Absorption and Transport of Phosphorus in Nodulated Soybean Plants and Diagnosis of Phosphorus Status Using Xylem Sap Analysis

Abstract

Phosphorus (P) is an essential major element for plants. The absorption and transport of P are important for soybean growth and yield, including nodule growth and N2 fixation. Through an analysis of xylem sap, we investigated how nodulated soybean plants absorb PO4 via the roots and transport it to the shoot. The nodulated soybean plants were treated with 0, 50, and 250 μM PO4 concentrations for 1, 3, 7, and 15 days. The PO4 concentration in the xylem sap significantly decreased after 1 day of P deprivation, and then it gradually decreased for 15 days. The high-concentration (250 μM PO4) treatment increased the PO4 concentrations in the xylem sap at 7- and 15-day timepoints but not at the 1- or 3-day timepoints. The soybean plants were treated with 0, 25, 50, 100, 150, 250, and 500 μM PO4 for 3 days. The PO4 absorption rate increased consistently in conjunction with the increase in the PO4 concentration; however, the PO4 concentrations in the xylem sap increased only from 0 to 50 μM PO4 but were constant under higher P concentrations. The soybean plants accumulated extra PO4 in the roots. The PO4 concentration in the xylem sap immediately reflected the P deficiency conditions; thus, this index may be used as an indicator for the diagnosis of P deficiency.