POTASSIUM-LOSS FROM STOMATAL GUARD-CELLS AT LOW WATER POTENTIALS

The potassium content of guard cells and the resistance to viscous flow of air through the leaf were determined in sunflower (Helianthus annuus) subjected to low leaf water potentials under illuminated conditions. In intact plants desiccated slowly by withholding water from the soil, large losses in guard cell K occurred as leaf water potentials decreased. Leaf viscous resistance increased, indicating stomatal closure. Similar results were obtained when detached leaf segments were desiccated rapidly. Upon rehydration of leaves, no stomatal opening was observed initially, despite leaf water potentials at predesiccated levels. After several hours, however, re-entry of K occurred and stomata became fully open. Turgid leaf segments floated on an ABA solution showed losses of guard cell K and closure of stomata as rapidly and completely as those brought about by desiccation. It is concluded that stomatal closure at low water potentials under illuminated conditions is not controlled solely by water loss from the tissue but involves the loss of osmoticum from the guard cells as well. This in turn decreases the turgor difference between the guard cells and the surrounding cells, and closing occurs. © 1979 Oxford University Press.