The Addition of Calcium and Strontium Improves Salt Tolerance of Chinese Cabbage at the Germination Stage

Strontium has similar chemical properties to calcium and has recently been recognized as a non-essential beneficial element for plants. In order to compare the effects of strontium and calcium on improving salt tolerance of Chinese cabbage during the germination stage, 2, 4, and 8 mmol/L of SrCl2, CaCl2 or an equimolar mixture of both were added separately to a 150 mmol/L NaCl solution. The results showed that Ca-Sr addition significantly increased seed viability, seed vigor, seed germination rate and seed germination uniformity of Chinese cabbage compared with the salt-control group. The differences in germination percentage (GP) and germination energy (GE) among the Ca-addition and Sr-addition groups were not significant, and the differences in coefficient of rate of germination (CRG), index of rate of germination (IRG) and coefficient of variation of the germination time (CVT) were relatively small, but clear differences were observed in germination index (GI), vigor index (VI) and coefficient of uniformity of germination (CUG). The results of GI and VI indicated that the higher the concentration of Ca-addition or Sr-addition, the more significant the enhancement of seed vigor. Under saline stress (150 mmol/L NaCl), the Ca-Sr co-addition outperformed Sr-treatment alone, and Ca-addition achieved the highest seed vigor at equivalent concentrations. Furthermore, all Ca-Sr treatments significantly enhanced the uniformity of Chinese cabbage sprouts exposed to 150 mM NaCl, with the best uniformity improved by the addition of 2 and 4 mmol/L SrCl2. Ca-Sr treatments increased the salt tolerance of Chinese cabbage sprouts during the germination stage mainly because the Ca2+ and Sr2+ significantly enhanced plasma membrane stability and reduced oxidative stress (as indicated by decreased contents of malondialdehyde and O2·− contents) in sprouts. The decrease of soluble sugar and proline content caused by Ca-Sr addition implies that elevated levels of these osmolytes were not the primary contributors to improved seed germinability in Chinese cabbage. These findings demonstrate that Sr is a beneficial element for enhancing salt tolerance in plants, laying a theoretical foundation for the development and application of strontium in agriculture.