Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO

It is anticipated that enrichment of the atmosphere with CO 2 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 ([CO 2 ]) anticipated for 2050 ( ~ 550 m mol mol 1 ) will stimulate leaf photosynthetic carbon assimilation ( A ) by 20 to 40%. Simultaneously, concentrations of tropospheric ozone ([O 3 ]) are expected to increase by 2050, and growth in controlled environments at elevated [O 3 ] 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 m mol m 2 s 1 . At elevated [CO 2 ], 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 [CO 2 ], which accounted for much of the variation in CO 2 effect among days. The effect of elevated [CO 2 ] on photosynthesis also tended to be greater under water stress conditions. The elevated [O 3 ] treatment had no statistically significant effect on mean A , g s or J PSII on newly expanded leaves. Combined elevation of [CO 2 ] and [O 3 ] resulted in a slightly smaller increase in average A than when [CO 2 ] 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. Key-words : air pollution; chlorophyll fluorescence; climate change; free-air gas concentration enrichment (FACE); global change.