Uptake of 15N-nitrate by citrus trees in winter and repartitioning in spring

The amount of 15N-nitrate taken up by a satsuma mandarin tree (Citrus unshiu Marc.) in the coldest season (minimum temp. -30C, mean temp. 30C) was about one tenth of that taken up in summer (maximum temp. 280C, mean temp. 230C). In gravel culture, 90% or more of total nitrogen taken up during the cold season was kept in the roots, especially in the fine roots. A large portion of it began to translocate upwards after late February/early March and was used, preferentially, to build up new organs, such as flower, leaves and twigs. This accounted for 40% of the total nitrogen taken up during the cold season; another 36% was partitioned to the existing organs in the top and 24% to the roots. This partitioning pattern is close to that in soil culture. One-year-old leaves acted as a sink for 15N even at the developing stage of flushed shoots. Data on 15N abundances in the amino acids and nitrate indicate that nitrate, which is taken up in winter, translocate upward mainly as nitrate and asparagine in the following spring. (author)