Cell turgor, osmotic pressure and water potential in the upper epidermis of barley leaves in relation to cell location and in response to NaCl and air humidity

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Previous single-cell studies on the upper epidermis of barley leaves have shown that cells differ systematically in their solute concentrations depending on their location relative to stomatal pores and veins and that during NaCI stress, gradients in osmotic pressure (π) develop (Fricke et al., 1995, 1996; Hinde, 1994). The objective of the present study was to address the question to which degree these intercellular differences in solute concentrations and π are associated with intercellular differences in turgor or water potential (ψ). Epidermal cells analysed were located at various positions within the ridge regions overlying large lateral or intermediate veins, in the trough regions between those veins or in between stomata (i.e. interstomatal cells). Turgor pressure of cells was measured using a cell pressure probe, and π of extracted cell sap was determined by picolitre osmometry. For both large and intermediate lateral veins, there were no systematic differences in turgor between cells located at the base, mid or top of ridges, regardless of whether plants were analysed at low or high PAR (≤ 10 or 300-400 μmol photons m -2 s -1 ). However, turgor within a ridge region was not necessarily uniform, but could vary by up to 0.14 MPa (1.4 bar) between adjacent cells. In 60 out of 63 plants, turgor of ridge cells was either slightly or significantly higher than turgor of trough (lowest turgor) or interstomatal cells (intermediate turgor). The significance and magnitude of turgor differences was higher in plants analysed under high PAR or local air flow than in plants analysed under low PAR. The largest (up to 0.41 MPa) and consistently significant differences in turgor were found in plants treated for 3-9 d prior to analysis with 100 mM NaCI. For both NaCI-treated and non-treated (control) plants, differences in turgor between cell types were mainly due to differences in π since differences in ψ were negligible (0.01-0.04 MPa). Epidermal cell ψ in NaCI-treated plants was about 0.38 MPa more negative than in control plants due to higher π. Turgor pressures were similar. Following a sudden change in rooting-medium ψ or air humidity, turgor of both ridge and trough cells responded within seconds and followed the same time-course of relaxation. The half time (T 1/2 ) of turgor relaxation was not limited by the cell's T 1/2 for water exchange.