Visualization of seasonal shifts of phosphorus re-translocation and overwintering recycling in a shortened annual cycle system of the deciduous poplar tree (Populus alba L.).

(1) the research conducted, including the rationale: Phosphorus (P) is an essential macronutrient for plant growth. In deciduous trees, P is remobilized from senescing leaves and stored in perennial tissues during winter for further growth. To clarify how deciduous trees utilize the internal P through a year, seasonal re-translocation routes and the reuse of P over winter were investigated. (2) methods: We analyzed the seasonal re-translocation and accumulation of P in poplar trees (Populus alba. L) cultivated under 9a shortened annual cycle system9, which mimicked seasonal phenology in a laboratory. The real-time radioisotope imaging and the macro- and micro-autoradiography with 32P and 33P were used to reveal the organ and tissue level P allocation. (3) key results: The direction of phosphate re-translocation changed seasonally. In the growing season, P was mainly re-translocated from a leaf to upper parts via phloem. During senescence, P was re-translocated to lower perennial parts, and also to inner xylem parenchyma cells. Phloem-xylem exchange of re-translocated P occurred in the stem. After bud burst in spring, stored P was re-translocated to the whole plant and mainly accumulated in new shoots. (4) the main conclusion, including the key points of discussion: Poplar trees change the routes of P re-translocation longitudinally and radially depending on the season, and recycle internal P throughout the year.