Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO 2 and ozone concentrations for 3 years under fully open-air field conditions

It is anticipated that enrichment of the atmosphere with CO 2 will increase photosynthetic carbon assimilation in C3 plants. Analysis of controlled environment studies con- ducted 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 assimila- tion ( 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 condi- tions have never been tested. Over three consecutive growing seasons > 4700 individual measurements of A , photosynthetic electron transport ( J PSII ) and stomatal con- ductance ( g s ) were measured on Glycine max (L.) Merr. (soybean). Experimental treatments used free-air gas con- centration enrichment (FACE) technology in a fully repli- cated, 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 sta- tistically significant on 80% of days. There was a strong positive correlation between daytime maximum tempera- tures 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.