Abstract. Mutual effects between Mn. Ca, and Mg were studied during steady-state absorption experiments with excised barley roots. Calcium appeared to enhance the rate of Mn absorption; whereas, Mg had a highly depressive effect. The combination of both Ca and Mg was even more inhibitory to Mn absorption than Mg alone. Manganese had no effect on the usual negligible Ca absorption by this tissue, but effectively inhibited the absorption of Mg. Although divalent cation absorption from the Ca-Mg-Mn system was essentially nil, K absorption was greatly stimulated in the presence of these cations.These mutual effects and others reported in the literature are explained by the hypothesis that selectivity in ion absorption results from cation-induced conformational changes in the structure of the carrier molecule.In a previous paper (10), evidence was presented for the metabolic transport of Mn into excised barley roots. Absorption was relatively rapid and was affected bv concentration, pH, etc., much as that of other inorganic cations. To further study the mechanism of Mn absorption and its similarities to that of other metabolically absorbed cations, the influence of several monovalent, divalent and trivalent cations on the absorption of Mn was examined. The effects of 2 of these cations, Ca and Mg, are discussed here.The role of Ca in regulating the absorption of ions is well recognized. Its effects on the uptake of the monovalent, al,kali cations range from highly stimulatorv to strongly inhibitory, depending on the ion and the H+ concentration (8). Furthermore, Ca has been found to be highly effective in controlling the relative selectivity of absorption of these ions from mixed solutions (4, 7). involves configurational changes, the hypothesis is promoted that selectivity in ion transport results from cation-induced conforniational changes in the structure of the membrane or ion carrier(s). Materials and MethodsExcised roots of 5-day-old barley seedlings (Hordeum zulgare L., var. Trebi) were used in all experiments. The planits were growwn and the roots were prepared and treated as described previously (10). The experimlental temperature was 25 ± 0.50 and the pH was 5.0 ± 0.2 maintained by small additions of 0.1 N HCl. Distilled water and analvzed reagent-grade chloride salts were used in all experiments. Absorption periods of 1 and 6 hr were chosen oIn the basis of the previous work so that rates of metabolic absorption occurring during the steady-state phase could be measured (10

Abstract. Short-term absorption studies with 5-day-old excised barley roots revealed that the basic aspects of Mn absorption were similar to those of other metab-olically absorbed cations. Following an initial non-metabolic equilibration with the root, Mn was absorbed for several hours at a slower steady-state rate comparable to that of other inorganic cations. Complete or nearly complete inhibition of the steady-state phase by low temperature, dinitrophenol, and azide provides strong evidence that Mn transport into this tissue was metabolically mediated. Within limits, the rate of transport was strongly dependent upon the concentrations of Mn and the hydrogen ions in the ambient solution. Absorption increased rapidly with increasing concentrations of Mn up to 1 meq per liter. Above this concentration, the rate leveled off, apparently due to a saturation of the transport mechanism. Within the physiologioal pH range in which Mn is soluble (below pH 7), absorption increased greatly with decreasing hydrogen-ion concentration.Altho,ugh numerous investigations of ion uptake by excised root tissue have been conducted, relative:ly 1little attention has been given to Mn. In 1934, Laine (4) studied the absorption of Mn and other ions by decapitated roots of Phaseoluts multiflorus. Analyses of both the exudate and the roots revealed an accumu,lation and retention of Mn by the roots. Using slices of carrolt tissue, Stiles and Skelding (10) found that Mn uptake occurred in 29 distinct phases, a rapid initial uptake followed by a pro,longed slower absorption. This 2-step process, characteristic of the uptake of other cations, was in(terpreted as consi,sting of the usual nonmetabo,lic and metabolic phases, respectively. Su'bsequent experiments with red beet discs cionfirmed this interpretation (9). More recently, Page and Dainty (6) 'have reported -that the uptake of !Mn by excised oat rooits is nonmetabolic. Both the characteristic rapid initial uptake and the folllowing slower steadystate tuptake occturretd independentlly of metabolic .ac-tivity. 6.0 or greater, nylon bags containing analytical grade Bio-Rad AG1-X8 anion exchange resin in the OH-form were dipped into the solution (7). All salts used were chlorides of analyzed reagenit grade.At the end of the absorption periood, the roots were coillected on a nylon screen and washed by pouring 3 4liters of distilled water over them. After drying overnight at 700, the roots were digested in nitric and perchloric acids and diluited to 100 ml. 527 www.plantphysiol.org on May 11, 2018 -Published by Downloaded from

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