Abstract
Water-relation parameters of leaf mesophyll cells of the CAM plant Kalanchoë daigremontiana have been determined directly in cells of tissue slices using the pressure-probe technique. Turgor pressures measured in cells of the second to fourth layer from the cut surface showed an average of 1.82 +/- 0.62 bar (mean +/- sd; n = 157 cells). This was lower than expected from measurements of the osmotic pressure of the cell sap. The half-time (T(1/2)) for water-flux equilibration of individual cells was 2.5 to 8.8 seconds. This is the fastest T(1/2) found so far for higher-plant cells. The calculated values of the hydraulic conductivity were in the range of 0.20 to 1.6 x 10(-5) centimeters second(-1) bar(-1), with an average of (0.69 +/- 0.46) x 10(-5) centimeters second(-1) bar(-1) (mean +/- sd; n = 8 cells). The T(1/2) values of water exchange of individual cells are consistent with the overall rates of water-flux equilibration measured for tissue slices.The volumetric elastic moduli (in) of individual cells were in the range 13 to 128 bar for turgor pressures between 0.0 and 3.4 bar; the average in value was 42.4 +/- 27.7 bar (mean +/- sd; n = 21 cells). This in value is similar to that observed for other higher-plant cells.The water-storage capacity of individual cells, calculated as C(c) = V/(in + pi(i)) (where V = cell volume and pi(i) = internal osmotic pressure) was 9.1 x 10(-9) cubic centimeters bar(-1) per cell, and the capacity for the tissue was 2.2 x 10(-2) cubic centimeters bar(-1) gram(-1) fresh weight. The significance of the water-relation parameters determined at the cellular level is discussed in terms of the water relations of whole leaves and the high water-use efficiency characteristic of CAM plants.