Effects of atmospheric CO2 on photosynthetic characteristics of soybean leaves

Soybean (Glycine max. cv. McCall) plants were grown at 500, 1000, and 2000 umol mol (exp -1) CO2 for 35 days with a photosynthetic photon flux of 300 umol m (exp -2) s (-1). Individual leaves were exposed to step changes of photosynthetic photon flux to study CO2 assimilation rates (CAR), i.e., leaf net photosynthesis. In general, CAR increased when CO2 increased from 500 to 1000 umol mol (exp -1), but not from 1000 to 2000 umol mol (exp -1). Regardless of the CO2 level, all leaves showed similar CAR at similar CO2 and PPF. This observation contrasts with reports that plants tend to become 'lazy' at elevated CO2 levels over time. Although leaf stomatal conductance (to water vapor) showed diurnal rhythms entrained to the photoperiod, leaf CAR did not show these rhythms and remained constant across the light period, indicating that stomatal conductance had little effect on CAR. Such measurements suggest that short-term changes in CO2 exchange dynamics for a controlled ecological life support system can be closely predicted for an actively growing soybean crop.