VACUOLAR SOLUTES IN THE UPPER EPIDERMIS OF BARLEY LEAVES - INTERCELLULAR DIFFERENCES FOLLOW PATTERNS

The concentrations of vacuolar solutes in different cells of the upper epidermis of the third leaf of barley (Hordeum vulgare L.) were studi ed in leaves of different ages grown under different irradiances (120 or 400 mu mol photons . m(-2). s(-1)). Vacuolar saps were extracted fr om individual cells located at various positions between adjacent vein s and were analysed for their osmolality and the concentrations of K+, Ca2+, Cl-, NO3- and malate. Each ion showed a cell-specific distribut ion within the epidermis that was both quantitatively and qualitativel y dependent on the leaf developmental stage and on the light level. Du ring leaf ageing, Ca2+ accumulated preferentially in interstomatal cel ls (i.e. those located between longitudinally adjacent stomata) at con centrations up to 180 mM. Under low light conditions, this was accompa nied by a more or less equal decrease in K+ concentration. Epidermal m alate was found only in plants grown continuously or transiently under the high irradiance and reached highest concentrations in trough and interstomatal cells (60 to 150 mM). Chloride concentration was highest in cells overlying the veins (designated as ridge cells) and lowest i n cells located between the veins (trough cells), while NO3- exhibited the reverse distribution, although the precise patterns were age-depe ndent. 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 sur rounding 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.