Warm air temperatures increase photosynthetic acclimation to elevated CO2 concentrations in rice under field conditions

Photosynthetic acclimation to elevated atmospheric CO2 concentration ([CO2]) accompanies decreased leaf nitrogen (N) content. Elevated air temperatures may enhance crop N nutrient content. Whether enhanced leaf N content at high temperatures relieves photosynthetic acclimation to high [CO2] in rice is unclear, so we investigated the effects of elevated [CO2] (eC; ambient [CO2]+200 mu mol mol(-1) CO2 and ambient temperature), elevated air temperature (eT; ambient+1 degrees C), and elevated temperature and [CO2] together (eT + eC; ambient+1 degrees C +200 mu mol mol(-1) CO2) compared to ambient [CO2] and temperature (Ambient) on photosynthetic and physiological parameters, biomass, and N accumulation and distribution throughout rice grain filling stages in 2015 and 2016. Net leaf photosynthesis (Pn) increased under eC, but the magnitude of Pn increase decreased as grain filling progressed, which was exacerbated under eT + eC. The total leaf N (TLN) content decreased by 7.1 % and increased by 4.7 % on average during the whole grain filling stage under eC and eT compared with Ambient, respectively. However, increasing TLN under elevated temperature did not offset the TLN reduction under [CO2] enrichment, similar to the effects of elevated [CO2] and air temperature on chlorophyll (Chl) and carotenoid (Caro) content. The percentage decrease in Rubisco content was larger under individual changes or the combined elevation of [CO2] and air temperature than the percentage change in TLN under eC. Meanwhile, the maximum rate of Rubisco carboxylation at 25 degrees C (V-cmax25) and the maximum rate of electron transport driving RuBP regeneration at 25 degrees C (J(max25)) declined significantly under eT + eC. V-cmax25 had a positive relationship with Rubisco content, and J(max25) had a positive relationship with Chl and Caro content. At the crop level, eC enhanced biomass but reduced N distribution in leaves. Furthermore, the decrease in biomass had a greater effect than the increase in TLN under eT, reducing N distribution in leaves. Photosynthetic acclimation was mainly due to the reduction in TLN and crop N distribution and the increased reduction in leaf N distribution to Rubisco under eT + eC. Therefore, an air temperature increase of approximately 1 degrees C exacerbated photosynthetic acclimation under eC. These results further elucidated the photosynthesis responses in rice to future climate conditions.

Automated summary

Increasing leaf nitrogen content may not offset the reduction in photosynthetic acclimation under high CO2 concentrations and elevated temperatures in rice. The effects of elevated CO2 and air temperature on chlorophyll and carotenoid content were similar to the changes in total leaf nitrogen content.