Journal of Experimental Botany Relative water content and water potential of tissue-

Relative water content (RWC) and water potential as measured with the pressure chamber were evaluated as indicators of water status of tissue-cultured apple shoots and plantlets (shoots with roots). During the hydration required for RWC measurement, both water content and water potential exhibited the same hydra-tion kinetics, indicating that 10 h were required for full hydration. Once full hydration was reached, shoot mass remained relatively constant. Moisture release characteristics were also constructed and the associ-ated shoot and plantlet water relations parameters were estimated. Under in vitro conditions, both shoot and plantlet water potential were similar to the water potential of the culture medium in which they were grown. The moisture release characteristic of shoots and plantlets was consistent with that expected for typical plant tissues, and gave estimates of maximum modulus of elasticity (6.20 + 1.14 MPa), osmotic poten-tial at saturation ( — 0.85 + 0.10 MPa), osmotic potential at zero turgor (-1.16±0.14 MPa) and RWC at zero turgor (78 ±2%) which were similar to values in the literature. Higher values of leaf conductance and RWC were found in shoots and plantlets placed at 95 % RH (21 °C) compared to those at 90 % RH. Plantlets had higher values of both conductance and RWC compared to shoots, suggesting that in vitro roots are functional in water uptake. Relative water content was related to measures of physiological activity such as leaf conductance, and it was also easier to measure than water potential. Relative water content is suggested as a sound index of water status in tissue culture plants