CELLS OF THE UPPER AND LOWER EPIDERMIS OF BARLEY (HORDEUM-VULGARE L) LEAVES EXHIBIT DISTINCT PATTERNS OF VACUOLAR SOLUTES

Vacuolar saps were extracted from individual, anatomically uniform cel ls of the upper (adaxial) and lower (abaxial) epidermis of the third l eaf of barley (Hordeum vulgare L.) using a modified pressure probe. Sa ps (volume 80-200 pL) were sampled at various times between 3 d before and 7 d after full-leaf expansion and were analyzed for their osmolal ity and their concentrations of NO3-, malate, Cl-, K+, and Ca2+. The o smolalities of upper and lower epidermis both increased with time but were similar to each other. In young leaves, K+ and Ca2+ were evenly d istributed between the two epidermal layers, but as the leaf aged, the upper epidermis accumulated high (40-100 mM) Ca2+, whereas cells of t he lower epidermis accumulated K+ instead. Nitrate concentration was 1 00 to 150 mM higher in the upper than in the lower epidermis, whereas Cl- was 50 to 120 mM higher in the lower epidermis. These differences did not depend on the leaf developmental stage. The uneven distributio n of epidermal NO3- and Cl- was maintained over a wide range of epider mal sap concentrations of these ions and was not affected by NO3- or C l- starvation or by an increase in the light intensity from 120 to 400 mu mol m(-2) s(-1). However, the latter did cause a decrease in epide rmal NO3- and the appearance and accumulation of epidermal malate, par ticularly in the upper epidermis. The physiological implications of th e results for solute storage in leaves and for the pathways of ion dis tribution to the epidermis are discussed.