Intrinsic water use efficiency at the pollination stage as a parameter for drought tolerance selection in Phaseolus vulgaris

Genotype differences in gas exchange during ontogeny and water stress responses at the vegetative and pollination stages were evaluated in four lines of Phaseolus vulgaris L. In the cultivar Carioca, net photosynthetic rate ( A ) and stomatal conductance ( g _s) were lower at the vegetative stage (20 days after sowing [DAS]) and maximum at the pollination stage (39 DAS), followed by a decrease at the flowering stage (46 DAS) and a dramatic fall at the grain-filling stage (60 DAS). Among the lines studied, the stomata of A320 closed faster than those of the other lines when water stress occurred at 20 or 39 DAS. The cultivar SC-90298823 had greater stomatal conductance at 39 DAS and a higher photosynthetic level than the other lines. Stomata of Ouro negro remained partially open during the water stress at the pollination stage (39 DAS) and supported a positive net photosynthetic rate ( A ). Differences were also found between lines in intrinsic water use efficiency (IWUE) at 39 DAS, but not at 20 DAS. The possibility of using IWUE at the pollination stage is discussed, in view of its use as one of the parameters for a drought tolerance breeding program in bean lines.