Transport and partitioning of phosphorus in wheat as affected by P withdrawal during flag-leaf expansion

Increasing P-use efficiency within the plant is one of the acclimations to P-limiting conditions. In this work, we studied the effects of P withdrawal during flag-leaf expansion on sink-source relationships and P-use efficiency in two detillered wheat cultivars (Triticum aestivum L., CA9325 and JM2) under controlled conditions. The study period was divided into two phases of one month each. In the first period after withdrawing P from the medium, the rates of dry weight gain were unaffected compared with the control plant. However, the net dry matter deposition in the ear, and P remobilization within the plant were accelerated in both cultivars. In control plants and in the first period, P transported in the xylem came mainly from the roots’ current uptake in both cultivars; in the second period, however, phloem retranslocation of P from the shoot and cycling through the root contributed 86% in CA9325 and 95% in JM2 to the xylem-transported P. In the P-deficient plants of both cultivars, almost all of the P transported in the xylem was remobilized, exported from vegetative organs and recycled through the phloem. Over the entire duration of the experiment, the net dry matter deposition and P allocation to grains were not synchronous, indicating independent regulatory processes. Although withdrawing P from the medium markedly reduced the net dry weight gain of whole plants in both cultivars, the final dry weight of the grains was hardly influenced. The percentage of grain dry weight to whole plant dry weight increased from 42.5% in control plants to 44.7% in P-deficient plants in CA9325, and from 41.0% to 45.0% in JM2, and that of P increased from 24.8% to 87.7% and from 25.5% to 84.3%, respectively. The results showed that withdrawing P from the medium during flag-leaf expansion did not influence grain growth and its final P content. The possible mechanisms to regulate P redistribution and reutilization in plants are discussed.