Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

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Abstract
From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, Km and Imax (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in Imax (up to four times), without concomitant change in Km, which remained between 5 and 7 μM. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the Km (i.e. an increased apparent affinity for potassium) from 53 μM to 11 μM, without alteration to Imax. After longer periods of potassium starvation, Imax increased (about two times) while the Km remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.