Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO₂ and ozone concentrations for 3 years under fully open-air field conditions

It is anticipated that enrichment of the atmosphere with CO₂ will increase photosynthetic carbon assimilation in C3 plants. Analysis of controlled environment studies conducted to date indicates that plant growth at concentrations of carbon dioxide ([CO₂]) anticipated for 2050 (∼ 550 μmol mol^-1) will stimulate leaf photosynthetic carbon assimilation (A) by 20 to 40%. Simultaneously, concentrations of tropospheric ozone ([O₃]) are expected to increase by 2050, and growth in controlled environments at elevated [O₃] significantly reduces A. However, the simultaneous effects of both increases on a major crop under open-air conditions have never been tested. Over three consecutive growing seasons > 4700 individual measurements of A, photosynthetic electron transport (J_PSII) and stomatal conductance (g_s) were measured on Glycine max (L.) Merr. (soybean). Experimental treatments used free-air gas concentration enrichment (FACE) technology in a fully replicated, factorial complete block design. The mean A in the control plots was 14.5 μmol m^-2 s^-1. At elevated [CO₂], mean A was 24% higher and the treatment effect was statistically significant on 80% of days. There was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated [CO₂], which accounted for much of the variation in CO₂ effect among days. The effect of elevated [CO₂] on photosynthesis also tended to be greater under water stress conditions. The elevated [O₃] treatment had no statistically significant effect on mean A, g_s or J_PSII on newly expanded leaves. Combined elevation of [CO₂] and [O ₃] resulted in a slightly smaller increase in average A than when [CO₂] alone was elevated, and was significantly greater than the control on 67% of days. Thus, the change in atmospheric composition predicted for the middle of this century will, based on the results of a 3 year open-air field experiment, have smaller effects on photosynthesis, g_s and whole chain electron transport through photosystem II than predicted by the substantial literature on relevant controlled environment studies on soybean and likely most other C3 plants.