Effects of foliar application of calcium nitrate on growth and physiological attributes of cowpea (Vigna unguiculata L. Walp.) grown under salt stress

Cowpea plants were grown in a glasshouse pot experiment to investigate the effects of NaCl salinity stress and foliar applications of Ca(NO3)2. The plants were subjected to the following four treatments: (1) control (nutrient solution alone), (2) 10 mmol L−1 Ca(NO3)2 as a foliar application + nutrient solution (FA + C), (3) 50 mmol L−1 NaCl + nutrient solution (NA + C), and (4) 50 mmol L−1 NaCl + 10 mmol L−1 Ca(NO3)2 as a foliar application + nutrient solution (NA + FA + C) twice weekly. The results showed that salt-stressed plants had less dry matter in the root and shoot, the concentrations of sodium and chloride in both plant organs increased, while those of Ca2+, Mg2+, and K+ decreased in the high NaCl treatment. No significant differences in stomatal conductance, transpiration, net photosynthesis, and intercellular CO2 were noted among treatments; hence, none of these variables was improved with the foliar Ca(NO3)2 sprays. However, chlorophyll fluorescence parameters of the NaCl-stressed plants had higher values following foliar Ca(NO3)2 sprays, suggesting that the spray was effective in partially alleviating adverse effects of salinity on these parameters. In conclusion, our overall results did not support the hypothesis that supplemental calcium would ameliorate the inhibitory effects of NaCl stress in cowpea plants.