Abstract
In the coldest season, uptake, reduction, organization and polymerization of 15N-nitrate by a satsuma mandarin tree (Citrus unshiu Marc.) were examined. A satsuma mandarin tree was capable of nitrate-uptake and reduction to amino acids in a low temperature regime (minimum temp. -4°C, maximum temp. 9°C. mean temp. 2.5°C in both air and gravel bed). Nitrate reductive activity of fine roots in vivo showed 24% of the maximum activity at 5°C and 10% at 0.5°C. No accumulation of nitrate was found in the fine roots.
Asparagine was most abundantly formed from 15N-nitrate in the fine roots in cold conditions, followed by γ-aminobutyrate, alanine, aspartate and glutamate. Proline formation was relatively low as compared with that in summer. In the leaves 15N-amino acids were formed in negligible amounts in cold temperatures, showing a sharp contrast to the intensive incorporation of 15N into amino acids in summer.
Incorporation of the newly synthesized amino acids into protein proceeded steadily in the fine roots. Leucine (+isoleucine), phenylalanine, arginine and proline were abundantly incorporated as compared with in summer. This indicates that protein formed in the cold season is qualitatively different from that formed in summer.
