Responses of the stomata of Aster tripolium to calcium and sodium ions in relation to salinity tolerance

In previous work, the stomata of the maritime halophyte Astertripolium L. were shown to close when NaCl concentrations risein the vicinity of the guard cells. Further studies have nowrevealed important effects of calcium on the ionic responsesof the stomata. When the guard cells were presented with KCl,Ca²⁺ suppressed opening in a manner similar to that which hasbecome familiar in other species such as Commelina communisL. However, in the presence of NaCl, Ca²⁺ had the opposite effect,reducing the closing response to NaCl. This pattern of behaviouris discussed in relation to known salt effects on membranes,but the underlying physiological basis remains obscure. A previous study led to the hypothesis that the closing responseof the stomata to Na⁺ ions may make an important contributionto the salinity tolerance of this species. Here we report thatincreasing supplies of Ca²⁺ ions reduce the effect of salinityon stomatal conductance in the whole plant as well as in theisolated epidermis. This finding is consistent with the wellestablished role of calcium in increasing resistance to salinity:in the presence of high calcium the plant can tolerate a greatersalt intake, and hence there is a reduced need for transpirationto be restricted by partial stomatal closure. Key words: Sodium, calcium, Aster tripolium, stomata, salinity tolerance