Water-relation Parameters of Individual Mesophyll Cells of the Crassulacean Acid Metabolism Plant Kalanchoë daigremontiana

Abstract

Water-relation parameters of leaf mesophyll cells of the CAM plant Kalanchoe 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 (T1/2) for water-flux equilibration of individual cells was 2.5 to 8.8 seconds. This is the fastest T1/2 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 T1/2 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 ([isin]) of individual cells were in the range 13 to 128 bar for turgor pressures between 0.0 and 3.4 bar; the average [isin] value was 42.4 {+/-} 27.7 bar (mean {+/-} SD; n = 21 cells). This [isin] value is similar to that observed for other higher-plant cells. The water-storage capacity of individual cells, calculated as Cc = V/([isin] + {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.