Assessing drought tolerance of snap bean (Phaseolus vulgaris) from genotypic differences in leaf water relations, shoot growth and photosynthetic parameters

The leaf water relations, photosynthetic parameters and shoot growth of five snap bean cultivars were assessed during the drought period to determine their role in alleviating plant water deficit imposed by withholding irrigation at flowering. Soil water content of irrigated plants was 18-20% while that of unirrigated plants was 6-10% at 60 days after seeding (DAS). Leaf water potential was approximately 0.15MPa lower and relative water content was approximately 5% lower in unirrigated plants than in irrigated plants at 57 DAS. Unirrigated plants had a lower stomatal conductance (g(s)) and intercellular CO2 concentration (C-i). Reduced leaf water potential and relative water content were associated with a decreased stem elongation rate. Plants with a lower stem elongation rate had a higher specific leaf weight and succulence index (SucI). Significant differences among five cultivars of snap bean were found for all parameters measured. Decreased leaf water potential and stem elongation rate resulting from drought participated in preserving relative water content and improving specific leaf weight and SucI. Maintenance of higher relative water content increased g(s) and C-i. Cultivars that maintained a high relative water content when leaf water potential and stem elongation rate were decreased markedly, were more tolerant to drought than those which a reduced relative water content and the leaf water potential and stem elongation rate were only slightly lowered. Reduced yield (pods per plant and seed biomass) resulting from drought was associated with reduced relative water content.