Integrated photosynthesis during fluctuating irradiance is correlated with steady-state photosynthesis for rice grown in field environments with elevated CO2 and temperature.

Photosynthesis under fluctuating irradiance is understudied, especially with regard to its response to elevated CO2 and temperature in field environments. We measured photosynthesis after a single-step irradiance change and during series of lightflecks for rice (Oryza sativa L.) grown under combinations of two CO2 levels (ambient and elevated by 200 μmol mol-1) and two temperature levels (ambient and increased by 2.0 °C) in temperature by free-air CO2 enrichment systems in two years. The cumulative CO2 fixation (CCF) during a single-step irradiance increase and during lightflecks were linearly correlated with final steady-state photosynthesis at the high irradiance (Anf) under elevated CO2 and temperature in both years. Across developmental stages, responses of CCF during a single-step irradiance increase and during lightflecks to elevated growth CO2, to elevated growth temperature, to elevated measurement CO2, and to elevated measurement temperature, were all in the same magnitude as responses of Anf to these variables. Lightfleck utilization efficiency (LfUE), defined as the ratio of CCF to assimilation expected from equivalent steady-state irradiance levels, showed large variation among lightfleck serial number, and this was related to the variation of stomatal conductance (gs) among the serial number. Thus, steady-state photosynthesis models may not result in critical errors in predicting the effects of elevated CO2 and temperature on dynamic photosynthesis provided that gs in responses to fluctuating irradiance can be quantified reliably.