Role of Guard Cells in Foliar Absorption

Abstract

MANY experiments on foliar absorption have shown that after solutions have been sprayed on the surface of leaves then the upper leaf-blades, which are poor or entirely lacking in guard cells, absorb smaller quantities of solution than the abaxial leaf-blades, which are rich in guard cells^1,2. Although foliar absorption cannot occur exclusively through guard cells, since surfaces lacking them also absorb applied substances, it seemed clear that guard cells must play a special part in foliar absorption. Several authors supposed that the stomatal pores were the places for entrance of solutes sprayed on to the cuticle. But the stomatal pores are continuations of the air-filled intercellular system of plant organs and normally function for gas exchange only³. Furthermore, all external and internal surfaces of aerial plant organs which border on air-filled spaces are covered with a layer of lipoidal substances which form a cuticle. These lipoidal substances prevent wetting by hydrophilic solutions. Thus only lipophilic solutes such as oils can penetrate the pores and reach the intercellular spaces⁴. Under normal conditions, that is, without adding detergents to spray solutions, watery liquids, therefore, cannot be absorbed through the stomatal pores.