A strategy of Ca2+ alleviating Na+ toxicity in salt-treated Cyclocarya paliurus seedlings: photosynthetic and nutritional responses

Abstract

Cyclocarya paliurus seedlings were subjected to 85 mM NaCl and 0, 6, 12 or 18 mM Ca(NO3)2 treatments to study changes in plant growth, photosynthetic parameters and distribution and/or accumulation of organic and inorganic solutes. Na+ toxicity symptoms were observed in plants non-treated with Ca(NO3)2, while 12 mM Ca(NO3)2 supplementation produced a significant promotion of shoot growth; meanwhile chlorophyll content, photosynthetic rate and optimum quantum yield of photosystem II (PSII), represented by the Fv/Fm ratio and pigments content as well as proline and soluble sugars, significantly increased. Ca(NO3)2 supply increased K+ and Ca2+ concentration, whereas the Na+ transport to the shoot was inhibited. There was a strong increase in the K+/Na+ ratio in shoot of Ca(NO3)2-treated plants. X-Ray microanalysis of roots showed that K+, Ca2+ and Na+ accumulated mainly in the epidermal cells and cortical cells of roots with 12 mM Ca(NO3)2 supply, and low accumulation was observed in stelar parenchyma, indicating exogenous Ca2+ possibly induced or strengthened effects of Casparian bands on ion transport. These results suggest that Ca(NO3)2 supplement increased inorganic and organic solutes accumulation in shoot and leaf, and restricted Na+ transport to the shoot by reinforcing barrier effects for attenuating salt injuries in plants, which could be a strategy of Ca2+ alleviating Na+ toxicity in C. paliurus seedlings subjected to salt stress.