Auxin changes both the extensibility and the yield threshold of the cell wall of Vigna hypocotyls

Elongation of plant stem is governed by two simultaneous processes: irreversible yielding of the cell wall and uptake of water. Among many candidates for the parameters that regulate and/ or restrict growth, we focused on the mechanical properties of the cell wall and determined those parameters that govern the process of IAA-induced growth by means of the pressure-jump method combined with the pressure-probe technique. The elongation growth of segments excised from the elongation zone of Vigna hypocotyls was accelerated by xylem perfusion with 10−4 M IAA. During the promotion of growth, both the extensibility (Φ) of the cell wall and the effective turgor (Pi–Y) increased while only a little or no change in the intracellular pressure (Pi) occurred. These results indicate that IAA increases not only the extensibility of the cell wall but also the effective turgor, i.e., the driving force for yielding of the cell wall. However, the driving force is not increased by the increase in Pi but by the decrease in the yield threshold (Y). These results suggest that Y is adjustable during the regulation of growth.