Auxin-Induced Water Uptake by Avena Coleoptile Sections.

Abstract

It is known that auxin induces the uptake of water by plant tissues. Three principal suggestions have been made concerning the mechanism of such auxininduced net water uptake. The first proposes that auxin in some manner plasticizes the cell wall. The second suggests that auxin brings about active synthesis of cell wall material. These two mechanisms suppose osmotic entry of water into the cell in response to lowered wall pressure. The third is that auxin brings about a non-osmotic transport of water into the tissue. Thimann (22) has grouped these into two hypotheses in the form of models. One model visualizes a pump (active transport) and the other a piston arrangement (cell wall pressure reduction). Such active transport has been rigorously defined by Rosenberg (21) as the movement of a substance against a chemical potential or concentration gradient, i.e., an uptake of water against a diffusion gradient as defined by Levitt (16). Active transport of water would consist then of water movement against an osmotic gradient. A fourth possibility in principle would be that an auxin-induced increase in the concentration of osmotically active solutes might occur within the cell. The cells of plant tissues are surrounded by relatively rigid cell walls and one must discuss water movement in terms of diffusion pressure gradients rather than in terms of osmotic pressure. In the terminology proposed by Meyer and Anderson (19) and by Crafts et al (10), DPD = OP-WP where DPD = diffusion pressure deficit, OP = osmotic pressure of cell contents, and WP = wall pressure. The DPD gradient from an external solution to the inside of the tissue can then be expressed as ? DPD = DPDj -DPDe(DPDe = OPt?) where i= internal and e = external. If ? DPD is negative, the gradient is outward; if ? DPI) is positive, the gradient is inward. To demonstrate a net active water uptake it is necessary to show that water uptake occurs while the purely osmotic ? DPD is negative. When the external solution is hypertonic, wall pressure becomes zero and DPDi = OPi. The net uptake of water from hypertonic solution would then involve movement against an osmotic gradient. It will be shown below that cells of Avena coleoptile sections are essentially in diffusion pressure equilibrium with the external solution over a wide range of external solute concentrations both in the presence and in the absence of auxin. There is no detectable net uptake of water from hypertonic solution by this tissue. It is concluded that water uptake by Avena coleoptile sections both in the presence and absence of auxin is a purely osmotic phenomenon. Materials and Methods