EFFECTS OF IN VITRO-FORMED ROOTS AND ACCLIMATIZATION ON WATER STATUS AND GAS-EXCHANGE OF TISSUE-CULTURED APPLE SHOOTS

Little is known about the physiological changes that occur during accl imatization and how these changes influence plant survival and growth in the new environment, In particular, it is unclear to what extent in vitro-formed roots are functional in water uptake, particularly when the plantlet is exposed to conditions of increasing evaporative demand , Tissue-cultured shoots and plantlets (shoots with roots) were acclim atized by exposing them to a linear reduction in relative humidity (RH ) from 99% to 75% over 4 days, When conductance was measured at 95% RH (21C), in vitro shoots and plantlets showed a very high initial condu ctance, followed by a gradual decline, reaching steady state in 12 hou rs, Acclimatized shoots and plantlets had a 50% lower initial conducta nce compared to nonacclimatized ones, and reached steady state in 4 ho urs, The reduction in conductance as a result of acclimatization most likely contributes to a reduced transpiration under conditions of incr eased evaporative demand. Roots formed in vitro were associated with a higher plant water status, suggesting that these roots were functiona l in water uptake. Relative mater content of the shoot was positively correlated with leaf conductance and net photosynthesis, We suggest th at tissue-cultured plantlets behave as hydraulically integrated units, in which there must be a coordination between control of water loss b y the shoot and uptake of water by the root to maintain a favorable pl ant water balance, Our results also indicate that methods that use exc ised shoots or leaves to determine transpiration gravimetrically may n ot accurately represent the stomatal water loss characteristics of tis sue-cultured plants.