COMPARTMENTATION AND TRANSPORT OF ABSCISIC-ACID IN MESOPHYLL-CELLS OF INTACT LEAVES OF VALERIANELLA-LOCUSTA

When leaf disks of Valerianella locusta were incubated with 1O-2 mol m-3 of the growth retardant tetcyclacis (an inhibitor of cytochrome P 450 dependent monoxygenases) oxidative ABA degradation to phaseic acid was inhibited, whereas formation of the ABA glucoseester remained unaffected. Therefore efflux compartmental analysis with green tissue could be performed, because in the presence of tetcyclacis, the most important prerequisite for compartmental analysis, equilibrium between endogenous and exogenous ABA could be achieved. Flux experiments showed that ABA distribution between apoplast, cytoplasm and vacuole occurs exclusively according to the intracellular pH gradients and the anion trap concept. There is no evidence for a saturable transport component neither at the plasma membrane nor at the tonoplast. The permeability coefficients of ABA for Valerianella mesophyll plasma membrane and tonoplast are 3 X 10-8 ms-1 and 5 X 10-9 ms-1, respectively. ABA distribution data were used to calculate cytoplasmic and vacuolar pH values that were 7.3-7.4 and 6.4-6.6, respectively. Osmotic stress reduced the cytoplasmic pH slightly, which resulted in a release of 15% of the cytoplasmic ABA. High SO2 concentrations in the medium reduced dominantly the vacuolar pH. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.