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

Previous single-cell studies on the upper epidermis of barley leaves h ave shown that cells differ systematically in their solute concentrati ons depending on their location relative to stomatal pores and veins a nd that during NaCl stress, gradients in osmotic pressure (pi) develop (Fricke et al., 1995, 1996; Hinde, 1994). The objective of the presen t study was to address the question to which degree these intercellula r differences in solute concentrations and pi are associated with inte rcellular differences in turgor or water potential (psi). Epidermal ce lls analysed were located at various positions within the ridge region s 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, an d pi of extracted cell sap was determined by picolitre osmometry. For both large and intermediate lateral veins, there were no systematic di fferences in turgor between cells located at the base, mid or top of r idges, regardless of whether plants were analysed at low or high PAR ( less than or equal to 10 or 300-400 mu mol photons m(-2) s(-1)). Howev er, turgor within a ridge region was not necessarily uniform, but coul d vary by up to 0.14 MPa (1.4 bar) between adjacent cells. In 60 out o f 63 plants, turgor of ridge cells was either slightly or significantl y higher than turgor of trough (lowest turgor) or interstomatal cells (intermediate turgor). The significance and magnitude of turgor differ ences was higher in plants analysed under high PAR or local air flow t han in plants analysed under low PAR. The largest (up to 0.41 MPa) and consistently significant differences in turgor were found in plants t reated for 3-9 d prior to analysis with 100 mM NaCl. For both NaCl-tre ated and non-treated (control) plants, differences in turgor between c ell types were mainly due to differences in pi since differences in ps i were negligible (0.01-0.04 MPa). Epidermal cell psi in NaCl-treated plants was about 0.38 MPa more negative than in control plants due to higher pi. Turgor pressures were similar. Following a sudden change in rooting-medium psi or air humidity, turgor of both ridge and trough c ells responded within seconds and followed the same time-course of rel axation. The half time (T(1/)2) of turgor relaxation was not limited b y the cell's T-1/2 for water exchange.