An Anomaly in Potassium Accumulation by Barley Roots

Abstract

When excised barley roots were incubated for 2 to 24 hr in KCI solutions of 0.01 to 0.1 mM concentration, the K+ accumulation curve exhibited an anomalous appearance (8, 9). As KC1 concentration was increased from nil to 0.04 mm, K+ uptake increased as expected. However, further increase in KCI concentration to 0.06 mm resulted in decreased absorption of K+. This accumulation peak always occurred at approximately 0.04 mm and was not shifted appreciably by changes in Na+ concentration or species of counteranion.The literature is replete with examples of absorption against concentration curves which do not exhibit this phenomenon (2,3,5,6,16). Generally the absorption curves for salt concentrations less than 1 mm are reported to be hyperbolic with near maximal rates of the low concentration mechanism observed at approxiniately 0.2 mm. The investigations cited above were concerned primarily with absorption kinetics and were limited to absorption periods of 10 min to 3 hr. In most kinetic studies of K absorption, K uptake is followed by the use of 86Rb as a tracer.This (8,9). Figure 1 shows the results of a 0.1 mm KCl-NaCl replacement series experiment in which CaS04 was omitted. Accumulation curves for high K+-low Na+ solutions were not changed markedly by the omission of Ca2+ (compare with Fig. 1 of Ref. 9); however, omission of Ca2+ from low K+-high Na+ solutions strikingly reduced the apparent preference for K+ over Na+. In the absence of Ca2+ the peak of the K+ accumulation curve occurred at 0.01 mM, and the roots actually lost K to solutions containing 0.02 mm K+ and 0.08 mm Na+. Figure 2 shows the effect of Ca2+ concentration on K+ accumulation in 24 hr from KCI solutions varied in concentration from 0.005 to 0.2 mz. In the absence of Ca2+, K+ accumulation showed a peak at the lowest KCI concentration used (0.005 mM) and a trough at 0.015 mm. With KCl concentrations greater than 0.02 mm the KCl accumulation curve was hyperbolic. The peak in the K+ accumulation curve was shifted to higher KCl concentrations as the solution Ca2+ concentration was increased. Varying the Ca2+ concentration from 0.02 to 1.0 mm had little effect on K+ accumulation from KCI solutions greater than 0.1 mm.Since the studies of Viets (17) on the stimulating effect of calcium on ion uptake, considerable attention has been given to the relationship of Ca2+ to ion uptake. The results have not always been consistent, with both reported stimulation and inhibition of K+ uptake by Ca2+, and the foregoing results might partially explain these observations. Part of the data from Figure 2 is plotted in a different manner in Figure 3. In Figure 3

Aspects of Potassium Accumulation by Stomata of Vicla Faba

“…Most other higher plant systems show stimulatory effects of calcium on potassium uptake and especially retention, although there are some exceptions reported for excised roots (Hiatt 1970), stems (Rains and Floyd 1970), and leaves (Osmond 1968). Figure 14 does indicate that as external potassium concentration increases, the inhibitory effect of calcium is reduced, suggesting that potassium and calcium are competing in some way.…”

“…However, several potential sources of error had become evident in the approach, including tracer uptake by epidermal cells which remain intact in the epidermal strips (Humble and Hsiao 1970), and the unsuitability of 86Rb as a tracer for potassium (Hiatt 1970). Also conclusions about the role of potassium required accurate estimations of changes in guard cell potassium content and hence knowledge of equilibration times for internal specific activity.…”