Compartmental distribution and redistribution of abscisic acid in intact leaves

Abstract

A computer model written for whole leaves and described in the preceding publication (Slovik et al. 1992, this volume) has been developed for calculating the distribution and fluxes of weak acids or bases amongst different leaf tissues and their compartments, considering membrane transport, transpiration-driven mass transport, symplasmic and apoplasmic diffusion, and metabolic turnover rates in specified compartments. The model is used to analyse flux equilibria and the transport behaviour of the phytohormone abscisic acid (ABA) in unstressed and stressed leaves. We compare experimental data of unstressed Valerianella locusta L. leaves and expectations based on the detailed analysis of the data. (i) The mean daily influx of ABA into the leaf lamina via the xylem sap is about 10 nmol · m⁻² · day⁻¹. It is balanced by the sum of an export of ABA via the phloem sap (0.7%), possibly also by a basipetal ABA transport in the petiole parenchyma of young leaves (up to 18%), by an irreversible conjugation of ABA (0.4–4%) and by net degradation of ABA in the leaf lamina (80–95%). (ii) The estimated kinetic parameters of this net degradation are for the mesophyll apoplasm: apparent K _m = 3.7 nM and V _max = 12.9 nmol · m⁻³ · s⁻¹, or for the mesophyll cytosol: apparent K _m = 8.1 nM and V _max = 32.3 nmol · m⁻³ · s⁻¹. (iii) The dynamic ABA concentration in the phloem sap of Valerianella is 2.8 nM. This is only 5.5% of the static ABA equilibrium concentration in excised leaves or 70% of the ABA concentration in the mesophyll apoplasm, and it equilibrates within a few hours after source concentrations in the mesophyll apoplasm are changed under stress. Thus, the phloem sap is a flexible medium for transporting ‘new phytohormone information’ from the lamina to the shoot and roots, (iv) Measured compartmental ABA concentrations are close to calculated equilibrium concentrations in unstressed leaves. We conclude that model calculations are close to reality, (v) pH gradients within the apoplasm influence the apoplasmic distribution of ABA. Its concentration is maximally about twofold higher in guard-cell walls relative to the mesophyll apoplasm. (vi) Unexpectedly, all compartmental equilibrium concentrations of ABA in the leaf lamina depend on plasmalemma conductances for undissociated ABA and on the transport properties of the plasmodesmata. This is a consequence of the cyclic diffusion pathway: mesophyll cytosol — mesophyll plasmalemma — mesophyll apoplasm — epidermal apoplasm — epidermal plasmalemma — epidermal cytosol — plasmodesmata — mesophyll cytosol (in this direction), if there are different apoplasmic or cytosolic pH values in both tissues. The cyclisation rate is 42 fmol · s⁻¹ · m⁻² leaf area, which corresponds to a turnover time = 11.0 h for the total ABA content within the leaf lamina. A decrease of the epidermal plasmalemma conductance by 90% yields a threefold ABA concentration in the guard-cell free space, (vii) Compartmental relaxation-time coefficients are estimated and summarised for all leaf tissues and its major compartments. They range from 1.5 min for chloroplasts up to 3.3 d for mesophyll vacuoles, (viii) The highest ABA concentration, which can be expected in any leaf compartment, is 7 mM in the guard-cell cytoplasm of certain plant species, (ix) We employed circadian changes (equal day + night, 12 h each = equinoctium) of the stromal pH ± 0.3 in C³ plants, and for Crassulacean acid metabolism (CAM) plants, additionally, vacuolar pH ± 2.5 changes, and calculated the consequences for ABA redistribution within the lamina. In plants of both photosynthesis types, the ABA concentration in guard-cell walls is only 1.5 times higher in the night relative to the day. We conclude that stomata may not be regulated by ABA in a night-day regime. The influence of the extreme vacuolar pH changes on ABA distribution is small in CAM plants for two reasons: the ABA content in CAM mesophyll vacuoles is low (maximum 2.7% of the total ABA mass per unit leaf area) and there is only a 6.5-fold increase of the mole fraction of undissociated ABA when the the vacuolar pH is lowered from 5.5 to 3.0 (importance of the absolute pK_a = 4.75 of ABA).