Direct turgor pressure measurements in individual leaf cells of Tradescantia virginiana.

The water relations of leaves of Tradescantia virginiana were studied using the miniaturized pressure probe (Hüsken, E. Steudle, Zimmermann, 1978 Plant Physiol. 61, 158-163). Under well-watered conditions cell turgor pressures, P o, ranged from 2 to 8 bar in epidermal cells. In subsidiary cells P o was about 1.5 to 4.5 bar and in mesophyll cells about 2 to 3.5 bar. From the turgor pressure, relaxation induced in individual cells by changing the turgor pressure directly by means of the pressure probe, the half-time of water exchange was measured to be between 3 and 100 s for the epidermal, subsidiary, and mesophyll cells. The volumetric elastic modulus, ε, of individual cells was determined by changing the cell volume by a defined amount and simultaneously measuring the corresponding change in cell turgor pressure. The values for the elastic modulus for epidermal, subsidiary, and mesophyll cells are in the range of 40 to 240 bar, 30 to 200 bar, and 6 to 14 bar, respectively. Using these values, the hydraulic conductivity, L p, for the epidermal, subsidiary, and mesophyll cells is calculated from the turgor pressure relaxation process (on the basis of the thermodynamics of irreversible processes) to be between 1 and 55·10(-7) cm s(-1) bar(-1). The data for the volumetric elastic modulus of epidermal and subsidiary cells indicate that the corresponding elastic modulus for the guard cells should be considerably lower due to the large volume changes of these cells during opening or closing. Recalculation of experimental data obtained by K. Raschke (1979, Encycl. Plant Physiol. N.S., vol. 7, pp 383-441) on epidermal strips of Vicia faba indicates that the elastic modulus of guard cells of V. faba is in the order of 40-80 bar for closed stomata. However, with increasing stomatal opening, i.e., increasing guard cell volume, ε decreases. Therefore, in our opinion Raschke's results would indicate a relationship between guard cell volume and ε which would be inverse to that for plant cells known in the literature. ε assumes values between 20-40 bar when the guard cell colume is soubled.