Determination of critical K concentrations in sap from individual leaves or from whole plants using K-interruption experiments

Summary A new method is described for estimating critical K concentrations from K interruption experiments using only 2 treatments. Frequent measurements are made of the growth and K concentration of plants subjected to either continued or interrupted K supply and the data used to define the relation between relative yield and K concentration for the K-deficient plants. Critical concentrations are estimated from the results using a mathematical model of plant growth to interpolate over the critical concentration region of the curve. The method has the advantage that the critical concentrations are determined at the exact time that growth is affected. The method was tested using data from previously published experiments with lettuce in which the concentrations of K were measured in sap from both the total shoot and from individual leaf petioles. The model accurately predicted the form of the relationship between relative yield and K concentration for the total shoot and for young expanding leaves, but consistently deviated from the data for recently matured ones. Average estimates of critical concentration ranged fromca. 18 to 34 mmoll⁻¹ in the young leaves and from 48 to 67 mmoll⁻¹ in the mature ones when Na salts were present or absent respectively. The values for total shoot sap were similar to those for mature leaves. The critical concentrations for young expanding leaves were virtually identical to the minimum believed to be needed for the maintenance of important biochemical processes in individual cells, and suggests that a single critical K concentration for plant sap might apply to a wide range of crops provided an actively growing part of the plant is sampled.