Effects of winter buds on winter nitrogen uptake and allocation in Pinus densiflora saplings

Abstract

Studies of nitrogen (N) use by plants have confirmed some winter N uptake; however, the mode of regulation of plant N use in winter is unknown. The regulation of N use by plants during winter may differ from that in the growing season, as plant growth strongly affects N use. We investigated the effects of winter buds on winter N use by Japanese red pine (Pinus densiflora), as a previous study demonstrated that N absorbed during winter contributes significantly to leaf growth in the following spring. We conducted a bud pruning experiment during winter to examine the effects of winter buds on winter N uptake and allocation among plant organs using 15N labeling. Over a three-week labeling period, the 15N content in roots increased to 0.20 ± 0.12 mg N g DW−1, which is equivalent to 1.8 ± 1.1 % of the total N content in the roots. However, this absorbed 15N rarely appeared in needles and buds. Bud pruning did not affect 15N uptake and allocation. On the other hand, significant total N retranslocation was found within the crowns of saplings without bud pruning, but N was not retranslocated in bud-pruned plants. The bud pruning experiment indicated that N was retranslocated from needles into winter buds. Since soil N availability changes dramatically and is unstable in many forest ecosystems, N contained in needles would be a more stable source of N than newly absorbed N.