Nitrogen use efficiency, photosynthesis and photorespiration in cotton plants exposed to excess energy and N-deprivation

Abstract

Adverse environmental conditions might cause damage to plants whose severity also depends on the inherent characteristics of the plant. High light and nitrogen oscillation in soil are one of the most important abiotic stresses that plants often have to deal with. Therefore, to understand how plants efficiently manage the nitrogen to minimize losses in photosynthesis and productivity under excess light and N fluctuation is crucial to crop production. First, a research was conducted to test 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 in hydroponic medium were supplied with 10 mM NO3- (high nitrate supply-HN) or NO3- - deprivation (ND) 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 constraints caused on PSII integrity, HN-plants displayed higher electron flux through PSII and enhancement in CO2 assimilation, which was positively related to the photosynthetic nitrogen use efficiency (PNUE). These plants also exhibited increase in nitrate reductase (NR) 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. These results have raised another question: How do HN plants followed by N deprivation efficiently use nitrogen in order to preserve photosynthesis? To solver this issue a second research was conducted on which cotton plants were exposed to three conditions in nutrient solution: (a) previous exposure to high nitrate supply (10 mM) for long-term (8 days); (b) nitrate deprivation (NO3- withdrawal) for four days followed by (c) an early N-deficiency during four days. Plants supplied with nitrate excess were able to display increment in shoot nitrogen use efficiency (NUE), whereas PNUE did not change, evidencing that excess N was not able to additionally improve CO2 assimilation. NR activity was crucial to remobilize stored nitrate through deprivation phase, whereas free amino acids, total proteins, and chlorophylls were essential to N-remobilization. Despite the great decrease in chlorophyll content, PSII and PSI activities were kept stable until the onset of early N-deficiency. In conclusion under high light condition the high nitrate supply stimulates nitrate assimilation, which induces photorespiration and these two processes act synergistically favouring the photosynthetic efficiency, mainly improving PSII efficiency and CO2 assimilation. On the other hand, nitrate deprivation plants in non-photoinhibitory conditions present high NUE associated with NR activity and N remobilization to maintain the photosystems integrity even in the initial nitrogen deficiency.