Compartmental distribution and redistribution of abscisic acid in intact leaves : I. Mathematical formulation.

Using experimental information obtained in earlier studies on the permeabilities of mesophyll and guard-cell membranes to abscisic acid (ABA), and on stress-induced pH shifts in the apoplasm and in symplasmic compartments (Hartung et al., 1988, Plant Physiol. 86, 908-913; Hartung et al. 1990, BPGRG Monogr. 215-235), a mathematical model is presented which will permit computer analysis of the stress-induced redistribution of ABA amongst different leaf cell types (mesophyll, epidermis, guard cells, phloem cells) and their compartments (cell wall, cytosol, chloroplast stroma, vacuole). Metabolism and conjugation of ABA and its transport in the xylem and the phloem are also taken into consideration. We ask whether the stressinduced redistribution of ABA is fast and intensive enough to induce stomatal closure within a few minutes. The model can be adapted to any other weak acid or base, e.g. to other phytohormones (auxins, gibberellins), which differ from ABA, e.g. by their membrane conductances, anion permeabilities and pKa values. Our wholeleaf model can predict the time course and the compartmentation of, for example, phytohormone concentrations as a function of changing source-sink patterns (e.g. by compartmental pH shifts in the leaf lamina). An analysis of the present knowledge of the ABA physiology of leaves and studies on stress effects are presented in subsequent publications. In this communication we describe the whole-leaf model and present and discuss all necessary morphological (volumes, surfaces etc.) and physiological (pH, membrane conductances etc.) parameters of an unstressed leaf of Valerianella locusta L. We draw fundamental conclusions by comparing determined and calculated ABA concentrations in the leaf-cell compartments. We found that the model predictions are close to measured data, and we conclude that in unstressed leaves ABA is close to flux equilibrium amongst the different tissues and compartments.