Ion fluxes in plants related to acid growth

The walls of plant cells are considered as a system containing weak acid polymers where the interaction of ions on the polymers obeys Manning condensation theory and Donnan theory (the Weak Acid Donnan Manning, or WADM, model). When protons are extruded to the walls during the growth of plant cells, the pH of the walls and therefore the ionic conditions of the walls change with time. The Microelectrode Ion Flux Estimation (MthE) technique was used to measure simultaneously proton and calcium fluxes during the fusicoccin- or IAA-induced growth of 4-day-old split coleoptile segments of Avena sativa L. and 6- to 7-day-old peeled epicotyl segments of Pisum sativurn L. line 107. The WADM model for fluxes is now extended to analyse the change of the ionic condition in the walls, as well as the ion fluxes outside the walls, during proton extrusion from the cells to the cell walls. The analysis shows that when there is calcium condensation in the walls, proton extrusion from the cells to the cell walls causes effluxes of both protons and calcium outside the walls. However when the walls are considered as a classical Dorman system only, which is a special case of the WADM model for fluxes, proton extrusion from the cells to the cell walls would not cause any calcium release from the walls. Fusicoccin induces immediate and transient proton and calcium effluxes from oat segments preincubated 4.5 hours. The fusicoccin-induced proton efflux saturates at 115 nmol. m-2 s-1 for 10-3 mole m-3 fusicoccin. The fusicoccininduced calcium efflux saturates at about 120 nmol. m-2 s-1. On peas, the fusicoccin-induced proton and calcium effluxes are smaller than on oats. When the WADM model for fluxes is applied to the observed data of both oats and peas, the calculated proton and calcium effluxes match well with the observed effluxes. This indicates that proton-calcium exchange happens in the walls during fusicoccin-induced proton extrusion across the plasmalemma and that the observed calcium efflux is the evidence for the validity of the model. IAA at 10-2 mole m-3 induces proton efflux with a lag of about 13 minutes on oats. The proton efflux is smaller when the preincubation time is shorter (15 or 5 nmol. m-2 s-1 for 4.5 or 1 hour preincubation). IAA also induces biphasic calcium efflux but without a noticeable lag. The increase of the calcium efflux induced by IAA is about 30 nmol. On pea segments preincubated for 4.5 hours, IAA at 10-2 mole m-3 causes, without a noticeable lag, transient proton and calcium effluxes, lasting about 15 minutes. The IAA-induced proton and calcium effluxes are about 5 and 25 nmol. m-2 s-1 respectively. The responses to IAA of oats and peas are also consistent with the proton-calcium exchange in the walls during IAA-induced proton extrusion. The WADM model does not specify the function of condensed calcium in the walls that is released during proton extrusion from the cells to the cell walls. In this present work, it is discussed that condensed calcium may serve a role of stiffening the walls and therefore the proton-calcium exchange during proton extrusion has some role, as well as other processes, in causing wall loosening. Proton flux determination by measuring the electrochemical potential gradient of protons can be in error when the measurements are carried out in buffered solutions and the error depends on the pH of the solutions. Analysis is done to quantify this error and take it into account. Besides proton and calcium fluxes, potassium flux and membrane potential were also measured. Fusicoccin induces immediate potassium efflux and membrane hyperpolarization on both oats and peas where their magnitudes depend on the fusicoccin concentration. On both plants, IAA does not cause any significant effects on potassium flux. IAA induces transient membrane hyperpolarization with a lag of about 7 minutes on oats. On peas, IAA causes transient membrane hyperpolarization without a noticeable lag and it is followed by membrane depolarization. Solid-state chloride rnicroelectrodes were specially developed to measure the flux of chloride during IAA action on oats. IAA does not cause any significant change on chloride flux. The effects of some treatments, such as cutting, preincubation time and solution change are also discussed.