Effects Of Exogenous Ca²⁺ On Photosynthesis And Nitrogen Metabolism Of Nitraria Tangutorum Bobr.Under Salt Stress

Nitraria tangutorum Bobr. is a shrub belonging to the Nitraria genus in Nitrariaceae. It possesses superior tolerance to severe drought, high salinity and alkalinity. It is an important edificator in desert and semi-desert and can be used as exceptional plants to improve saline soil. N. tangutorum was used to study the effects of different concentrations exogenous calcium on the gas exchange parameters, chlorophyll content and chlorophyll fluorescence parameters of photosynthesis during salt stress. Interference of nitrogen metabolism of N. tangutorum with different concentrations exogenous calcium under salt stress was discussed. The results were showed as follows:（1） With the increase of Na Cl concentration, leaf relative water content was fluctuated and chlorophyll content increased first and then decreased. Chlorophyll content declined sharply when NaCl concentration was higher than 300 mmol?L^-1. Applying appropriate calcium(≤15 mmol?L^-1) was able to increase the leaf relative water content and chlorophyll content in the treatment group which NaCl concentration did not exceed 300 mmol?L^-1. The results suggested that calcium with certain concentration was able to relieve shortage of water and increase chlorophyll content in leaves. Guaranteed the vitality of N. tangutorum in a higher level. But the alleviating effect was not obvious under high calcium concentration(>15 mmol?L^-1), even showed inhibition to plant.（2） Gas exchange parameters of N. tangutorum was influenced by salt stress. Net photosynthetic rate, stomatal conductance and transpiration rate increased, but intercellular carbon dioxide concentration declined when NaCl concentration was higher than 200 mmol?L^-1. Transpiration rate, the maximum photochemical efficiency and photochemical quenching increased first and then decreased, non-photochemical quenching decreased first and then increased with the increase of salt concentration. Chlorophyll fluorescence parameters was much influenced when Na Cl concentration was higher than 300 mmol?L^-1. The results showed that gas exchange of N. tangutorum reduced when Na Cl concentration was overstepped. Utilization of carbon dioxide and electronic transfer of PSII declined, more light energy transferred into heat energy, photosynthetic carbon cycle route was damaged under salt stress.（3） Calcium with certain concentration was able to promote stomatal opening and increased photochemical efficiency. Applying appropriate calcium was able to increase Net photosynthetic rate, stomatal conductance and transpiration rate increased, while intercellular carbon dioxide concentration declined. It has the best efficiency when calcium concentration was 15 mmol?L^-1. The alleviating effect was not obvious under high calcium concentration(>15 mmol?L^-1). Moreover, the maximum photochemical efficiency, real photochemical efficiency and photochemical quenching increased, non-photochemical quenching decreased when calcium concentration was 15 mmol?L^-1. The results showed that calcium with certain concentration was able to promote electronic transfer of PSII, increase the capability of heat dissipation and photochemical efficiency, mitigate photoinhibition caused by salt stress.（4） Nitrogen metabolism was influenced by the increase of Na Cl concentration. NO3--N content, nitrate reductase activity and glutamine synthetase activity were gradually decreased, while NH₄⁺-N content and soluble protein content were increased under salt stress. It is because that salt stress with high concentration was able to decrease utilization and assimilation on NO3--N of N. tangutorum, reduced nitrate reductase and glutamine synthetase activities, produced toxic effects on plants. NO3--N content, nitrate reductase activity and glutamine synthetase activity increased first and then decreased, while NH₄⁺-N content was opposite with the increase of calcium concentration. The results suggested that calcium was able to promote enzyme activity of nitrogen metabolism, accelerated utilization and assimilation on NO3--N, decreased NH₄⁺-N content in plants and kept nitrogen metabolism away from injury of salt stress.