RADIOISOTOPES

Phosphorus (P) is preferentially allocated to the young leaves and the shoot apex for efficient utilization in plants, however, the mechanisms underlying this P allocation remain unclear. In this study, we performed nondestructive sequential imaging and autoradiography to observe and quantify the dynamics of P transport from roots to individual leaves in a model tree, poplar. Within 1 h after ³²P application, the ³²P activity per unit leaf area was nearly uniform across all leaf positions. By 3 h after ³²P application, the ratio of ³²P activity per unit leaf area in the upper leaves gradually increased. These results suggest that preferential allocation of P absorbed through the roots to upper leaves may occur through the following process: first, xylem-mediated transport after absorption, and second, subsequent phloem-mediated translocation.