Response of Soybean to Low Concentrations of Ozone: I. Reductions in Leaf and Whole Plant Net Photosynthesis and Leaf Chlorophyll Content

Abstract

Abstract Since most commercially grown soybean plants in the USA are subjected to low or moderate levels of ambient O 3 pollution it is important to understand whether exposure to O 3 influences CO 2 exchange in this species. Therefore, for 8 weeks soybean ( Glycine max cv. Hodgson) plants were exposed to O 3 for 6.8 h daily in controlled environment chambers. Four treatments were used: 0.01, 0.05, 0.09, and 0.13 µ L L −1 O 3 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 CO 2 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 µ L L −1 treatments, respectively, compared with the 0.01 µ L L −1 treatment. Among individual leaves of similar ages, exposure to O 3 also resulted in declines in P n and this occurred at all leaf ages. There was a significant linear relationship between P n and O 3 concentration for both individual leaves and for whole plants. Chlorophyll contents of whole plants and individual leaves were also reduced by O 3 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 O 3 treatment on dark respiration. The observed reduction in P n at low levels of O 3 (coupled with reports of O 3 ‐induced reduction in growth and yield) suggests that ambient O 3 pollution in the USA is currently causing decreased P n in field‐grown soybean which can lead to a significant loss in yield.