Effects of elevated ozone on photosynthesis and stomatal conductance of two soybean varieties: a case study to assess impacts of one component of predicted global climate change.

Global climatic change scenarios predict a significant increase in future tropospheric ozone (O(3)) concentrations. The present investigation was done to assess the effects of elevated O(3) (70 and 100 ppb) on electron transport, carbon fixation, stomatal conductance and pigment concentrations in two tropical soybean (Glycine max L.) varieties, PK 472 and Bragg. Plants were exposed to O(3) for 4 h.day(-1) from 10:00 to 14:00 from germination to maturity. Photosynthesis of both varieties were adversely affected, but the reduction was higher in PK 472 than Bragg. A comparison of chlorophyll a fluorescence kinetics with carbon fixation suggested greater sensitivity of dark reactions than light reactions of photosynthesis to O(3) stress. The O(3)-induced uncoupling between photosynthesis and stomatal conductance in PK 472 suggests the reduction in photosynthesis may be attributed to a factor other than reduced stomatal conductance. An increase in internal CO(2) concentration in both O(3)-treated soybean varieties compared suggests that the reduction in photosynthesis was due to damage to the photosynthetic apparatus, leading to accumulation of internal CO(2) and stomatal closure. The adverse impact of O(3) stress increased at higher O(3) concentrations in both soybean varieties leading to large reductions in photosynthesis. This study suggests that O(3)-induced reductions in photosynthesis in tropical and temperate varieties are similar.