Influence of exogenous calcium on the physiological, biochemical, phytochemical and ionic homeostasis of tea plants (Camellia sinensis (L.) O. Kuntze) subjected to fluorine stress

Tea plant (Camellia sinensis) may hyperaccumulate fluorine (F) in its leaves, which may cause fluorosis in tea consumers. Recent studies have implied that exogenous calcium (Ca) may reduce F in tea leaves, although our mechanistic understanding of this phenomenon remains limited. Here, the effects of exogenous Ca on the physiological, biochemical and ionic homeostasis of tea leaves were investigated in the presence and absence of F. Elevated levels of malondialdehyde (MDA) and impaired cellular ultrastructure indicated that exogenous F induced stress in tea plants subjected to deficient Ca (0.01, 0.05mM) and extremely excessive Ca (10mM) treatments. Additionally, more F were accumulated in leaves compared to the control when tea plants were treated with 0.5mM Ca. The lowest levels of MDA and F were observed at an optimal level of 5mM Ca. F increased the levels of caffeine, polyphenols, and catechins, but decreased the content of soluble sugars and gallic acid when the level of Ca was within 5mM. Moreover, based on a multivariate analysis on ionic composition, the Ca-regulated disorder in the homeostasis of B, Al, Cu, and Zn was strongly correlated with the accumulation of F. Our results demonstrate that within a range of concentrations, exogenous Ca was able to reduce F content and enhance F tolerance in tea leaves. These effects of exogenous Ca on F tolerance may be related to ionic homeostasis.