitored and described [12]. 2.3. Data Analysis The statistical analysis was accomplished using SAS Table 1. Microclimate variables in the studied full sunlight and moderately shaded coffee nursery site at Jimma Research Cen- ter. Temperature (˚C) Variable RH (%) Air Soil Time of day * ** Ns Night 80.97 ± 9.97a 16.56 ± 1.74b 19.10 ± 7.16 Day 70.82 ± 6.03b 20.47 ± 2.01a 23.83 ± 4.23 Shading level Ns Ns Ns Full sun 73.42 ± 8.57 18.75 ± 3.17 24.03 ± 5.95 Shaded 78.36 ± 9.52 18.28 ± 2.46 18.90 ± 5.60 Mean 75.89 18.51 21.46 CV (%) 3.17 3.58 30.42 Time* shade Ns Ns Ns Ns = Not significant; *P < 0.05; **P < 0.001. Means with the same letter is not different from each other according to Tukey test at P = 0.05. for Windows version 8.1 (SAS Institute Inc., Cary, NC). Two-way analysis of variance (ANOVA) was computed for each growth variable considered in a factorial ex- periment arranged in a randomized complete block de- sign with three replications. Principal component analy- sis was computed to compare the variability among the coffee germplasm accessions. Moreover, treatment means were ranked according to Tukey test at P = 0.05, when- ever the F-test showed significant differences. Figures of significant interactions were made with the Sigma Plot SPW9.0 (SYS TAT So ft wa r e, Inc.) . 3. RESULTS 3.1. Biomass Production 3.1.1. Shoot Biomass Significantly (P < 0.001) higher stem (main stem and branch) biomass (17.48 ± 2.49 g) was obtained from un- shaded plots than from those in partial shade environ- ments. In addition, insignificantly higher leaf dry weight (13.58 g) was found for sun-exposed seedlings (Tab le 2). Similarly, most shoot growth parameters were signifi- cantly higher for water deficit subjected seedlings. Hence, significantly (P < 0.05) higher leaf dry mass and main stem plus branch (P < 0.05) were obtained for non-irri- gated than for well-watered seedlings. With regard to shade or irrigation, stem biomass was higher (17.48 g) in the open sun than the shade (15.60 g) with about 11% difference. Coffee accessions significantly (P < 0.05) differed in stem dry weigh t, with average results ranging from 14.36 ± 1.85 to 19.08 ± 2.01 g for Berhane-Kontir (III-2) and Harenna (I-1) seedlings, respectively (Table 2). Most accessions had relatively lower growth re- sponses in shade cond itions as compared to the su n plots. As a result, higher dry mass of leaf, root and total dry matter were recorded for seedlings exposed to direct sunlight (Figure 1). The results depict significant differ- ences among accessions in root dry mass and root to shoot ratio; the values followed the descending order of Harenna > Yayu > Bonga > Berhane-Kontir accessions. 3.1.2. Root Biomass A significantly ( P < 0.01) higher root dry weig ht (8.93 g) was obtained from unshaded than from partially shaded seedlings (7.89 g), where a reduction of about 12% was noted. The difference between watering re- gimes was also significant (P < 0.01) for root dry mass and was higher for drought-stressed (8.90 g) than for well-irrigated (7.92 g) seedlings. In the same manner, coffee accessions significantly differed (P < 0.01) in root dry mass. Consequently, the lowest (III-1 = 6.48 g) and highest (I-2 = 10.43 g) average values were obtained from Berhane-Kontir and Harenna seedlings, respec- Copyright © 2012 SciRes. OPEN AC CESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 Copyright © 2012 SciRes. 838 Table 2. Shoot and root growth (means ± S.D) of seedlings of arabica coffee genotypes under varying irradiance and irrigation re- gimes. Treatment LDW (g) SDW (g) RDW (g) R:S TDM (g) Irradiance Ns ** ** Ns ** Open sun 13.58 ± 1.59 17.48 ± 2.49a 8.93 ± 1.64a 0.29 ± 0.03 39.99 ± 5.31a Moderate shade 13.06 ± 0.91 15.60 ± 1.55b 7.89 ± 1.52b 0.28 ± 0.04 36.54 ± 3.13b Irrigation * * ** Ns ** Water stressed 13.68 ± 1.40a 17.14 ± 2.53a 8.90 ± 1.52a 0.29 ± 0.03 39.72 ± 4.95a Well-watered 12.95 ± 1.12b 15.94 ± 1.81b 7.92 ± 1.66b 0.27 ± 0.04 36.81 ± 3.92b Genotype Ns * ** ** * I-1 13.25 ± 0.80 19.08 ± 2.01a 10.02 ± 1.1 1a 0 .31 ± 0.04ab 42.34 ± 3.43a I-2 13.06 ± 0.89 18.47 ± 2.09ab 10.43 ± 1.23a 0.33 ± 0.01a 41.95 ± 4.03ab I-3 13.73 ± 1.85 16.63 ± 1.78ab 9.22 ± 0.95ab 0.31 ± 0.02ab 39.57 ± 4.52ab II-1 13.63 ± 1.78 16.65 ± 2.47ab 8.30 ± 1.44abc 0.28 ± 0.02abc 38.58 ± 5.57ab II-2 12.34 ± 0.66 15.52 ± 1.33ab 7.97 ± 1.11abc 0.28 ± 0.03abc 35.82 ± 2.90ab II-3 12.81 ± 0.68 15.29 ± 1.66ab 8.15 ± 1.34abc 0.29 ± 0.04abc 36.24 ± 2.84ab III-1 11.91 ± 0.51 14.84 ± 1.45ab 6.48 ± 1.09c 0.25 ± 0.04 bc 33.23 ± 1.93b III-2 12.97 ± 1.29 14.36 ± 1.85b 7.22 ± 1.66bc 0.27 ± 0.04abc 34.54 ± 4.2 1a b III-3 14.37 ± 1.4 5 16.11 ± 2.41ab 6.86 ± 1. 2 0b c 0.23 ± 0.03 c 37.33 ± 4.65ab IV-1 13.87 ± 0.76 17.16 ± 1.50ab 8.67 ± 0.30abc 0.28 ± 0.02abc 39.69 ± 2.09ab IV-2 14.41 ± 0.67 18.06 ± 1.24ab 9.39 ± 1.00ab 0.29 ± 0.01abc 41.87 ± 2.81ab IV-3 13.52 ± 2.12 16.29 ± 3.37ab 8.23 ± 2.43abc 0.27 ± 0.05abc 38.03 ± 7.63ab Mean 13.32 16.54 8.41 0.28 38.26 CV (%) 7.95 9.35 10.72 9.31 8.20 Ns = Not significant; *, ** and *** = significant at P < 0.05, P < 0.01 and P < 0.001, respectively. Means followed by same letter with in a column are not differ- ent from each other (Tukey test at P = 0.05). Abbreviations: LDW = leaf dry weight, SDW = stem dry weight, RDW= root dry weight, R:S = root to shoot ratio, TDM = total dry matter. F igure 1. Biomass yield and root to shoot ratio of coffee accessions under full sunlight and shaded conditions. OPEN A CCESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 839 tively (Table 2). In general, the Harenna seedlings had a higher root mass than the others, particularly the Ber- hane-Kontir accessions, which had a low root biomass ranging between 6.48 and 7.22 g. This was less than the overall average root dry mass of 8.41 g. The ratios of root to shoot dry mass of the seedlings also significantly differed among the coffee accessions, but not between irradiance and irrigation levels. The absence of signifi- cant differences due to shade and irrigation suggests that the duration of the treatments was too short to considera- bly change root biomass accumulation. However, root to shoot ratio of some seedlings surpassed those in shadow conditions. The significantly lowest (III-3 = 0.23) and highest (I-2 = 0.33) root to shoot values were determined for the Berhane-Kontir and Harenna accessions, respec- tively. The root to shoot ratio result was higher for the Harenna as compared with those from Berhane-Kontir coffee germpl asm (Figure 2). 3.1.3. Total Biomass The analysis of variance comparing the total dry matter production of coffee seedlings depicts significant (P < 0.01) differences between the two shade and irrigation treatments as well as due to accessions (P < 0.05). Sig- nificantly (P < 0.001) higher total dry matter yield (3 9.99 ± 5.31 g) was obtained from the full sun light than from the shaded plots. There was abou t 9% reducetion in total dry matter production of seedlings in moderately shaded seedlings as compared to those in open sunlight. Simi- larly, significantly (P < 0.01) higher total dry matter was obtained from drought-stressed than well-watered seed- lings. The results also reveal sign ificantly lowest (III-1 = 33.23 g) and highest (I-1 = 42.34 g) total biomass from the Berhane-Kontir and Harenna coffee accessions, re- spectively (Table 2, Figure 2). Accessions from Harenna (I-2 = 41.95 g) and Berhane-Kontir (III-2 = 34.54 g) had the next maximum and low average values, respectively, though no significance was detected among the other accessions. The results of the destructive parameters manifested that the coffee germplasm accessions were grouped into three broad classes. The first group con- sisted of a mixture of accessions from Yayu (IV-1 and IV-2), Bonga (II-1), Berhane-Kontir (III-3) and Harenna (I-3). Wher eas, the Bong a and Harenna populatio ns were classified into the second and the third group (Figure 3). 3.2. Biomass Partitioning The amount of total dry mass partitioned to the root part was comparable between shade treatments, though it was higher in full sun irradiance (22.22% ± 1.89%) and drought stressed (22.35% ± 2.02%) seedlings. In contrast, leaf partitioning was significantly different according to the level of shade (P < 0.001) and irrigation (P < 0.05). As a result, the mean values ranged from 34.11% ± 2.19% Figure 2. Root and shoot dry mass for seedlings of wild coffee accessions under (a) full sunlight and (b) shaded conditions. Figure 3. Principal component analysis for destructive growth parameters of coffee seedlings maintained under optimal envi- ronmental conditions. to 35.89% ± 2.92% and 34.63% ± 2.50% to 35.37% ± 2.90% between full sun and shade and drought-stressed and irrigated seedlings, respectively. Furthermore, in full sunlight, the stem plus branch part shared significantly (P < 0.05) more of the total assimilates (43.67% ± 1.55%) as opposed to seedlings in partial shade conditions (Ta- ble 3). The total dry matter yield distributed to leaf and total shoot part was, however, not altered due to irrigation regimes. Hence, the average leaf and shoot shares ranged between 43.03% ± 1.75% and 43.30% ± 1.53% and 77.66% ± 2.02% and 78.67% ± 2.68 %, respectiv ely. Th is was similar to the slightly higher assimilate amounts (78.55% ± 2.81%) stored in the shoot part of shaded seedlings, though the allocation to the root part was low (Table 3). This corresponds with the more luxurious shoot growth of coffee seedlings at resource rich envi- ronments as opposed to deep root systems in drought- stressed situations. The coffee accessions from Berhane- ontir had significantly (P < 0.01) the lowest (18.3%) K Copyright © 2012 SciRes. OPEN AC CESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 840 Ta b le 3. Total dry matter partitioning patterns (means ± SD) in seedlings of Arabica coffee genotypes under varying nursery envi- ronments. Treatment Root Leaf Stem Whole-shoot Irradiance Ns *** * Ns Open sun 22.22 ± 1.89 34.1 1 ± 2.19b 43.67 ± 1.55a 77.78 ± 1.90 Moderate shade 21.45 ± 2.81 35.89 ± 2.92a 4 2.66 ± 1.58b 78.55 ± 2.81 Irrigation Ns * Ns Ns Water stressed 22.35 ± 2.02 34.63 ± 2.50b 43.03 ± 1.75 77.66 ± 2.02 Well-watered 21.33 ± 2.68 35.37 ± 2.90a 4 3.30 ± 1.53 78.67 ± 2.68 Genotype ** *** Ns ** I-1 23.68 ± 1.89ab 31.33 ± 1.14de 45.00 ± 1.55 76.33 ± 1.89bc I-2 24.82 ± 0.87a 31.21 ± 1.61e 43.98 ± 1.19 75.18 ± 0.87c I-3 23.33 ± 0.66ab 34.62 ± 0.99bc 42.05 ± 0.73 76.67 ± 0.66bc II-1 21.48 ± 1.19abc 35.38 ± 0.92bc 43.15 ± 1.05 7 8.53 ± 1.19abc II-2 22.16 ± 1.45abc 34.52 ± 1.50c 43.32 ± 1.48 77.84 ± 1.44abc II-3 22.37 ± 2.04abc 35.51 ± 3.48abc 42.13 ± 2.20 77.64 ± 2.04abc III-1 19.50 ± 3.15bc 35.87 ± 1.09abc 44.64 ± 2.90 80.50 ± 3.15ab III-2 20.75 ± 2.89abc 37.71 ± 3.00ab 41.54 ± 0.57 79. 25 ± 2 .89abc III-3 18.34 ± 1.86c 38.62 ± 2.52a 43.05 ± 1.16 81.66 ± 1.85a IV-1 21.88 ± 1.32abc 34.94 ± 1.00bc 43.18 ± 1.73 78.13 ± 1.33abc IV-2 22.39 ± 0.92abc 34.47 ± 1.07cd 43.15 ± 0.76 77.62 ± 0.93abc IV-3 21.38 ± 2.81abc 35.81 ± 2.85abc 42.82 ± 0.74 78.62 ± 2.81abc Mean 21.84 35.00 43.17 78.16 CV (%) 7.67 3.18 3.30 2.14 Ns = Not significant; *, ** and *** = significant at P < 0.05, P < 0.01 and P < 0.001, respectively. Means followed by the same letter(s) with in a column are not different from each other (Tukey test at P = 0.05). root partitioning as compared to the Harenna seedlings, which had the highest (38.6%) root share. The results indicate significant differences among accessions in the patterns of total biomass partitioned in leaf (P < 0.001) and shoot (P < 0.01) growth organs. Consequently, the values ranged between 31.2% and 38.6% for leaves and between 75.2% and 81.7% for the shoot. Under ideal shade and irrigation regimes, differences among acces- sions in the allocation of total biomass were not signifi- cant. However, low leaf partitioning was observed in th e Harenna accessions as compared to those from the Bonga and Berhane-Kontir forests. Maximum leaf por- tioning was also observed in one of the accessions from the Yayu populations (IV-3 = 35.8%). On the other hand, stem plus branch partitioning was comparable among coffee genotypes, with the values ranging between 41.5 and 44.6% for III-2 and III-3 of the Berhane-Kontir ac- cessions, respectively (Table 3, Figure 4). 4. DISCUSSION 4.1. Biomass Assimilation The results show significantly higher shoot dry mass Figure 4. Total dry matter partitioning to root and shoot parts in seedlings of Arabica coffee germplasm accessions. production in unshaded and progressively droughted coffee seedlings. This could in part be related to low stomatal conductance and concomitantly lower rate of net carbon assimilation in shade and irrigated plants compared to short-term drought-stressed plants [15]. Moreover, a reduced leaf growth was noted in shade compared to sun plots, and such smaller leaf area may alter assimilate partitioning among the tree organs and Copyright © 2012 SciRes. OPEN ACC ESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 841 decrease shoot dry matter yield, in part indicating the effect of micro-climatic variables at coffee nurseries (Table 1). This suggests that a major mode of adjustment to reduced soil water demand by the coffee seedlings, at least in the early stages of soil moisture deficit irrigation, could consist of the maintenance of nearly constant as- similation on an area ba sis through a reductio n in the rate of increase in average leaf size. This indicates that leaf expansion can be accompanied by growth and develop- ment as well as accumulation and synthesis of the leaf cell components. Similar results were also reported for field-grown robusta coffee [16]. On the other hand, coffee accessions significantly dif- fered in stem dry weight with the lowest and highest av- erage results obtained from the Berhane-Kontir and Harenna seedlings, respectively. This could reflect the variations among the coffee genotypes in growth rate and productivity, which in turn depend upon stem nature (stiff or flexible) and size of water conducting tissue [17]. The results of dry matter production and partitioning are in line with the maximum hydraulic conductance meas- ured in full sun light and irrigated seedlings [12]. More- over, other finding [18] also indicated the enhanced wa- ter use of the arabica coffee populations with a decreased rainfall gradient in Ethiopia. This could be associated, among others, with the specific growth responses in- cluding increased number of lateral roots, higher root to shoot ratio as well as decreased leaf-specific area in drought-stressed coffee seedlings [19]. In this regard, many other authors [20-22] pointed out that plants withstand drought stress by drought tolerance (higher biomass allocation to vegetative organs and root to shoot ratio) , whereas drough t escape strategies invo lve early flower set and leaf senescence. According to the work done [16], water deficit led to marked decreases in net carbon assimilation rate and to a lesser extent in stomatal conductance, regardless of the low and high nitrogen levels. In Ethiopia, variations among coffee varieties were found [23] due to soil moisture deficit irrigation and their capacity to establish in more stressful field conditions. This concurs with the previous findings [13] that underlined similar patterns of total dry matter production and allocation in Arabica coffee seedlings due to varied potting media blends. The response of the cof- fee seedlings could also suggest a drought-stress resis- tance strategy investing more of the daily biomass pro- duction in the root system, while penalizing the shoot system. This may be explained due to the differences in root and shoot growth habits and thus hydraulic conduc- tance in coffee seedlings as elucidated [19]. 4.2. Biomass Partitioning The amount of total dry mass partitioned to the root part was comparable between the shade and irrigations regimes, though the results were higher for partially shaded and well-irrigated seedlings. By contrast, the cof- fee accessions significantly (P < 0.001) differ in total biomass share to root, leaf and whole shoot part. For maximum rate of production of dry matter within the plant as a whole it is important that a high proportion of assimilates as possible should be returned to the leaf tis- sue, which will further increase the productive capacity of the plant, and that expenditure of dry matter on the rest of the plant (stems, petioles and roots) should be no more than is required to efficiently support the leaves and supply sufficient mineral nutrients and water. The overall rate of utilization of assimilates in leaf production depend upon the rate of new leaf initiation, the rate of growth and final leaf size and the branching habit [24]. Under field conditions, assimilate utilizatio n may be lim- ited by various external factors including temperature, and the supply of water and mineral elements [1]. How- ever, there is evidently some homeostatic mechanism maintaining a given root-shoot balance under any given set of conditions, and this in turn implies an interplay between growth in root and shoot components [25]. Changes in the partitioning of dry matter between roots and shoots brought about by environmental factors sug- gest that when the size of the source is reduced, the growth of organs most remote from it is often particu- larly affected [26]. Similarly, assimilate allocation in plants was found to be affected by water deficit stress conditions [15]. Hence, the manner in which dry matter is partitioned between the different parts of the plant is clearly of great importance both in natural vegetation and in crop plants. In essence, the growth and seed production is a result of the integrated processes of photosynthetic carbon di- oxide assimilation and subsequent partitioning and utili- zation of the assimilated carbon. This requires efficient translocation of photo-assimilates to the developing plant parts [8,9]. A method of studying energy allocation in plants that requires separating individual plants into component tissues has been reported [27] according to their function, and expressing energy allocation as a proportion of total biomass stored in each tissue type. Ther e are several r easons why weight is used to measu re biomass allocation patterns. According to available in- formation [8,20], energy content and dry weight equally reflect energy allocation pattern and dry weight is re- ported to reflect the integration of all physiological processes throughout the growing season. In addition, dry weight reflects the functional aspects of all assimila- tion [28]. In many crop species, increased yields of improved cultivars have been related to changes in partitioning as opposed to increase in total biomass [29]. In the tall, Copyright © 2012 SciRes. OPEN AC CESS
T. Kufa / Agricultural Sciences 3 (2012) 835-843 842 mature field-grown Arabica coffee trees in Kenya, shoot growth is reported to be associated with seasonal changes in dry matter distribution within the [7]. Green-house- grown Arabica coffee cultivar-Ruiru 11 showed that dry matter partitioning to above- and below-ground parts varied with accessions [27]. The present finding could be explained in terms of the variations in morphological growth natures in seedlings of the same coffee popula- tions [30]. This corroborates with the prev ious results [13] on the influence of potting media on dry matter yield and allocation in coffee seedlings. Cognizant of the impor- tance of coffee, its high genetic erosion in the centers of origin and minimal conservation efforts emphasized the need for immediate conservation meas- ures to safeguard the sustainability of the global coffee industry [10]. 5. CONCLUSION The study depicted that biomass production and dis- tribution patterns in coffee seedlings varied due to geno- types and environmental modifications, singly and/or in combinations. The present findings provide an insight research information to evaluate and character ize growth response of coffee germ plasm accessions to the induced heat and water stress environments. This contributes to identify and develop drought tolerant arabica coffee cul- tivars against the expansion of coffee cultivation to open sun fields and marginal sites coupled with changing cli- matic variables. It can be concluded that dry matter pro- duction and partitioning patterns in the various growth organs of coffee seedlings can be considered and used as selection criteria in future breeding program. 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