Interpretation of the Dual Isotherm for Ion Absorption in Beet Tissue

Abstract

Abstract. Beet discs aged in 0.5 mM CaS04 develop a capacity to absorb K+ and Cl-from solutions of low concentration. The initial influx of these ions is described by a hyperbolic relationship with concentration in the range 0.01 to 0.5 mM KCI, which is identical with the system 1 absorption isotherm found in other tissues. A second hyperbolic isotherm, attributable to system 2, i's found at higher concentrations (1-50 mm Ka).When the transport of labeled ion to the vacuole is studied by wash-exchanging the bulk of the cytoplasmic label following the absorption period, it is noted that in the range of system 1, isotope influx to the vaouole increases with time as the concentration of labeled ions in the cytoplasm increases, while in die range of system 2, influx to the vacuole is oonstant from the beginning. Diminution of the cytoplasmic specific activity during radioisotope absorption by prefilling the cytoplasm with the analogous unlabeled salt, markedly reduces subsequent radioisotope uptake to the vacuole only in the range of system 1. These experiments suggest that the cytoplasm serves as a mixing chamber, and that the plasma membrane controls ion uptake to the tissue at 'low concentrations, indicating thazt the system 1 isotherm reflects ion movement into the cytoplasm through the plasma membrane. Flux experiments support this conclusion, showing that development with age of the system 1 isotherm corresponds to a quantitatively similar increase in plasma membrane influx in 0.2 mM KC1.At higher concentrations the outer membrane no longer rate-limits entry of ions to the vacuole. Isotope influx under these conditions, described by the system 2 isotherm, presumably reflects movement across the tonoplast.The initial rates of absorption of labeled ions by plant tissues over a wide range of concentrations show 2 or more hyperbolic isotherms which presumably represent carrier mechanisms having different apparent K. values (5). Torii and Laties (23) recently suggested a spatial separation of the 2 principal ion absorption svstems, which differ in their apparent affinities for ions by almost 3 orders of magnitude. On the basis of a comparison of the ion absorption isotherms in largely non-vacuolated root tips and proximal vacuolated cells, they proposed that system 1 (0.01-0.5 mM) reflects ion transport across the plasma membrane, whereas system 2 (1-50 mM) reflects transport across the tonoplast. -Other distinctions, with regard to inhibitor sensitivity '(12), counter ion effects (10) and organic acid synthesis )(24), have supported this separation. The dual mechanism of ion transport, developed so extensively by Epstein (5, 6) was further explored in relation to symplastic transport

Sodium Absorption by Intact Sugar Beet Plants

“…Such an observation would seem to be easily explained by assuming that the second mechanism (operating at Na concentration higher than 1 meq/liter) is not active in Na absorption during early stages of growth (3). The characteristics of ion absorption can change considerably under artificial conditions, i.e., aging (10,15). Rains (15) reported that in freshly sliced bean stem tissue, mechanisms 1 and 2 operate for Na absorption; when the tissue is aged, Na transport is mediated only by mechanism 2. The inability of the second mechanism to absorb Na during the early period of growth could be due to the lack of Na transport to the shoots until the plants are beyond the 10-leaf stage.…”

Compartmentation of Malate in Relation to Ion Absorption in Beet

“…These observations show that the exchange was specific, and suggest that metabolism of malate-14C was of minor importance, at least at 20. At 250 malate-14C was more rapidly exchanged from the tissue, as would be expected (3,19,21), but concurrent metabolisnm complicated the interpretation, as shown by changes in the specific activity of other acids (table II). The time curves for malate-14C exchange are shown, in figure 4.…”

Carrier-mediated Potassium Efflux Across the Cell Membrane of Red Beet

“…On the basis of this analysis, Pitman an,d Saddler suggested that the active transport of potassium occurs at the plasmalemma. Although active potassium uptake, as opposed to a diffusion process, is not firmly established for red beet, Osmond and Laties (9) have demonstrated a typical highatffinity uptake mechanisimi for potassitumr and chloride ions in this tissue, anid have given evidence that this uptake mechanism is located at the plasmalemnma.…”

“…The present study investigates the na-ture of the potassium efflux across the plasmalemma of beet cells, at an external concentration (0.6 mM) such that uptake by the high-affiiity mechanism of salt accumulation is maximal while uptake by the lowaffinity mechanism is relatively slow ' (3,9).…”