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Résumé :
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The aim of the present study was to test the accuracy of the pressure-chamber technique as a method for estimating leaf-cell turgor pressures. To this end, pressure-probe measurements of cell turgor pressure (P-cell) were made on mesophyll cells of intact, attached leaves of Kalanchoe daigremontiana. Immediately following these measurements, leaves were excised and placed in a pressure chamber for the determination of balance pressure (p(bal)). Cell-sap osmotic pressure (Pi(cell)) and xylem-sap osmotic pressure (Pi(xyl)) were also measured, and an average cell turgor pressure calculated as P-cell = Pi(cell)-Pi(xyl)-P-bal. The apparent value of P-bal was positively correlated with the rate of increase of chamber pressure, and there was also a time-dependent increase associated with water loss. On expressing sap from the xylem, Pi(xyl) fell to a plateau value that was positively correlated with Pi(cell). Correcting for these effects yielded estimates of P-bal and Pi(xyl) at the time of leaf excision. On average, the values of P-cell obtained with the two techniques agreed to within +/-0.02 MPa (errors are approximate 95% confidence limits). If Pi xyl were ignored, however, the calculated turgor pressures would exceed the measured values by an average of 0.074 +/- 0.012 MPa, or 48% at the mean measured pressure of 0.155 MPa. We conclude that the pressure-chamber technique allows a good estimate to be made of turgor pressure in mesophyll cells of K. daigremontiana, provided that Pi(xyl) is included in the determination. The 1:1 relationship between the measured and calculated turgor pressures also implies that the weighted-average reflection coefficient for the mesophyll cell membranes is close to unity.
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