Stress-dependent redistribution of abscisic acid (ABA) in Hordeum vulgare L leaves: The role of epidermal ABA metabolism, tonoplastic transport and the cuticle

When C-14-labelled abscisic acid ([C-14]ABA) was supplied to isolated protoplasts of the barley leaf at pH 6, initial rates of metabolism were about five times higher in epidermal cell protoplasts than in mesophyll cell protoplasts if equal cytosolic volumes were considered, In spite of the fact that epidermal cells make up only about 35% of the total water space in barley leaves, and despite the small cytosolic volume of these cells, in intact leaves all epidermal cells would thus metabolize half as much ABA per unit time as the mesophyll cells (0.27 and 0.51 mmol h(-1) m(-3) leaf water), Therefore, under these conditions epiderm mal cells seem to be a stronger sink than mesophyll cells for ABA that arrives via the transpiration stream, However, at an apoplastic pH of 7.25, which occurs in stressed leaves, the proportion of total metabolized ABA would be much smaller in epidermal than in mesophyll cells (0.029 and 0.204 mmol h(-1) m(-3) leaf water), Our results indicate that under conditions of slightly alkaline apoplastic pH the epidermis may serve as the main source for fast stress-dependent ABA redistribution into the guard cell apoplast, This is partly the result of ABA transport across the epidermal tonoplast, which is dependent on the apoplastic pH and possibly on the cytosolic calcium concentration, The cuticle seems to be of no particular importance in stress-induced apoplastic ABA shifts and cannot be regarded as a significant sink for high ABA concentrations under stress.