A Comparison of Methods for Measuring Turgor Pressures and Osmotic Pressures of Cells of Red Beet Storage Tissue

Abstract

Two methods for measuring the turgor pressures of cells in discs of storage tissue of red beet (Beta vulgaris L.) were compared, and a centrifugation method for extracting sap from frozen and thawed tissue was evaluated. Turgor pressures were measured directly using a pressure probe, or indirectly using a vapour pressure osmometer. With the latter, discs were placed directly in the osmometer chamber and turgor was calculated as the difference in osmotic pressure before and after freezing and thawing. Turgor in freshly cut discs, measured with the pressure probe, was between 0 012 MPa and 0-118 MPa with a mean ±s.d. of 0 092 + 0-032 MPa (n = 24). That measured with the osmometer varied between 0-08 MPa and 0-12 MPa with a mean ±s.d. of 0-09 ±0-10 MPa(n = 54). After vacuum infiltration of discs with distilled water, the turgor measured with the pressure probe increased to 1-05 1-12 MPa. Turgor measured with the osmometer also increased after vacuum infiltration but was, on average, 12% lower than that measured with the pressure probe. Overall, the results suggest that for routine measurements, the osmometer can provide reasonable estimates of the turgor of cells in beet discs. This is because a number of factors that, potentially, could interfere with this method have only a small effect in this tissue. None of the measured turgors is indicative of that occurring in intact storage roots because both excision and vacuum infiltration of discs alter the concentrations of solutes in the extracellular space. The osmotic pressure of sap extracted by centrifugation from frozen and thawed discs was not significantly different from that measured by placing frozen and thawed discs directly in the osmometer. Solute concentrations in the sap were not significantly different from those measured by chemical extraction of discs.