Influence of atmospheric vapour pressure deficit on ozone responses of snap bean (Phaseolus vulgaris L.) genotypes

Environmental conditions influence plant responses to ozone (O ₃), but few studies have evaluated individual factors directly. In this study, the effect of O₃ at high and low atmospheric vapour pressure deficit (VPD) was evaluated in two genotypes of snap bean (Phaseolus vulgaris L.) (R123 and S156) used as O₃ bioindicator plants. Plants were grown in outdoor controlled-environment chambers in charcoal-filtered air containing 0 or 60 nl l^-1 O₃ (12 h average) at two VPDs (1.26 and 1.96 kPa) and sampled for biomass, leaf area, daily water loss, and seed yield. VPD clearly influenced O₃ effects. At low VPD, O ₃ reduced biomass, leaf area, and seed yield substantially in both genotypes, while at high VPD, O₃ had no significant effect on these components. In clean air, high VPD reduced biomass and yield by similar fractions in both genotypes compared with low VPD. Data suggest that a stomatal response to VPD per se may be lacking in both genotypes and it is hypothesized that the high VPD resulted in unsustainable transpiration and water deficits that resulted in reduced growth and yield. High VPD-and water-stress-induced stomatal responses may have reduced the O₃ flux into the leaves, which contributed to a higher yield compared to the low VPD treatment in both genotypes. At low VPD, transpiration increased in the O₃ treatment relative to the clean air treatment, suggesting that whole-plant conductance was increased by O₃ exposure. Ozone-related biomass reductions at low VPD were proportionally higher in S156 than in R123, indicating that differential O₃ sensitivity of these bioindicator plants remained evident when environmental conditions were conducive for O₃ effects. Assessments of potential O₃ impacts on vegetation should incorporate interacting factors such as VPD.