Effects of elevated O3 exposure on seed yield, N concentration and photosynthesis of nine soybean cultivars (Glycine max (L.) Merr.) in Northeast China.

Nine soybean cultivars widely cultivated in Northeast China were investigated in present study to assess their O3 sensitivities on the basis of the response of photosynthesis and seed yield to ambient and future ozone (O3) concentrations, and determine whether the effects of O3 vary with the developmental stages (flowering and seed filling stages). Relative to charcoal-filtered air (CF), elevated O3 concentration (E-O3, ambient air+40 ppb) significantly reduced soybean yields by 40%, with a range of 32-46% among cultivars. E-O3 also induced significant decreases in pigment contents, net photosynthetic rate and chlorophyll a fluorescence at both flowering and seed filling stages in most cultivars. Except net photosynthetic rate and stomatal conductance (gs) at seed filling stage, all variables showed no significant interaction between O3 and cultivar, suggesting that all tested cultivars had similar sensitivities to O3. The responses of seed N content to E-O3 differed among cultivars. Ambient O3 concentration (mean of daily concentration of 19 ppb) did not induce any change relative to CF. Significant positive relationship between endogenous gs in CF and yield loss among cultivars was found only at seed filling stage. Positive correlation between effects of E-O3 on leaf N content and effects on light saturated photosynthetic rate (Asat) indicated that gs and leaf N content at seed filling stage contributes to yield loss and decreased photosynthesis by E-O3, respectively. It can be inferred that E-O3 had a larger negative effects on seed filling stage than flowering stage of soybean.