Apoplastic Transport in the Leaf Epidermis in Relation to Stomatal Activity

Abstract

Summary Potassium selective micro-electrodes have been used to follow changes in K+ activity in the apoplast of the epidermis of leaves of Commelina communis. When the stomata opened after the leaves had been transferred to light it was observed that potassium activity in the apoplast of the guard cell fell rapidly. This was followed by a further rise and fall in activity which appeared to be due to potassium coming from the surrounding epidermal cells. Stomatal closure resulted in a rise in potassium activity in the guard cell apoplast followed by a decline later. These results indicated that a large amount of potassium is transported between the guard cells and the epidermal cells by way of the apoplast during stomatal movements. Experiments were carried out which showed that epidermal cells released potassium in acid, but not alkaline, conditions. This was considered to be evidence in favour of the hypothesis that after initiation of stomatal opening there is a progressive acidification of the apoplast due to proton extrusion by the guard cells. This results in a release by the epidermal cells of potassium which diffuses in the apoplast to the guard cells where it is accumulated. The reverse process is assumed to occur on stomatal closure.