A critical comparison of the pressure‐probe and pressure‐chamber techniques for estimating leaf‐ceil turgor pressure in Kalanchoë daigremontiana
MURPHY R., SMITH JAC.
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 Kalanchoë 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 (? cell ) and xylem‐sap osmotic pressure (? xyl ) were also measured, and an average cell turgor pressure calculated as P cell =? cell –? 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, ? xyl fell to a plateau value that was positively correlated with ? cell . Correcting for these effects yielded estimates of P bal and ? xyl at the time of leaf excision. On average, the values of P cell obtained with the two techniques agreed to within ±002 MPa (errors are approximate 95% confidence limits). If ? xyl were ignored, however, the calculated turgor pressures would exceed the measured values by an average of 0.074 ± 0.012MPa, 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 ? 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. Copyright © 1994, Wiley Blackwell. All rights reserved