Do all leaf photosynthesis parameters of rice acclimate to elevated CO , elevated temperature, and their combination, in FACE environments?

Leaf photosynthesis of crops acclimates to elevated CO and temperature, but studies quantifying responses of leaf photosynthetic parameters to combined CO and temperature increases under field conditions are scarce. We measured leaf photosynthesis of rice cultivars Changyou 5 and Nanjing 9108 grown in two free-air CO enrichment (FACE) systems, respectively, installed in paddy fields. Each FACE system had four combinations of two levels of CO (ambient and enriched) and two levels of canopy temperature (no warming and warmed by 1.0-2.0°C). Parameters of the C photosynthesis model of Farquhar, von Caemmerer and Berry (the FvCB model), and of a stomatal conductance (g ) model were estimated for the four conditions. Most photosynthetic parameters acclimated to elevated CO , elevated temperature, and their combination. The combination of elevated CO and temperature changed the functional relationships between biochemical parameters and leaf nitrogen content for Changyou 5. The g model significantly underestimated g under the combination of elevated CO and temperature by 19% for Changyou 5 and by 10% for Nanjing 9108 if no acclimation was assumed. However, our further analysis applying the coupled g -FvCB model to an independent, previously published FACE experiment showed that including such an acclimation response of g hardly improved prediction of leaf photosynthesis under the four combinations of CO and temperature. Therefore, the typical procedure that crop models using the FvCB and g models are parameterized from plants grown under current ambient conditions may not result in critical errors in projecting productivity of paddy rice under future global change.