Recovery of turgor by wilted, excised cabbage leaves in the absence of water uptake : 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.