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

Abstract

Global climatic change scenarios predict a significant increase in future tropospheric ozone (O3) concentrations. The present investigation was done to assess the effects of elevated O3 (70 and 100ppb) 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 O3 for 4h·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 O3 stress. The O3-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 CO2 concentration in both O3-treated soybean varieties compared suggests that the reduction in photosynthesis was due to damage to the photosynthetic apparatus, leading to accumulation of internal CO2 and stomatal closure. The adverse impact of O3 stress increased at higher O3 concentrations in both soybean varieties leading to large reductions in photosynthesis. This study suggests that O3-induced reductions in photosynthesis in tropical and temperate varieties are similar. © 2009 German Botanical Society and The Royal Botanical Society of the Netherlands.