Studies of phosphorus and potassium deficiencies in Trifolium subterraneum based on electrical measurements

Abstract

Electrical measurements were used to indicate changes in plant tissue due to alterations in nutrient status. An experiment with Trifolium subterraneum L., grown in soil at five rates of applied P, showed that inadequate P produced high values for (1) resistance to low frequency current (RLM) and (2) radius of the impedance locus (r). With increasing P the values of these two parameters fell, reaching a minimum at the P level giving maximum growth. Increasing the P supply above the optimum had no further effect.A study of K-deficient and of P-deficient plants showed the feasibility of diagnosing either deficiency or of discriminating between the two deficiencies, from changes in certain impedance parameters of the petiole tissue following correction of the deficiencies. Changes in membrane phase angle were consistent with K deficiency delaying and P deficiency hastening aging of the plasma membrane. RLF of both petioles and leaflets was increased by P deficiency, and the value for attached but not detached petioles from P-deficient plants was reduced by treatment with phosphate. RLF values for attached P-deficient leaflets decreased in response to phosphate treatment during leaf development but no longer responded (within 6 days of treatment) after a certain stage of maturity.Changes in RLF and r resulting from the correction of P deficiency show that some factor in addition to increase in tissue K is involved, most likely the plasmodesmata. The differences between RLF changes due to correction of nutrient deficiencies and those due to injury are noted.