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 acclimatization 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 acclimatized 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 (21 C), in vitro shoots and plantlets showed a very high initial conductance, followed by a gradual decline, reaching steady state in 12 hours. Acclimatized shoots and plantlets had a 50% lower initial conductance compared to nonacclimatized ones, and reached steady state in 4 hours. The reduction in conductance as a result of acclimatization most likely contributes to a reduced transpiration under conditions of increased evaporative demand. Roots formed in vitro were associated with a higher plant water status, suggesting that these roots were functional in water uptake. Relative water content of the shoot was positively correlated with leaf conductance and net photosynthesis. We suggest that tissue-cultured plantlets behave as hydraulically integrated units, in which there must be a coordination between control of water loss by the shoot and uptake of water by the root to maintain a favorable plant water balance. Our results also indicate that methods that use excised shoots or leaves to determine transpiration gravimetrically may not accurately represent the stomatal water loss characteristics of tissue-cultur ed plants. In vitro shoots and plantlets are grown at low evaporative demand conditions of high relative humidity (RH) and low light (30 to 75 µmol photons/m 2 per see). Under these conditions, there may be a low rate of transpiration even though stomata are widely open (Shackel et al., 1990). When plantlets are exposed to lower humidities, as occurs during transplanting, they may show high transpiration rates because of their high initial leaf conductance. In many woody species, transpiration after transplanting is often excessive, leading to severe plant water deficits. These water deficits are one of the main causes of plant mortality after transfer from culture to the greenhouse (Preece and Sutter, 1991). To ameliorate the impact of plant dehydration during transplanting, tissue-cultured plants are acclimatized for several weeks by gradu- ally decreasing RH and increasing luminosity. During acclimati- zation, plants may undergo physiological and morphological changes in response to such alterations in the environment. Despite the intense research in micropropagat ion, however, little is known about the physiological changes that occur during acclimatization and how these changes influence plant survival and growth in the new environment. Water deficits in plants will increase whenever transpiration exceeds water uptake. Transpiration is under the control primarily