Effects of Organic Acids on Ion Uptake and Retention in Barley Roots

Abstract

Effects of several organic acids on ion uptake and retention and on respiration in barley roots having low and high KCI contents were assayed by measurements of K+, Na+, Ca2+, Cl-, and oxygen uptake. Organic acids with high pKa values increase the permeability of roots to ions and decrease respiration when present in sufficient concentrations at pH 5 but have no inhibitory effects at pH 7. Absence of respiratory inhibition in short times and at lower organic acid concentrations, under conditions that immediately produce a permeability increase, indicate that the permeability change is not a result of respiratory inhibition. Effects of formate, acetate, propionate, and glutarate are attributed to entry of undissociated acid molecules into the effective membranes. Lack of a permeability increase with succinate, which has lower distribution coefficients to lipid solvents than do the aliphatic acids, can be explained by failure of sufficient amounts of the hydrophilic succinic acid molecules to penetrate the membranes involved. These experiments suggest that undissociated acid in root membranes can increase permeability of the roots.Entry of weak organic acids into plants has been known for many years (1,4,18). They are generally thought to enter by distribution to the lipid-protein of the cellular membranes (see Ref. 1, for review), in part because of enhanced effects with decrease in pH. Most of these acids, however, are toxic at moderate concentrations and low pH at which inorganic salts are tolerated (7,16). Evidence that organic acids are taken up as anions by plant roots has only recently been advanced (9). Considerations of the entry of organic anions require some clarification of the nature of the interaction of the undissociated acid with roots.Measurements of salt loss from roots in the presence of several weak organic acids as shown here indicate that the limiting membranes are modified under usual experimental conditions. Changes in metabolic rates are shown to be secondary to these modifications. The conclusion is that the acid anion can enter the root without evident modification of the root membranes. The undissociated acid in sufficient concentration, however, modifies the membrane so as to alter the response toward inorganic salts and possibly toward the acid itself. MATERIALS AND METHODSThe roots were from 6-day-old seedlings of barley (Hordewn vulgare, var. Compana and var. Trebi) which had been dark grown in aerated 2 X 10-4 M CaSO4, pH 5.6 at 25 C. High KCl roots were from seedlings grown in 10-3 M KCI + 2 X 10-4 M CaSO4 at pH 5.6 for 8 hr on the 5th day. Roots were excised and rinsed several times just before use.Details of procedure and conditions for the experiments were the same as those used previously (8). Briefly, the roots were maintained in aerated organic acid salt solutions at 23 to 25 C, from which successive root samples were withdrawn periodically, rinsed with water four times, and weighed. Sampling times varied from 5 min to 24 hr. Half-gram root samples were dried in the ...

Effect of Secondary Metabolites Associated with Anaerobic Soil Conditions on Ion Fluxes and Electrophysiology in Barley Roots

“…1B); of these, propionic acid caused the largest K 1 efflux, followed by acetic acid, and formic acid the least, proportional to the amount of undissociated acid in the bath solution (Table II). This is consistent with previous reports on the adverse effects of these acids on the K 1 uptake (Jackson and Taylor, 1970;Jackson and St. John, 1980). These authors also suggested that changes in membrane lipid composition might be responsible for the observed leak of K 1 and Ca 21 from roots treated with monocarboxylic acids.…”

“…Similar to phenolics, lipid-soluble undissociated forms of the volatile monocarboxylic acids are often regarded as the most toxic (Jackson and Taylor, 1970;Jackson and St. John, 1980). Given that the concentration of monocarboxylic acids used in our experiment is much higher than that of the phenolic acids, the concentration of H 1 in the cell cytosol would be much higher than in phenolic acid-treated plants.…”

“…5C). Thus, it is highly unlikely that general changes in membrane permeability were involved as was suggested by Jackson and coauthors (Jackson and Taylor, 1970;Jackson and St. John, 1980). Strong membrane depolarization (Fig.…”

“…1B). Such changes in permeability are usually associated with a change in membrane lipid components (Jackson and Taylor, 1970;Glass, 1974;Jackson and St. John, 1980), the latter process most likely operating on a slower time scale. Importantly, the above changes in membrane permeability are believed to be nonspecific (Glass and Dunlop, 1974), thus, a mirror image kinetics for K 1 efflux and Ca 21 uptake (according to electrochemical potential for each ion) should be observed.…”

Membrane-surfactant Interactions in Lipid Micelles Labeled with l-Anilino-8-naphthalenesulfonate

“…Organic anions that inhibit membrane permeability reduce the fluorescence of ANS with the same order of effectiveness (17). Organic cations have the opposite effect (17), and several undissociated organic acids increase the ionic permeability of barley roots (10). Radda and Vanderkooi (17), the separation of these effects into those produced by changes in polarity, accessibility, and orientation constraint is difficult at this time.…”