Increase in assimilatory nitrate reduction and photorespiration enhances CO2 assimilation under high light-induced photoinhibition in cotton

The mechanisms by which nitrate assimilation might favour photorespiration and how these pathways could cooperatively improve photosynthesis are still little understood, especially under excess light. Here, we tested the hypothesis that increased assimilatory nitrate reduction is able to stimulate photorespiration and these two processes together are capable to enhance photosynthesis in presence of high light. Cotton plants were supplied with 10 mM NO3- (high nitrate supply-HN) or NO3-deprivation for short-term and subsequently exposed to high light-HL or low light-LL. HL induced photoinhibition in both N-treatments but strongly enhanced nitrate assimilation and photorespiration rates in HN. Despite these apparent constraints caused by HL on PSII integrity, HN-plants displayed higher electron flux through PSII, in parallel to enhancement in CO2 assimilation, which was positively related to the photosynthetic nitrogen utilization efficiency. These plants also exhibited increase in nitrate reductase and glutamine synthetase activities, which were associated with large accumulation of free amino acids and ammonia, in parallel to high NO3- reduction rates. The increase in photorespiration was greatly dependent on light intensity and, in a minor extent, on nitrate supply. The changes in the alternative electron sink strength towards nitrate assimilation and photorespiration were corroborated by decrease in the acceptor side limitation of PSI. In addition, the correlation study evidenced that CO2 assimilation, nitrate reductase activity, and photorespiration rates were greatly correlated. Thus, our data clearly show that nitrate assimilatory reduction and photorespiration are mutually up regulated by high light, favoring CO2 assimilation and photochemical activity under a photoinhibitory condition.