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Response of Soybean to Low Concentrations of Ozone: I. Reductions in Leaf and Whole Plant Net Photosynthesis and Leaf Chlorophyll Content¹

ABSTRACT

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.

Key Words: air pollution • oxidant pollution • CO₂ uptake • CO₂ exchange • Glycine max

¹ Contribution of the Boyce Thompson Institute for Plant Research, Cornell Univ., Ithaca, NY 14853. This project was supported in part by funds from the U.S. Environmental Protection Agency.

² Assistant Professor, Dep. of Forestry, Univ. of Wisconsin, Madison, WI 53706; Plant Physiologist, Rocky Mountain Forest and Range Exp. Stn., Fort Collins, CO 80526; Research Specialist and Research Associate, Boyce Thompson Inst., Cornell Univ., Ithaca, NY 14853.

Received for publication April 26, 1985.