Exogenous Calcium Enhances Cold Tolerance in Satsuma Mandarin (Citrus unshiu cv. Miyagawa)

Abstract

This study was conducted to examine the effects of exogenous calcium spray on enhancing cold tolerance of Miyagawa Satsuma mandarin during winter and the possibility of practical uses of exogenous calcium in open-field. Malondialdehyde (MDA) content, an indicative for oxidative damage in cell membrane, and soluble sugar content, highly correlated with cold tolerance enhancement, increased in early February but decreased in late February. Especially, the differences of the accumulation were substantially large in soluble sugar content. The change of expressions of the most cold related genes did not show a distinct trend, although the expressions of O71 and O8 genes, down-regulated in cold acclimated Satsuma mandarin in previous research, were considerably lower in early February than in early December. Calcium content slightly declined during winter. In order to determine the effective calcium concentration, leaves attached to branch were treated with 0, 2, 20, and 200 mM Ca(NO3)2 in the laboratory. At 24 h after treatment, LT50 was lowest in leaves treated with 20 mM Ca(NO3)2, but there was no significant difference in exothermal point among the different Ca(NO3)2 concentrations. The leaves were treated with 20 mM Ca(NO3)2 in the laboratory and then treated at 7oC for 6 h to simulate the condition in Seogwipo. Exogenous calcium treatment decreased MDA content, but increased glucose, fructose, and calcium contents. Exogenous calcium treatment increased O71 expression, but did not influence the other gene expression. To examine whether exogenous calcium treatment would be practical in open-field, Satsuma mandarin trees were sprayed with 20 mM Ca(NO3)2 till running off weekly up to four times from January. After trees were exposed to cold, exogenous calcium treatment decreased MDA content in leaves at 2 and 4 weeks after the last calcium treatment. However, exogenous calcium treatment had no effect on soluble sugar and calcium contents. Exogenous calcium treatment increased O71 expression at 2 weeks after the last treatment, but did not influence the other gene expression at 4 weeks after the treatment. These results indicated that exogenous calcium treatment enhanced cold tolerance by decreasing LT50 and cell membrane injury, and by increasing soluble sugar content. However, expressions of many cold related genes were not significantly affected. Further investigation is needed on genes related with lipid degradation in the membranes and calcium signaling. Detailed and effective calcium uses in open-field are also needed to be determined.