Response of soybean to low concentrations of ozone: I. Reductions in leaf and whole plant net photosynthesis and leaf chlorophyll content

Since most commercially grown soybean plants in the USA are subjected to low or moderate levels of ambient O₃ pollution it is important to understand whether exposure to O₃ influences CO₂ exchange in this species. Therefore, for 8 weeks soybean (Glycine max cv. Hodgson) plants were exposed to O₃ for 6.8 h daily in controlled environment chambers. Four treatments were used: 0.01, 0.05, 0.09, and 0.13 μL L^-1 O₃ in filtered air. Net photosynthesis (P(n)), dark respiration, and chlorophyll content were measured in all treatments for both individual leaves of various ages and for whole plants. Measurements of CO₂ exchange were made nondestructively with an infrared gas analysis system using either plexiglass cuvettes (leaves) or continuously stirred tank reactors (plants). The P(n) of whole plants was reduced by 10, 11, and 22% in the 0.05, 0.09, and 0.13 μL L^-1 treatments, respectively, compared with the 0.01 μL L^-1 treatment. Among individual leaves of similar ages, exposure to O₃ also resulted in declines in P(n) and this occurred at all leaf ages. There was a significant linear relationship between P(n) and O₃ concentration for both individual leaves and for whole plants. Chlorophyll contents of whole plants and individual leaves were also reduced by O₃ exposure. Chlorophyll content and P(n) were correlated with each other for both individual leaves and whole plants. Changes with leaf age were typical for P(n) and for chlorophyll content. There was no apparent effect of O₃ treatment on dark respiration. The observed reduction in P(n) at low levels of O₃ (coupled with reports of O₃-induced reduction in growth and yield) suggests that ambient O₃ pollution in the USA is currently causing decreased P(n) in field-grown soybean which can lead to a significant loss in yield.