Compartmental distribution and redistribution of abscisic acid in intact leaves : III. Analysis of the stress-signal chain.

Using a computer model written for whole leaves (Slovik et al. 1992, Planta 187, 14-25) we present in this paper calculations of abscisic acid (ABA) redistribution among different leaf tissues and their compartments in relation to stomatal regulation under drought stress. The model calculations are based on experimental data and biophysical laws. They yield the following results and postulates: (i) Under stress, compartmental pH-shifts come about as a consequence of the inhibition of the pH component of proton-motive forces at the plasmalemma. There is a decrease of net proton fluxes by about 8.6 nmol · s(-1) · m(-2). (ii) Using stress-induced pH-shifts we demonstrate how 'stress intensities' can be quantified on a molecular basis. (iii) As the weak acid ABA is the only phytohormone which behaves in vivo and in vitro ideally according to the Henderson-Hasselbalch equation, pH-shifts induce a complicated redistribution amongst compartments in the model leaf. (iv) The final accumulation of ABA in guard-cell walls is intensive: up to 16.1-fold compared with only up to 3.4-fold in the guard-cell cytosol. We propose that the binding site of the guard-cell ABA receptor faces the apoplasm. (v) A twoto three-fold ABA accumulation in guard-cell walls is sufficient to induce closure of stomata. (vi) The minimum time lag until stomata start to close is 1-5 min; it depends on the stress intensity and on the guard-cell sensitivity to ABA: the more moderate the stress is, the later stomata start to close or they do not close at all. (vii) In the short term, there is almost no influence of the velocity of pH-shifts on the velocity of the ABA redistribution, (viii) Six hours after the termination of stress there is still an ABA concentration 1.4-fold the initial level in the guard-cell cytosol (delay of ABA relaxation, 'aftereffect'), (ix) The observed 'induction' of net ABA synthesis after onset of stress may be explained by a decrease in cytosolic ABA degradation. About 1 h after onset of stress the model leaf would start to synthesise ABA (and its conjugates) automatically, (x) This ABA net synthesis serves to 'inform roots' via an increased ABA concentration in the phloem sap. The stress-induced ABA redistribution is per se not sufficient to feed the ploem sap with ABA. (xi) The primary target membrane of 'stress' is the plasmalemma, not thylakoids. (xii) The effective 'stress sensor', which induces the proposed signal chain finally leading to stomatal closure, is located in epidermal cells. Mesophyll cells are not capable of creating a significant ABA signal to guard cells if the epidermal plasmalemma conductance to undissociated molecular species of ABA (HABA) is indeed higher than the plasmalemma conductance of the mesophyll (plasmodesmata open), (xiii) All model conclusions which can be compared with independent experimental data quantitatively fit to them. We conclude that the basic experimental data of the model are consistent. A stress-induced ABA redistribution in the leaf lamina elicits stomatal closure.