Physiological changes in, and phosphate uptake by potato plants during development of, and recovery from phosphate deficiency

Abstract

The development of phosphate deficiency (P‐stress) was observed in rooted sprouts of Solanum tuberosum L. cv. Desiree growing in solutions without phosphate. Shoot growth was inhibited by P‐stress within 3 to 5 days of terminating the phosphate supply, while significant effects on root growth were not recorded until 7 to 9 days. Thus, the shoot:root dry weight ratio decreased from 4.3 to 2.6 over a 10‐day period. Growth in the absence of an exogenous phosphate supply progressively diluted the phosphorus in the plant. The proportional decrease in concentration was similar in roots and shoots over a 7‐day period, even though the former were growing more quickly.The potential for phosphate uptake per unit weight of root increased rapidly during the first 3 days of P‐stress. When the plants were provided subsequently with a labelled, 1 mol m−3 phosphate solution, the absorption rate was 3 to 4‐fold greater than that of control plants which had received a continuous phosphate supply. The increased rate of uptake by P‐stressed plants was accounted for by an increase (3‐fold) in the Vmax of system 1 for phosphate transport and by a marked increase in the affinity of the system for phosphate (decrease in Km). In the early stages of P‐stress, before marked changes in growth were measured, the proportion of labelled phosphate translocated to the shoots increased slightly relative to the controls when a phosphate supply was restored. In the later stages of stress a greater proportion was retained in the root system of P‐stressed plants than in that of controls.In plants with roots divided between solutions containing or lacking a phosphate supply, the increased absorption rate was determined by the general demand for phosphate in the plant and not by the P‐status of the particular root where uptake was measured. By contrast, the poportion translocated was strongly dependent on the P‐status of the root.The restoration of a phosphate supply to P‐stressed plants was marked by a rapid increase in the P concentration in snoots and roots which returned to levels similar to unstressed controls within 24 h. The enhanced uptake rate persisted for at least 5 days, resulting in supra‐normal concentrations of P in both shoots and roots, and in the formation of extensive necrotic areas between the veins of mature leaves. Autoradiographs showed accumulations of 32P in these lesions and at the points where guttation droplets formed on leaves.