Turgor and growth at low water potentials.

Turgor affects cell enlargement but has not been measured in enlarging tissue of intact plants when growth is inhibited by inadequate water. Mature or excised tissue can be problematic for these measurements because turgor may not be the same as in intact enlarging cells. Therefore, we measured the average turgor in the elongating region of intact stems of soybean (Glycine max [L.] Merr.) while the seedlings were exposed to low water potentials by transplanting to vermiculite of low water content. Stem growth was completely inhibited by the transplanting, and the average turgor decreased in the mature stem tissue. However, it did not decrease in the elongating region whether measured in intact or excised tissue (total of four methods). At the cellular level, turgor was uniform in the elongating tissue except at transplanting, when turgor decreased in a small number of cortical cells near the xylem. The reduced turgor in these cells, but constant turgor in most of the cells, confirmed that no general turgor loss had occurred but indicated that gradients in water potential extending from the xylem into the enlarging tissue were reduced, thus decreasing the movement of water into the tissue for cell enlargement. A modest growth recovery occurred after 2 days and was preceded by a recovery of the gradient. This suggests that under these conditions, growth initially was inhibited not by turgor loss but by a collapse of the water potential gradient necessary for the growth process.