A NEW FACTOR IN DROUGHT ACCLIMATION

Cabbage leaves excised from a fully turgid plant wilt within 20 minutes to 2 hours (depending on plant age) with a loss of about 10% relative water content (RWC). If droughted for 2 to 4 days in a high relative humidity leaf chamber, they may acclimate, recovering their turgor without the absorption of water, in fact at a loss of 15 to 25% RWC. This turgor recovery in the absence of water uptake occurs only if (a) the rate of water loss is slow enough (about 1-5% RWC per day after the first 24 hours drought loss of about 15% RWC), (b) if the leaves are no longer growing actively. Osmotic adjustment accompanies the turgor adjustment, but cannot be the cause in the absence of water uptake. The recovery of turgor by wilted cabbage leaves in the absence of water uptake cannot be explained by (a) transfer of reserve water from apoplast to symplast either from the cell walls or from the vessel lumens by cavitation or (b) metabolic loss of dry matter and gain of water. It can be explained by a contraction of the cell walls around the partially dehydrated protoplasts, until they regain their elastic extensibility. These proposed cell wall changes during drought acclimation are therefore the opposite of those occurring during growth. This hypothesis therefore explains the long recognized inverse relation between growth and acclimation. Two predictions of this hypothesis were tested and substantiated. It has long been known (6) that slightly wilted leaves of droughted plants are capable of recovering their turgor without an addition of water to their roots. This kind of wilting is called 'temporary wilting' and recovery of turgor normally occurs at night. If recovery does not occur unless the drought is ended by an additional supply of water to the plant, this is called 'permanent wilting'. Temporary wilting is usually explained by a more rapid loss than uptake of water, and recovery-a more rapid uptake than loss. The following investigation describes another kind of temporary wilting-wilting from which the leaf can recover its turgor without any uptake of water, in fact when accompanied by a further slow loss of water. This recovery from wilting may be called TA,2 by analogy with OA. MATERIALS AND METHODS Cabbage (Brassica oleracea var capitata. Early Jersey Wakefield) plants were grown from seed in a growth chamber (4) for 'Carnegie Institution of Washington Department of Plant Biology Publication No. 918. 2 Abbreviations: TA, turgor adjustment; RWC, relative water content; OA, osmotic adjustment; TR, turgor recovery; WA, wall adjustment. 30 to 60 d. It was previously shown (5) that mature, moderately wilted cabbage leaves absorb water extremely slowly, and remain wilted even when floated on water in a covered dish for 24 h or more. However, when a fresh cut is made, water is absorbed rapidly through this surface by the floating leaf, and full turgor is recovered within minutes. It is thus obvious that the recovery of turgor in an intact leaf cannot be followed by a daily removal of samples (e.g. leaf discs for thermocouple psychrometry). Turgor recovery was therefore followed by a simple method which distinguishes between cell walls under tension (turgid leaf) and cell walls not under tension (wilted leaf). Leaves were excised from fully turgid plants, weighed, then allowed to lose water until distinctly limp (usually within 1 h). This was determined by holding the petiole in a horizontal position. In the turgid state, the leaf blade remained stiffly horizontal, whether held right-side up or down. In the wilted state, the blade bent limply downward, whether held either way. Due to the extremely small amount of mechanical tissue (sclerenchyma and fibrovascular bundles), any rigidity of the leaf can only be due to turgor and any loss of rigidity to loss of turgor. No attempt was made to measure degrees of turgor or wilting. The wilted leaves (numbered upward, number 1 being the first true leaf) were then acclimated to drought by transfer to leaf chambers with an atmosphere near 100% RH. This was accomplished as follows. Two plastic weighing dishes (12 cm square) were lined with a double layer of wet paper towel covered by a plastic screen. One dish was then inverted and allowed to rest on the other forming a high humidity leaf chamber. Several leaf chambers were then placed in the growth chamber, exposed to the same light regime as the growing plants but to a lower internal illumination due to the paper towel and the translucent plastic dishes. Each leaf rested on top of the screen. This prevented contact between the leaf and the wet paper. The two plates of the humidity chamber were not sealed, so as to permit gas exchange. This also ensured an atmosphere near, but not quite at, 100% RH. Since the interior of the leaf is also near saturation (about 99% when just limp), water exchange between the leaf and chamber was very slow-usually a loss of about 1 to 5% RWC/d, after a 1st d loss of about 5 to 10%. Since the lid was not clamped to the bottom half, water evaporated slowly from the leaf chamber. Every 24 h when the leaves were weighed, a few ml water were added to the wet towel to replace that lost. Rate of dehydration of the leaf was controlled, within limits, by the amount of water added. The rate of water loss could be further reduced by partial clamping of the lid to the bottom with paper clips. RWC was calculated in the usual way: RWC = fresh wt -dry wt Xl00 saturated wt dry wt 147 www.plantphysiol. org on April 30, 2019 Published by Downloaded from Copyright © 1986 American Society of Plant Biologists. All rights reserved. Plant Physiol. Vol. 82, 1986 Since the pots with the growing plants stood in trays of nutrient solutions, the leaves were assumed to be saturated (100% RWC). Tests with leaves floating on distilled H20 (cut petiole surface submerged) supported this assumption, although the leaves continued a very slow uptake beyond the 100% point due, presumably, to continued slow expansion of fully turgid leaves. Osmotic potential was measured by the method of incipient plasmolysis (5). This classical method has the advantage of measuring osmotic concentration of the cell sap at the point of zero turgor, regardless of the water status (wilted or turgid) of the leaf.