NITROGEN INCORPORATION AND REMOBILIZATION IN DIFFERENT SHOOT COMPONENTS OF FIELD-GROWN WINTER OILSEED RAPE (BRASSICA-NAPUS L) AS AFFECTED BY RATE OF NITROGEN APPLICATION AND IRRIGATION

The seasonal course of nitrogen uptake, incorporation and remobilizati on in different shoot components of winter oilseed rape (Brassica napu s L.) was studied under field conditions including three rates of N-15 labelled nitrogen application (0, 100 or 200 kg N ha(-1)) and two irr igation treatments (rainfed or watered at a deficit of 20 mm). The tot al amount of irrigation water applied was 260 mm, split over 13 occasi ons in a 7-week-period ranging from 1 week before onset of flowering u ntil 4 weeks after flowering. Nitrogen application and irrigation incr eased plant growth and nitrogen accumulation. Irrespective of N and ir rigation treatment more than 50% of total shoot N was present in the s tem when flowering started. At the end of flowering, pod walls were th e main N store containing about 30-40% of shoot N. The quantities of N remobilized from stems and pod walls amounted in all treatments to ab out 70% of the N present in these organs at mid-flowering. At harvest, stem and pod walls each contained about 10% of total shoot N, the rem aining 80% being incorporated into seeds. The main component contribut ing to the response of seed N accumulation to nitrogen application and irrigation was pods in axillary racemes. Up to 20 kg N ha(-1), corres ponding to about 10% of final shoot N content, was lost from the plant s by leaf drop. Irrigation increased the recovery at harvest of applie d N from 30% to about 50%, while the level of N application did not af fect the N recovery. N-15 labelled (fertilizer derived) nitrogen const ituted a greater proportion of the N content in old leaves than in you ng leaves and increased with age in the former, but not in the latter. Relative to soil N, fertilizer derived N also contributed more to the N content of vegetative than to that of reproductive shoot components . Small net changes in shoot N content after flowering reflected a bal ance between N import and export, leading to continuous dilution of N- 15 labelled N with unlabelled N.