Vacuolar solutes in the upper epidermis of barley leaves

The concentrations of vacuolar solutes in different cells of the upper epidermis of the third leaf of barley (Hordeum vulgare L.) were studied in leaves of different ages grown under different irradiances (120 or 400 mol photons·m^–2·s^–1). Vacuolar saps were extracted from individual cells located at various positions between adjacent veins and were analysed for their osmolality and the concentrations of K⁺, Ca²⁺, Cl^–, NO ₃ ^– and malate. Each ion showed a cell-specific distribution within the epidermis that was both quantitatively and qualitatively dependent on the leaf developmental stage and on the light level. During leaf ageing, Ca²⁺ accumulated preferentially in interstomatal cells (i.e. those located between longitudinally adjacent stomata) at concentrations up to 180 mM. Under low light conditions, this was accompanied by a more or less equal decrease in K⁺ concentration. Epidermal malate was found only in plants grown continuously or transiently under the high irradiance and reached highest concentrations in trough and interstomatal cells (60 to 150mM). Chloride concentration was highest in cells overlying the veins (designated as ridge cells) and lowest in cells located between the veins (trough cells), while NO ₃ ^– exhibited the reverse distribution, although the precise patterns were age-dependent. Epidermal osmolality increased with age, but the intercellular differences in the osmolalities were small compared to differences in vacuolar solute composition. A cell-to-cell analysis of the region surrounding the stomata showed that the steepest changes in the vacuolar solute composition of epidermal cells occurred at the boundary between ridge or trough cells and the adjacent near-stomatal cells.Abbreviations EDX analysis energy dispersive X-ray analysis We wish to thank Andrew Davies and Alison Bell (Bangor) for their technical advice. This work was financed as an Agricultural and Food Research Council Linked Research Group project between Bangor and Rothamsted (grants LR5/187 and 521).