Auxin action on growth in intact plants: Threshold turgor is regulated

The guillotine thermocouple psychrometer allows auxin action on cell enlargement to be investigated in intact plants. Because the technique measures all the physical parameters affecting enlargement in the same plants, close comparisons can be made of the changes brought about by this growth regulator. In etiolated seedlings of soybean (Glycine max L. Merr.), auxin was supplied endogenously by the intact plant or was depleted by removing the apical portion of the stem. We observed that, when stem growth was rapid in the intact plant, the water potential of the growing region was lower than in the nongrowing region but, as growth slowed during auxin depletion, the water potential rose until it became essentially the same as in the nongrowing region. This indicated that gradients in water potential had been induced by the demand for water during rapid growth but had decreased as growth decreased in the auxin-depleted cells. The turgor appeared to rise slightly as growth slowed which is in the wrong direction to account for the growth change unless compensating changes occurred in wall properties and/or synthesis. As growth ceased in the auxin-depleted tissue, the threshold turgor rose until it became nearly the same as the cell turgor, which indicates that auxin affected this wall parameter. The osmotic potential increased slightly, probably because of a dilution of the cell contents by the residual growth occurring after the stem apex (and cotyledons) had been removed. The hydraulic conductance for water was unaffected by auxin status whether it was measured in the whole enlarging region or in individual cortical cells from the region. It was concluded that auxin acts mainly on the metabolism of the cell walls manifested by the change in growth rate and threshold turgor. The other changes were passive responses to the changed growth rate.Abbreviations and Symbols G relative growth rate - L conductance of tissue - Lp hydraulic conductivity of cell - m extensibility of cell walls - T threshold turgor - t_1/2 halftime for turgor relaxation - V volume of water - bulk elastic modulus - _o water potential of nongrowing tissue - (_{o w}) growth-induced water potential - _p turgor - (_p T) growth-active turgor - _s osmotic potential - _w water potential of growing tissueThis work was supported by a grant from the Science and Technology Agency of Japan to S.M. and grants from the DuPont Company and the Department of Energy DE-FG02-87ER13776 to J.S.B. We thank Dr. Douglas Miller for help with the statistics.