Effects of Drought Stress on Growth, Water-Use Efficiency and Leaf Gas-Exchange of Common Bean (<i>Phaseolus vulgaris</i> L.) Genotypes Differing in Drought Resistance

Abstract

Growth, water-use efficiency (WUE) and leaf gas exchange responses of common bean (Phaseolus vulgaris L.) genotypes differing in degree of drought  resistance were assessed when plants were subjected to drought stress at vegetative and early pod filling stages. A drought-resistant inbred line (SEA 15)  and a drought-susceptible cultivar (BrSp) selected from an earlier screening trial were used for the study. The two genotypes were grown in a vegetation  hall under drought stress and non-stress conditions during the summer of 2004. Despite differential genotypic responses found to the stress imposed, drought initiated at both growth stages had an adverse effect on leaf area expansion, above-ground biomass yield and biomass partitioning of the two  genotypes. Seed yield reduction due to drought imposed during early pod-fill stage was 72% and 33% for BrSp and SEA 15, respectively. Drought stress at  the vegetative stage increased the WUE of both genotypes, although the increase was considerably higher for SEA 15 (3.12 mg g^-1) compared with  BrSp (2.45 mg g^-1). On the other hand, seed yield based water-use efficiency (WUESY) was significantly higher for SEA 15 (1.81 mg g^-1) compared with  BrSp (0.33 mg g^-1) under drought stress imposed at early pod-fill stage. Drought stress initiated during the reproductive phase significantly reduced the  net photosynthetic rate (A) and stomatal conductance (gs) of the two genotypes. Despite the similar response in leaf gas-exchange parameters (A and gs)  between the two genotypes, the stress resulted in a significant and consistent decrease in Ci/Ca ratio of SEA 15 implying that the stress thresholds at  which stomatal and metabolic limitations to A occurred differed between the two genotypes. Some of these specific adaptation traits related to growth,  artitioning and water-use can be used in tandem with seed  yield as selection criteria in breeding beans for drought conditions.