THE PHYSICAL-CHEMISTRY OF THE PRIMARY-CELL WALL - IMPLICATIONS FOR THE CONTROL OF EXPANSION RATE

The main macroscopic view of the mechanical properties of the elongati ng plant cell wall (the Lockhart model) embraces the notions of yield threshold (Y) and extensibility (Phi). The model would be especially p owerful if Y and Phi were parameters. Often, however, they behave more like variables that tend to maintain elongation rate constant despite changes in turgor. To understand the variable nature of Y and Phi req uires the macroscopic model to be augmented with a molecular one. This paper describes a molecular model of the expanding wall that mimics t he variable nature of Y and Phi. The main postulates are (1) that ther e are two functionally disparate populations of hemicellulose molecule s that tie the cellulose microfibrils of the wall together: those that are taut and load-bearing, and those that are slack and not load-bear ing; and (2) that there are enzymes that cleave or loosen the load-bea ring molecules. It is supposed that during the stretching of the wall slack molecules become taut and are recruited to the load-bearing cont ingent, thereby stiffening the wall. This stiffening is undone by enzy mes that loosen load-bearing molecules. The net result is that changes in turgor alter the distribution of the hemicellulose molecules betwe en the slack and taut populations without necessarily altering the rat es of recruitment from one to the other, except transiently. The expan sion rate depends on the frequency at which load-bearing molecules are loosened multiplied by the amount of expansion that occurs before the next load-bearing molecule is recruited. Effects of changing water st atus on expansion rate may be mediated not necessarily through turgor, but possibly through the hydration of the wall, which may shrink at l ow water potential, thereby inactivating enzyme molecules by restricti ng their freedom of movement.