Water Relations of Leaf Epidermal Cells of Tradescantia virginiana

Water-relation parameters (cell turgor pressure P], volumetric elastic modulus epsilon] and hydraulic conductivity Lp]) of individual leaf epidermal cells of Tradescantia virginiana have been determined with the pressure-probe technique. Turgor was 4.5 +/- 2.1 41] bar (mean +/- sd; in brackets the number of cells) and ranged from 0.9 to 9.6 bar. By vacuum infiltration with nutrient solution, it was raised to 7.5 +/- 1.5 5] bar (range: 5.3-8.8 bar). There was a large variability in the absolute value of epsilon of individual cells. epsilon ranged from 40 to 360 bar; mean +/- sd: 135 +/- 83 bar; n = 50 cells. epsilon values of individual cells seemed to be rather independent of changes in cell turgor. A critical assessment of the errors incurred in determining epsilon by the technique is included. The half-times of water exchange of individual cells ranged from 1 to 35 seconds, which gave values of 0.2 to 11 x 10(-6) centimeters per second per bar for Lp (mean +/- sd: 3.1 +/- 2.3 x 10(-6) centimeters per second per bar; n = 39 cells). The large range in Lp and epsilon is believed to be due to the difficulties in determining the effective surface area of water exchange of the cells. Lp is not influenced by active salt pumping driven by respiration energy inasmuch as it was not altered by 0.1 millimolar KCN. The temperature dependence of Lp (T((1/2))) was measured for the first time in individual higher-plant cells. Lp increased by a factor of 2 to 4, when the temperature was increased by 10 C. The activation energy of water exchange was found to be between 50 and 186 kilojoules per mole. Within the large range of variation it was found that T((1/2)), Lp, and epsilon did not change under various experimental conditions (intact and excised tissue, water content and turgidity, age, etc.). Similar results were obtained for the epidermal cells of Tradescantia andersoniana. The measurements suggest that the entire epidermis would respond very rapidly (i.e. with a half-time of 1 to 30 s) to a demand for water from the stomata.