High supply of NO3 - mitigates salinity effects through an enhancement in the efficiency of photosystem II and CO2 assimilation in Jatropha curcas plants

This study was performed to determine if a high supply of N-NO3 (-) is capable of mitigating negative salinity effects on photosynthesis and growth through the stimulation of nitrate assimilation, which could act as an sink from photosynthetic electron transport chain and restrict the over reduction in thylakoid membrane in Jatropha curcas leaves. The experiment was arranged in a factorial design with two nitrate concentrations (1 and 10 mM) and two NaCl levels (0 and 100 mM). Salt-stressed plants supplied with high NO3 (-) demonstrated a higher nitrate uptake rate, nitrate reductase activity and soluble-protein content when compared with plants that presented low nitrate uptake. High nitrate assimilation was associated with higher leaf growth, CO2 assimilation and lower membrane damage in salt-stressed plants. The superior performance of salt-stressed plants grown with high NO3 (-) was indicated by a higher effective quantum yield of PSII and electron transport rate and lower energy excess at the PSII level and non-photochemical quenching. Interestingly, a high NO3 (-) level in the absence of NaCl did not alter the leaf growth, photochemical activity and gas exchange parameters when compared with plants supplied with low nitrate. The proline and glycinebetaine contents were similarly increased in both low- and high-NO3 (-) salt-stressed plants. Our data suggest that the favorable effects induced by high nitrate supply were possibly associated with stimulation in the nitrate assimilatory pathway. This process might have acted as a sink of electrons from the thylakoid membranes minimizing photo-damage and stimulating CO2 assimilation under salinity in J. curcas.