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

Abstract

Summanry. Radioactively labeled Na+ absorbed by barley roots wvas sequiesteredl in an intracelluilar compartmenit or compartmeints ("inner" spaces) in which it was onl) very slowly exchanigeable with exogenouis Na'. Absorptioni of this fraction proceede(l at a constant rate for at least 1 hotur. WVhen the rate of Na+ absorption was examinie(d over the range of concentrations, 0.005; to 50 mni1 the isotherm (lepicting the relationi showed (Iiial kiinetics as follows. Over the range, 0.005 to 0.2 mllM, a single AMichaelis-AMenten term (lescribes the relation 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 es-selntial 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.