Physiological response to drought stress of common bean (Phaseolus vulgaris L.) genotypes differing in drought resistance
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Drought stress is a major constraint to common bean (Phaseolus vulgaris L.) production worldwide. Understanding the physiological basis of drought resistance may help to target the key traits that limit yield of the crop under drought situations. The objective of this study was to test the hypotheses that I) differences exist in biomass accumulation, yield and water-use efficiency among common bean cultivars developed for wider agro-ecological adaptation and inbred lines selected for specific adaptation to drought situations when subjected to drought stress; II) a drought-resistant genotype has a higher sink strength than a susceptible genotype and the difference between the genotypes is related to the ability to maintain assimilate synthesis and availability of assimilates for metabolism in the reproductive sink organs under drought stress; III) drought stress induces higher accumulation of ABA in sink leaves of a drought-susceptible genotype than in the leaves of a resistant genotype; and IV) relative to non-stressed plants, drought stress alters the protein pattern in a mature bean leaf. Three adapted cultivars and three inbred lines of common bean were initially screened to assess seed yield-based drought resistance and water-use efficiency of the genotypes. A drought-resistant (SEA 15) and a susceptible genotype (Brown Speckled) were selected and used for subsequent experiments carried out thereafter. Drought stress was initiated at different growth stages (vegetative or reproductive phases) by withholding the amount of water applied in order to keep the moisture level at about 30% of the maximum water-holding capacity (WHC) of the soil. For control treatments, the soil moisture was maintained at 70% of the maximum WHC. Parameters related to growth, yield, water-use efficiency, leaf-water relations, gas-exchange, seed sink size, and the concentration of assimilates in various source and sink organs were determined. Changes in mature leaf proteins due to drought stress imposed during the vegetative phase were detected using 2D gel electophoresis. Drought stress initiated during the reproductive phase significantly reduced the seed yields of the tested genotypes. However, seed yield reductions owing to the stress imposed were considerably higher for the old adapted cultivars than for the inbred lines selected for specific adaptation to drought conditions. With ca. 30 and 53% decrease in seed yield, SEA 15 and Brown Speckled were the most drought-resistant and susceptible genotypes, respectively. Drought stress initiated during different growth phases of the crop adversely affected biomass accumulation, although apparent genotypic differences were lacking. However, the harvest index of the drought-susceptible genotype (Brown Speckled) was reduced by about 29% due to drought, whereas that of SEA 15 remained unaffected. The maintenance of higher sink strength (larger numbers of pods and seeds per plant) under drought stress of SEA 15 was partly contributed by the higher efficiency of the genotype to remobilize biomass from vegetative parts to reproductive organs. SEA 15 maintained higher water-use efficiency (WUE) than Brown Speckled when subjected to drought stress. Relatively smaller transpiration rate (less water consumption), efficient stomatal regulation, faster vegetative biomass accumulation and higher seed yield production accounted for the observed higher WUE of the drought-resistant genotype. These features may confer a fitness advantage for drought-resistant inbred lines over drought-susceptible cultivars under drought conditions. Drought stress commenced at the vegetative as well as reproductive growth phases decreased net photosynthetic rate of the bean genotypes differing in drought resistance. Drought-induced stomatal closure (limited availability of CO2) was the main factor responsible for the reduced carbon assimilation rate of both genotypes, although there was evidence of non-stomatal inhibition of photosynthesis for the drought-susceptible genotype. During the vegetative phase, drought-induced reductions in A corresponded with the decreases in leaf sugar concentrations of the bean genotypes. Thus, shortage of assimilate as a consequence of reduced carbon assimilation could be the growth-limiting factor during the vegetative phase. Similarly, drought initiated during the pod-filling stage reduced not only A but also the concentrations of sucrose in the leaves and seeds of Brown Speckled demonstrating that reproductive sink establishment and yield of the susceptible genotype was primarily source-limited. Moreover, higher pod sucrose to hexose sugars ratio found for Brown Speckled demonstrates that the capability to utilize the imported sucrose was inhibited due to drought. Drought stress initiated during the same period did not alter the availability of assimilates for the drought-resistant genotype (SEA 15) both at source and sink levels. The results suggest that at whole plant level of SEA 15, total A is less affected than total carbohydrate demand because the genotype was able to adjust sink demand with source supply under drought conditions. In addition to assimilate synthesis, availability and metabolism, the differences found in drought-induced ABA accumulation and the decrease in pod water concentration could also be responsible for the differential drought-sensitivity of the bean genotypes. Drought stress also negatively affected seed starch accumulation of the bean genotypes mainly through arresting the expansion growth of starch granules (amyloplasts). In summary, although apparent genotypic differences were observed for sink strength under drought stress, the underlying variation in sink establishment and ultimate yield of the bean genotypes reside in the capacity to supply assimilates by the source (source-strength). Drought stress imposed during the vegetative phase of Brown Speckled resulted in the differential expression of ca. 42% of the total leaf proteins detected. Out of these proteins, 1.5% disappeared, 1.8% were newly produced, 23.5% were down-regulated and 15.1% were up-regulated in response to drought. The sizes of the drought-responsive proteins were widely variable.Verknüpfung zu Publikationen oder weiteren Datensätzen
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Göttingen : http://www.cuvillier-verlag.de: Cuvillier; 2006