Malate-switch hypothesis to explain the action of stomata

Abstract

DURING the past ten years major advances have been made in our understanding of the mechanics of stomata. A most important discovery has been that potassium enters the guard cells in sufficient quantities to account for the decreases in osmotic potential which occur on stomatal opening¹. Association of potassium with stomatal movements has now been reported for at least 22 species². There is doubt, however, about the nature of the anions which accompany K⁺. Chloride has been found to balance about 40% of the K⁺ in Zea mays³ and considerably less in Commelina communis⁴. Malate could be the balancing anion as it has been found to accumulate in the guard cells on stomatal opening⁵ although it is not clear whether it is synthesised in the guard cells or is transported there from the surrounding cells. Clearly an understanding of the mechanism of the ion transport in and out of the guard cells is necessary as a first step to the unravelling of the stomatal mechanism. An hypothesis is proposed here which attempts to explain the transport aspect of stomatal function. It is based mainly on data obtained from studies with the monocotyledon C. communis but should be applicable to other species.