Ameliorating effects of exogenous calcium on the photosynthetic physiology of honeysuckle (Lonicera japonica) under salt stress

Calcium (Ca2+) plays pivotal roles in modulating plant growth, development and stress responses. This work was conducted to study the effects of 20 mM calcium on the biomass, malondialdehyde content, chlorophyll content, ion ratio, chlorophyll a fluorescence and gas-exchange parameters, gene expression of annual honeysuckle under 50, 100 and 200 mM NaCl. At the end of treatment, Na+ concentration was increased with the mounting salinity, but a higher ratio of K+/Na2+, Ca2+/Na+, Mg2+/Na+ were obtained after calcium addition. Salinity exerted an adverse effect on the dry weights and chlorophyll content, whereas CaCl2 played a positive role. Consistent with biomass reduction, the photosynthetic rate and stomatal conductance declined in leaves of honeysuckle exposed to elevated salinity. However, the extent of reduction was much less under CaCl2 combination treatments than one caused by NaCl treatments. Exogenous calcium also protects the photochemical activity of PSII by protecting reaction centre from inactivation and maintaining electron transport from QA– to QB–. Further, exogenous calcium promoted the overexpression of LHCB coding gene Cab and Rubisco large subunit coding gene rbcL under short-term stress. In conclusion, exogenous calcium was effective in improving the salt tolerance of honeysuckle in the photosynthetic base, thereby improving the growth of plants.