THE ROLE OF ACTIVE WATER ABSORPTION IN AUXIN-INDUCED WATER UPTAKE BY AERATED POTATO DISCS

Abstract

In recent years, the concept of active water absorption has been utilized to explain a variety of experimental results. Levitt (2) has shown that some of these explanations are thermodynamically impossible and based on incorrect interpretations. The auxin-induced water uptake by potato discs has also been attributed to active water absorption [Reinders (3), Commoner et al. (5)]. In favor of this interpretation is Reinders' discovery that the auxin usually very markedly increases the respiratory loss of dry matter (3, 4). However, she also obtained the opposite result under certain conditions, van Overbeek (7) concluded that the evidence available is insufficient to permit a choice between active water absorption, reduced wall pressure, or both. Obviously, both the concept of water absorption and the mechanism of auxin action would be better understood if this problem were cleared up. As van Overbefk (V) has pointed out, since the auxin-induced water uptake can occur from distilled water, it is not associated with active salt absorption. The only active absorption possible under these conditions is that of water. By definition, active absorption involves uptake against a diffusion gradient. The diffusion pressure of the water in the cell must therefore be greater than that of the almost pure water outside the cell?i.e., the D.P.D. must have a negative value. This can be true only if the turgor pressure (or wall pressure) is increased to a value greater than the osmotic pressure of the cell. But the absorption of water by the cell will be associated with an increased T.P. (or W.P.) only if the wall has not previously reached its elastic limit. Thus an elastic (reversible) stretch involves an increase in T.P. and W.P., a plastic (irreversible) stretch, does not. Consequently, from the point of view of the kind of wall extension involved, there are two possible modes of active water absorption. (1) The absorption may occur against a diffusion gradient accompanied by an elastic stretch of the cell wall, both being reversible. Energy must be continually expended to retain this actively absorbed water in the cell. (2) The absorption may at first occur against a diffusion gradient, accompanied by a temporary elastic stretch of the wall. This elastic stretch is soon converted into a permanent set?i.e., it becomes a plastic stretch. The increased wall pressure simultaneously disappears, as does the negative diffusion gradient, and the actively absorbed water is now retained in diffusion equilibrium with the surrounding water. No energy need be expended to retain this water but some must be used initially to drive it into the cell.