Turgor and Growth at Low Water Potentials

Abstract

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.Plants grow in size mostly by increasing in cell water content. The enlargement occurs because solute concentrations are high enough inside the cells to extract water osmotically from the surroundings. As a result, the pressure in the cells (turgor) rises and extends the walls irreversibly, enlarging the cell compartment.The dependency of these processes on water causes growth to respond to the supply of water in the soil. Effects are particularly apparent for organs in the shoot, which may be inhibited during vegetative or reproductive development (33) with large consequences for plants.The depletion of soil water decreases water potentials ('') in the plant and also can decrease the turgor. Effects on growth often are attributed to this turgor loss (2, 16). However, the discovery that plants adjust osmotically to low *I' (14, 23) was followed by reports that turgor was maintained, sometimes completely, while growth was inhibited (12,13,20,21,24,25,33). Ifturgor does not fall, the growth inhibition must be attributed to other factors. Several possibilities have been suggested (3,7,20,32) but doubt exists because the turgor measurements (12,13,20,21,24,25,33) involved excised tissue, sometimes from nongrowing parts of the plant.

Translatable RNA Populations Associated with Maintenance of Primary Root Elongation and Inhibition of Mesocotyl Elongation by Abscisic Acid in Maize Seedlings at Low Water Potentials

“…condition. It is possible that these events were triggered by a slight or transient reduction in turgor upon transfer of the seedlings to low t// w , perhaps in cells close to the xylem (Nonami and Boyer, 1989). In a similar situation in soybean, ABA accumulated in the hypocotyl elongation zone but with no apparent water loss (Creelman and Mullet, 1991a).…”

Abscisic Acid Accumulates at Positive Turgor Potential in Excised Soybean Seedling Growing Zones

“…Previously, increased accumulation of ABA has been correlated with turgor potential dropping to zero (6,18). However, this mechanism is unlikely to affect ABA levels in soybean growing zones since turgor does not decrease to zero in these regions when seedlings are grown in low water potential vermiculite (14). To understand the origin of ABA in growing zones, we determined the capacity of growing and nongrowing sections to accumulate ABA (Table I).…”

“…For example, flooding (10), fruit removal or stem girdling (1 1, 20), long photoperiod (22), ' 29'C and 100% RH as previously described (2,5,12,14). The following seedling sections were defined: hook and the zone of elongation the first 5 mm and next 15 mm along the hypocotyl below the cotyledon with the mature region of the hypocotyl the remaining portion of the hypocotyl.…”

Gradients of Turgor, Osmotic Pressure, and Water Potential in the Cortex of the Hypocotyl of Growing Ricinus Seedlings

“…The question is tied up with the question of the existence of growth-induced water potential gradients. The latter should occur ifthe mechanical relaxation of cell walls during growth is sufficiently rapid so that the water supply could limit growth (1)(2)(3)(18)(19)(20)(21). The concept of growth-induced water potential gradients has a sound physical basis (19).…”

“…In recent years, the biophysics of extension growth has been extensively discussed in terms oflimitations by water transport or by mechanical properties of the cell walls. Boyer and coworkers (1)(2)(3)(18)(19)(20)(21) have proposed some limitation or colimitation of growth by hydraulic resistances. Cosgrove and coworkers (4)(5)(6) have rejected this idea and have proposed that extension should be governed by mechanical restrictions.…”