STOMATAL RESPONSES TO SODIUM-IONS IN ASTER-TRIPOLIUM - A NEW HYPOTHESIS TO EXPLAIN SALINITY REGULATION IN ABOVEGROUND TISSUES

A study has been made of the ionic relations of stomata of Aster tripo lium L., a maritime halophyte which colonizes coastal saltmarshes. The results obtained allow us to add this species to the growing list for which an involvement of KC transport in stomatal movements has been d emonstrated. However, an additional and ecologically important charact eristic was found: there was a suppression of stomatal opening by incr easing NaCl concentrations. A new hypothesis is offered of the mechani sm for controlling salt and water relations in A. tripolium, a species which does not possess glands or other means of excreting salt. It is suggested that when the capacity of the tissues to accumulate salt in cell vacuoles is exceeded, the concentration of Na+ ions in the apopl ast around the guard cells begins to rise. This causes partial stomata l closure, reduces transpiration and increases water-use-efficiency Th erefore, the flow of salt into the leaves is reduced but growth (and t he manufacture of the new photosynthates required to support it) can c ontinue. Aster tripolium can be added to the small list of known speci es which readily yield isolated epidermis suitable for detailed stomat al studies. Throughout this study, we have compared its stomatal physi ology with C. communis, which has been thoroughly investigated in the past.