Effects of salt stress and exogenous Ca2+ on Na+ compartmentalization, ion pump activities of tonoplast and plasma membrane in Nitraria tangutorum Bobr. leaves

Nitraria tangutorum Bobr. is a typical halophyte with superior tolerance to salinity. However, little is known about its physiological adaptation mechanisms to the salt environment. In the present study, N. tangutorum seedlings were treated with different concentrations of NaCl (100, 200, 300 and 400 mmol L-1) combined with five levels of Ca2+ (0, 5, 10, 15 and 20 mmol L-1) to investigate the effects of salt stress and exogenous Ca2+ on Na+ compartmentalization and ion pump activities of tonoplast and plasma membrane (PM) in leaves. Na+ and Ca2+ treatments increased the fresh weight and dry weight of N. tangutorum seedlings. The absorption of Na+ in roots, stems and leaves was substantially increased with the increases of NaCl concentration, and Na+ was mainly accumulated in leaves. Exogenous Ca2+ reduced Na+ accumulation in roots but promoted Na+ accumulation in leaves. The absorption and transportation of Ca2+ in N. tangutorum seedlings were inhibited under NaCl treatments. Exogenous Ca2+ promoted Ca2+ accumulation in the plant. Na+ contents in apoplast and symplast of leaves were also significantly increased, and symplast was the main part of Na+ intracellular compartmentalization. The tonoplast H+-ATPase and H+-PPase activities were significantly promoted under salt stress (NaCl concentrations a parts per thousand currency sign300 mmol L-1). PM H+-ATPase activities gradually increased under salt stress (NaCl concentrations a parts per thousand currency sign200 mmol L-1) followed by decreases with NaCl concentration increasing. The tonoplast H+-ATPase, H+-PPase and PM H+-ATPase activities increased first with the increasing exogenous Ca2+ concentration, reached the maximums at 15 mmol L-1 Ca2+, and then decreased. The tonoplast and PM Ca2+-ATPase activities showed increasing trends with the increases of NaCl and Ca2+ concentration. These results suggested that certain concentrations of exogenous Ca2+ effectively enhanced ion pump activities of tonoplast and PM as well as promoted the intracellular Na+ compartmentalization to improve the salt tolerance of N. tangutorum.