Abstract: | Barley (Hordeum vulgare L.) varieties differed in their raponseto K+]0, in terms of their utilization efficiencies (UE = freshweight. concentration of K+]11). At low K+]0, Compana,an efficient-non-responder demonstrated superior utilizationof absorbed K+. On the other hand, at high K+]0, Fergus (anefficient responder) and BT 334 (an inefficient responder) hadhigher UE values for K+ than Compana which performed poorlyat this K+]0. Kinetic parameters for K+ activation of the enzyme pyruvatekinase from 12 barley varieties, representing a range of UEvalues, were determined. Varieties showed substantial differencesin their Vmax values (P<0·01). Compana, an efficientvariety, had the highest Vmax (31 µmol g1 freshwt. h1) which was about 50% higher than that of Mingo,an inefficient variety. By contrast, Km values for the enzymeswere not significantly different among varieties The mean valuesfor all varieties (3·9±0·15 mol m3K+) is far below the estimated cytoplasmic K+] (100-200 molm3). It is, therefore, unlikely that differences in theutilization of K+ by these varieties can be explained on thebasis of differential requirements for (K+) activation of theseenzymes. Alternative possibilities for differences in the utilizationof K+ are discussed. Key words: K+ utilization efficiency, Pyruvate kinase, Barley varieties |