Sodium Absorption by Barley Roots: Role of the Dual Mechanisms of Alkali Cation Transport

Abstract

Summatnry. Radioactively labeled Na+ absorbed by barley roots wvas sequiesteredl in an intracelluilar compartmenit or compartmeints ("inner" spaces) bettween the concentrationi of Na. and the rate of its absorptioni. The mechanism of Na' absorption operatiing over this range of concenitrationls, mechanism 1 of alklali catioin transport, is severely inhibited in the presenice of Ca2' and virtually rendered inoperative for Na+ transport by the combined presence of Ca2+ and K+. The iechanism is eqtually effective in Na+ transport whether Cl-or F-is the anion, but is somewhat inhibited when the anion is S042-.Over the high range of concenitrations, 0.5 to 50 mm Na+, a second, low-affinity mechanism of Na+ absorption comes into play. In the presence of. Ca2+ and K+, this mechainism 2 is the only onie to transport N,Ia+ effectively, silnce \a+ absorption via mechanism 1 is virtually abolished tinder these conditions. Anaerobic conditions, low temperatture, and the uncotupler, 2, 4-dinitrophenol, inhibit Nad absorption both at low and high Na+ concentrations.Sodium is the predomiiinant cation in the world's ocealns and(I the preponderanit soltible cation in many of the soils of those very large land areas lying in the ari(l and( semi-aril regions of the earth. However, its absorption by planits and plant tissnies has been stuldie(d muich less than that of potassiuim, the only alkali metal knowin to be uiniversally esselntial for all higher plants (7). In the present work, colncepts and findings gainied primarilv from experimelnts on the absorption of potassium (and rulbidiuim) have been applied to the stuidy of soditum absorption by excised barley roots.The prevriolus conclusions referre(d to are 2-fold. 1) Calcium ions in the medium play a crtucial, indeed an indispensable, role in selective ion transport by plant tisstues. This is so for absorption of

Influx of Na+, K+, and Ca2+ into Roots of Salt-Stressed Cotton Seedlings

“…3) is in agreement with other findings (10,15,20,26,27). This does not necessarily contradict the findings of Kent and Lauchli (18), who found that the tissue concentrations of Na in cotton seedlings were unaffected by external Ca2`concentrations at 200 mM NaCl.…”

Sodium Absorption by Intact Sugar Beet Plants

“…At high Na concentration, low K concentration did not have any effect on Na absorption, but at a high K concentration K competed with Na absorption. Therefore, to explain the present results we have to assume the operation of two absorption mechanisms in sugar beets for the Na ion, as reported earlier for excised barley roots (12)(13)(14). The first mechanism operates over a low range of Na concentrations; presence of K renders this mechanism completely unavailable for Na.…”

“…1 and 2), and earlier ones (3). Results like these have been interpreted to mean that, in solutions low in K, Na absorption depends on the K concentration if Na concentration is low, but not if Na concentration is high (4,12,13).…”

“…Since then the possibility has been raised (3) that in a long term study with intact sugar beet plants a dual mechanism might exist for Na absorption similar to the one reported in excised barley roots (12)(13)(14)16).…”

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

“…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 rubidium by excised barley roots is stimulated by CaCl2, CaBr2 and halide salts of several other polyvalent cations (7,14,20,21).…”