Short-term K+(86Rb) influx and its regulation by root K+concentration was studied in barley varieties, using plants grown in complete nutrient solution at constant concentrations. The varieties employed in this study exhibited substantial differences not only in K+influx but also in the intensity and the pattern of regulation of K+influx. In the high-potential growth-rate varieties these K+uptake characteristics were found to correlate well with their growth responses to K+supply reported earlier by Siddiqi and Glass. Predictions of K+influx, based upon kinetic constants and internal K+concentrations derived from steady-state growth experiments, were found to correspond well with the observed fluxes for plants grown under these conditions over a wide range of root K+concentrations. These predictions also provided good estimates of influx in CaSO4-grown plants for intermediate levels of root K+concentration. However, at low root K+concentration, predictions greatly overestimated observed fluxes, while at high root K+, influx was underestimated. Similarly, when kinetic constants derived from CaSO4-grown plants (whose root K+concentrations were increased by rapid loading) were applied to steady-state plants, predicted influx values were close to observed in the intermediate range of root K+concentration. However, at high root K+, influx was overestimated. These adjustments serve, in the steady state, to maintain tissue K+concentration within rather narrow limits.
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