Role of the Accompanying Anion in the Effect of Calcium Salts on Potassium Uptake by Excised Barley Roots

Abstract

The effect of addition of Ca salts on accumulation of K from 5 mN KCI or K2S04 solutions was found to depend on whether Ca was added as Cl or S04 salt. Chloride as well as K uptake was increased when Ca and Cl concentrations in culture solutions were increased. Pre-treatment of roots with CaC12 stimulated subsequent K uptake from K,,SO4 solutions as compared to pre-treatment with distilled water but pre-treatment with CaSO4 did not. The results indicate that addition of Ca salts to KCI or K2SO4 solutions increased anion uptake and the effect of the addition of the Ca salts on K uptake was in part the result of increased anion uptake and not entirely a direct effect of Ca. In contrast, accumulation of Ns and K from solutions containing these ions as SO4 or Cl salts was changed from preferential uptake of Na to preferential uptake of K by addition of either CaCl, or CaSO4. Thus, while Ca salts may influence K aecumulation partly as a result of effects on anion uptake, the selectivity for K uptake depends on the presence of Ca and is influenced little by the accompanying anion. Calcium salts exert several effects on the uptake of potassium and other ions by excised barley roots and, as emphasized by Rains and Epstein (18), at least small concentrations of calcium ions must be present in the culture medium for roots to function normally. Metabolic uptake of potassium and rubi-dium by excised barley roots is stimulated by CaCl2, CaBr2 and halide salts of several other polyvalent cations (7, 14,20,21). Uptake of bromide, chloride and sulfate by excised barley roots is also increased by addition of calcium to the culture solution (3, 7, 12, 20). The presence of calcium and of some other polyvalent cations in culture solutions also influences the ability of roots of barley and many other plants to accumulate potassium in preference to sodium from solutions containing both ions (5, 9). Over the concentration range of about 0.5 to 50 mm, uptake of monovalent cations may be influenced by the accompanying anion (6). When K uptake is limited by anion uptake, addition to culture solutions of calcium salts of anions such as chloride that are rapidly accumulated by excised barley roots may, in addition to any direct effect of calcium ions, stimulate potassium uptake as a result of increased anion uptake. Indeed, Elzam and Hodiges (4) observed that although the addition of CaCl2 to culture solutions increased potassium uptake by excised barley roots, the addition of CaSO4 had no effect. Also, Pitman (17) has shown that slices of beet root tissue accumulate more chloride from solutions containing both potassium and calcium than from KCl solutions at the same chloride concentration. Pitman (17) concluded that chloride uptake limited potassium uptake by the beet root tissue and that calcium acted to increase chloride uptake and consequently potassium uptake. The effects of calcium salts on accumulation of potassium and other ions however, are usually attributed to a direct effect of calcium ions without consideration of possible influences of the nature or concentration, of the accompanying anion. The purpose of this study was to attempt to separate the effeicts of calcium ions and the accompanying anion on potassium uptake, and the selectivity for potassium by excised barley roots. Materials and Methods Excised barley IHordeum vulgare L., var. Erie) roots were grown essentially as described by Jacob-son et al. (9). For each experiment, 50 grams of seed were treated with 75 ml of 10 % '(v/v) HO02 for 10 min. After discarding the H202 solution, the seeds were rinsed with distilled water and then soaked for 24 hr in 1500 ml of aerated distilled water. The seeds were then placed on cheesecloth supported by a nylon screen and covered with another nylon screen and cheesecloth. The screen suipporting the seeds was suspended over 8 liters of aerated nutrient solution in a polypropylene tray with the ends of the cheesecloth dipping into the solution. The nutrient solution contained 0.1 mmole per liter each of MgSO4, KH2PO, and Ca(NO,). 1639 PLANT PHYSIOLOGY The seedlings were grown in a dark chamber at 250. After 2 days the screen and cheesecloth covering the seedlings were removed. After the plants had grown for 3 days, the nutrient solution was replaced with fresh nutrient solution. On the fifth day the nutrient solution was replaced with distilled water and 24 hr later the roots were excised jus't below the screen. The excised roots were rinsed several times with flowing distilled water and placed in 4 liters of aerated distilled water for 1 hr before use in an experiment. The experiments were conducted at 250 using a ratio of 1 gram of roots (fresh wt) to 250 ml of solution. The solutions were aerated continuously and the pH adjusted to !6.0 and maintained at 6.0 during the experiments by adding IRA 400 anion exchange resin in the OH form at 30 min intervals (16). Results of preliminary experiments in which pH either was not controlled or was controlled by additions of KOH were essentially the same as those obtained when pH was controlled using the resin. The results were more reproducible when pH was controlled and it was found to be easier to maintain the desired pH by using the resin than by using KOH. After each experiment, the roots were collected on nylon screen, rinsed for 30 sec with flowing distilled water, and then blotted dry. For determination of cations a 1.0 gram sample of roots was