An assessment of the depletion technique for comparative measurement of phosphorus uptake in plants

This paper examines the depletion of a standard 32P solution by 221 plants, either cereals or clovers, as a means of determining the recognized parameters, Vmax, Km and Cmin, in the kinetics of phosphorus uptake, and attempts to relate them to comparative phosphorus uptake by the plants, with a view to recognizing the more efficient ones.No single depletion curve was found. The data were grouped into six distinct curve types, based on the change in solution concentration (C) with time (t) and the rate of change in concentration (dC/dt) with concentration. The six types were part of one general curve which could be separated into 3 phases.In Phase 1, solution concentration was non-limiting, and plant performance was the principal determinant in depleting the solution. In the second and third phases, plant performance became progressively less dominant and the solution concentration more dominant in determining the concentration of the remaining solution.For comparative phosphorus uptake purposes, it is suggested that only the initial rate of uptake (Phase 1) is worth measuring and that this has attendant difficulties; the issue whether these kinetic parameters are real, or measurements of the model selected, is raised and discussed.No relation was found between these kinetic parameters and the efficiency of phosphorus use by subterranean clover plants grown in soil. Despite these practical shortcomings, variability in uptake capability between individual plants, even of the same line, was demonstrated. This plant variability offers a new area of plant nutrition for exploitation.