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

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 N 2 fixation. Through an analysis of xylem sap, we investigated how nodulated soybean plants absorb PO 4 via the roots and transport it to the shoot. The nodulated soybean plants were treated with 0, 50, and 250 μM PO 4 concentrations for 1, 3, 7, and 15 days. The PO 4 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 PO 4 ) treatment increased the PO 4 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 PO 4 for 3 days. The PO 4 absorption rate increased consistently in conjunction with the increase in the PO 4 concentration; however, the PO 4 concentrations in the xylem sap increased only from 0 to 50 μM PO 4 but were constant under higher P concentrations. The soybean plants accumulated extra PO 4 in the roots. The PO 4 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.