Steady-state rates of potassium ion and sodium ion absorption by excised barley roots accompanied by various anions were compared with the rates of anion absorption and the concomitant H + and base release by the roots. The cation absorption rates were found to be independent of the identities, concentrations, and rates of absorption of the anions of the external solution, including bicarbonate. Absorption of the anion of the salt plus bicarbonate could not account for the cation absorption. H + is released during cation absorption and base during anion absorption. The magnitude by which one or the other predominates depends on the relative rates of anion and cation absorption under various conditions of pH, cation and anion concentration, and inhibitor concentrations. The conclusion is that potassium and sodium ions are absorbed independently of the anions of the absorption solution in exchange for H +, while anions are exchanged for a base. The H + release reflects a specificity between K + and Na + absorption such that it appears to be H + exchanged in the specific rate-limiting reactions of the cation absorption. I N T R O D U C T I O NMaintenance of a constant charge balance in the roots during salt absorption could require that cation absorption rates be equal to absorption rates of the accompanying anions. Absorption of anions and cations by barley roots is so closely related, according to Steward and Sutcliffe (16), that an effect on one results in an effect on the other. For example, stimulation of K + absorption by beet root disks at high pH is attributed to an increased uptake of the associated anions, bicarbonate and chloride (7). They agree with Lundegardh (12) that anion accumulation is limiting for cation absorption. Ulrich (18) also considers that cations absorbed in excess of the anion of the salt are absorbed in association with bicarbonate, resulting in the observed increase in organic acids.

BOTANY: HAGEN ET AL. sion are cited. A convincing analysis of repressed enzyme formation in the pathway leading to pyrimidine compounds has been carried out by R. A. Yates and A. B. Pardee (J. Biol. Chem. [in press] and personal communication). 2 Induction and repression may respectively be brought about by inducers and repressers acting separately. When, however, an inducer and a represser of one and the same enzyme are present simultaneously, an antagonistic effect involving the two regulators may be expected to result. More generally, it is possible that inducer or represser action is antagonizable by substances that, in themselves, may or may not be regulators.

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 ...

Previous work has shown that undissociated forms of organic acids, such as formic, acetic, and propionic acids, increase the permeability of barley roots to ions. The work here was undertaken to test whether these undissociated acids affect the lipids from the root membranes in such a way as to account for the permeability increase. Relative amounts of the principal fatty acids from barley root membranes were measured as a function of organic acid concentration, pH, and time of treatment of barley roots under conditions similar to those of the previous studies.Undissociated formic, acetic, and propionic acids al rapidly increase the proportions of palmitic, stearic, and oleic acids and decrease proportions of linoleic and linolenic acids. Only the undissociated species are effective.The effects on the fatty acids from membrane lpids parallel effects on Ion permeability. It is concluded that the increase in permeability produced by undissociated organic acid is due to changes in the lipids of barley root membranes.Previous work has shown that undissociated forms of organic acids, such as formic, acetic, propionic, butyric, and glutaric acids, increase the permeability of barley roots to ions (8). Relative effects of the acids increased according to their lipid solubility. This suggests a link between lipids in root membranes and areas where hydrophilic ions permeate. The work presented here was designed to test this hypothesis.The approach was to determine relative amounts of the principal fatty acids, i.e. palmitic (C 16:0), stearic (C 18:0), oleic (C 18: 1), linoleic (C 18:2), and linolenic (C 18:3), from the polar lipids of membranes (4) in the roots. The fatty acids were measured as a function of organic acid concentration, pH, and time of treatment under conditions similar to those of the previous studies of the organic acid effects on permeability of roots to ions (8). The work presented here is among the first to demonstrate that a chemical can cause an immediate change in membrane composition and, thereby, immediately affect functionality. MATERIALS AND METHODSThe roots were from 6-day-old seedlings of barley (Hordeum vulgare var. Trebi) which had been dark-grown in aerated 0.2 mM CaSO4 (pH 5.6) at 25 C. Roots were excised and rinsed several times with demineralized H20 just before the experiment.Details of procedures and conditions for the experiments have been described (5). Briefly, about 10 g roots were maintained in 4 liters aerated organic acid salt solution at 23 to 25 C for 15 min to 6 h. Then the roots were removed and freeze-dried. The pH was maintained during treatment by periodic titration with an appropriate base or acid.