Rapid Changes in Transpiration in Plants

Abstract

PROF. HEATH'S interpretation of my results is based on a hypothesis which, according to my experiments, is no longer valid. He assumes that water transport in the plant is mediated by a cell-to-cell mechanism and that it is the suction potential of the mesophyll cells which determines the rate of water movement through the plant. I imagine water transport from the root to the atmosphere in easily movable columns in the xylem and cell walls. The rate of transport then depends on the gradient of suction potential from the root medium to atmosphere and the permeability or resistance to water movement of the intervening stages. The cells of the plant obtain their water from the cell wall reservoir. The uptake of water does not depend on the suction potential of the mesophyll cells but on that of the air in the intercellular spaces, which is a function of that in the ambient air and the stomatal resistance. However, as the water content of both the mesophyll cells and the guard cells, at equilibrium, is determined by the cell-wall water potential (referred to earlier¹ as the cell-wall water deficit), the impression is obtained that the guard cells are regulated by the mesophyll cells. When equilibrium does not occur, as can happen when a stress is suddenly released, the water content of the guard cells and the mesophyll cells may vary independently, if they have different permeabilities. The results which have been obtained experimentally agree with the supposition that the guard cells are more permeable than the mesophyll cells.