Increased cucumber salt tolerance by grafting on pumpkin rootstock and after application of calcium

Self-grafted and pumpkin rootstock-grafted cucumber plants were subjected to the following four treatments: 1) aerated nutrient solution alone (control), 2) nutrient solution with 10 mM Ca(NO3)2 (Ca), 3) nutrient solution with 90 mM NaCl (NaCl), and 4) nutrient solution with 90 mM NaCl + 10 mM Ca(NO3)2 (NaCl+Ca). The NaCl treatment decreased the plant dry mass and content of Ca2+ and K+ but increased the Na+ content in roots and shoots. Smaller changes were observed in pumpkin rootstock-grafted plants. Supplementary Ca(NO3)2 ameliorated the negative effects of NaCl on plant dry mass, relative growth rate (RGR), as well as Ca2+, K+, and Na+ content especially for pumpkin rootstock-grafted plants. Supplementary Ca(NO3)2 distinctly stimulated the plasma membrane (PM) H+-ATPase activity which supplies the energy to remove excess Na+ from the cells. The expressions of gene encoding PM H+-ATPases (PMA) and gene encoding a PM Na+/H+ antiporter (SOS1) were up-regulated when Ca(NO3)2 was applied. The pumpkin rootstock-grafted plants had higher PM H+-ATPase activity as well as higher PMA and SOS1 expressions than the self-grafted plants under NaCl + Ca treatment. Therefore, the addition of Ca2+ in combination with pumpkin rootstock grafting is a powerful way to increase cucumber salt tolerance.