Ethylene Directly Inhibits Foliar Gas Exchange in Glycine max.

Gas exchange of individual attached leaves of soybean, Glycine max (L.) Merr cv Davis, was monitored during exposure to exogenous ethylene (C(2)H(4)) to test the hypothesis that the effects of C(2)H(4) on net photosynthesis (P(N)) and stomatal conductance to H(2)O vapor (g(s)) are direct and not mediated by changes in leaf orientation to light. Leaflets were held perpendicular to incident light in a temperature-controlled cuvette throughout a 5.5 hour exposure to 10 microliters per liter C(2)H(4). Declines in both P(N) and g(s) were evident within 2 hours and became more pronounced throughout the exposure period. In C(2)H(4) treated plants, P(N) and g(s) decreased to 80 and 62%, respectively, of the rates in control plants. Because epinastic movement of the leaflets was prohibited by the cuvette, the observed declines in P(N) and g(s) were a direct effect of C(2)H(4) rather than the result of reduced light interception caused by changing leaf angle.