Evidence for the uptake of large anions through stomatal pores

Experiments were conducted with leek (Allium porrum L.) leaves to investiga te whether aqueous solutions are able to penetrate stomata. Epidermal strip s were used for the determination of transport rates. Stomata were opened b y fusicoccin or closed by darkness or abscisic acid. A droplet containing t he anionic fluorescent dye, uranine, was placed on the physiologically oute r side of the epidermis and allowed to dry. With open stomata 30 times more uranine penetrated through the epidermal strips than with closed stomata ( comparison of medians). In another experiment droplets of uranine solution were placed on leaf segments and epidermal strips were removed after drying of the droplets. Penetration of uranine through stomata was detectable und er the microscope both with epidermal strips from the transport experiments and with strips obtained after application on leaf segments. As maximum up take rates occurred during the drying process, it is concluded that penetra tion took place via water films. These results show that the physical restr ictions preventing stomatal penetration of static droplets are not decisive for drying droplets and that stomatal uptake of dissolved ionic substances occurs under natural conditions, i.e. without surfactants or applied press ure.