Leaf photosynthesis and stomatal conductance acclimate to elevated [CO2] and temperature thus increasing dry matter productivity in a double rice cropping system

Changes in atmospheric CO2 concentration ([CO2]) and temperature have impacts on leaf photosynthesis (A(net)) and stomatal conductance (G(s)) of rice, which strongly affect dry matter productivity. Understanding of the acclimation responses of A(net) and G(s) under elevated [CO2] and/or temperature is limited. A field experiment of double rice (early rice-late rice rotation) was conducted using open-top chambers from 2013 to 2016 in Hubei Province, Central China, with two levels of [CO2] (ambient, ambient + 60 mu mol mol(-1)) and two levels of temperature (ambient, ambient + 2 degrees C). Averaging across the four-year experiment, elevated [CO2] and elevated temperature increased A(net) by 2.4 % -9.4 % and 2.4 %-10.7 %, respectively at pre-heading stages. A positive interaction was observed between elevated [CO2] and temperature in early rice which further increased A(net), while the interaction was not additive in late rice. Elevated [CO2] caused lower G(s) and partly offset the stimulation of elevated temperature on G(s) at tillering and jointing. At post-heading stages, photosynthetic acclimation to elevated [CO2] was observed as the stimulation of A(net) was not continued. Elevated temperature decreased A(net )by 1.7 %-16.6 % and further accelerated photosynthetic down-regulation by elevated [CO2]. Warming reduced G(s) at milking and maturity in early rice because of high ambient temperature, which resulted in stomatal limitation to photosynthesis, but its effects on G(s) in late rice were positive because the ambient temperature was low. Generally, G(s) acclimated to warming and CO2 enrichment in parallel with A(net) when there were no environmental constraints. Earlier leaf senescence, decreased leaf SPAD, lower seed-setting rate or larger sink capacity caused by elevated [CO2] and/or temperature might directly or indirectly explain the downregulation on photosynthesis after heading. Our results show that warming alters the acclimation of leaf photosynthesis and stomatal conductance to CO2 enrichment and the combined effects differ between growth stages, and indicate that co-elevation of [CO2] and temperature may increase dry matter productivity in the Chinese double rice cropping system.