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The seasonal course of nitrogen uptake, incorporation and remobilization in different shoot components of winter oilseed rape (Brassica napus L.) was studied under field conditions including three rates of 15N labelled nitrogen application (0, 100 or 200 kg N ha 1 ) and two irrigation treatments (rainfed or watered at a deficit of 20 mm) . The total amount of irrigation water applied was 260 mm, split over 13 occasions in a 7-week-period ranging from 1 week before onset of flowering until 4 weeks after flowering .Nitrogen application and irrigation increased plant growth and nitrogen accumulation . Irrespective of N and irrigation treatment more than 50% of total shoot N was present in the stem when flowering started . At the end of flowering, pod walls were the 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 about 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 remaining 80% being incorporated into seeds. The main component contributing to the response of seed N accumulation to nitrogen application and irrigation was pods in axillary racemes . Up to 20 kg N ha 1 , corresponding to about 10% of final shoot N content, was lost from the plants by leaf drop .Irrigation increased the recovery at harvest of applied N from 30% to about 50%, while the level of N application did not affect the N recovery . 15 N labelled (fertilizer derived) nitrogen constituted a greater proportion of the N content in old leaves than in young 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 balance between N import and export, leading to continuous dilution of 15 N labelled N with unlabelled N.

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The rate of photosynthesis and its relation to tissue nitrogen content was studied in leaves and siliques of winter oilseed rape (Brassica napus L.) growing under field conditions including three rates of nitrogen application (0, 100 or 200 kg N ha-1) and two levels of irrigation (rainfed or irrigated at a deficit of 20 mm).The predominant effect of increasing N application under conditions without water deficiency was enhanced expansion of photosynthetically active leaf and silique surfaces, while the rate of photosynthesis per unit leaf or silique surface area was similar in the different N treatments. Thus, oilseed rape did not increase N investment in leaf area expansion before a decline in photosynthetic rate per unit leaf area due to N deficiency could be avoided. Much less photosynthetically active radiation penetrated into high-N canopies than into low-N canopies. The specific leaf area increased markedly in low light conditions, causing leaves in shade to be less dense than leaves exposed to ample light.In both leaves and siliques the photosynthetic rate per unit surface area responded linearly to increasing N content up to about 2 g m -2, thus showing a constant rate of net CO2 assimilation per unit increment in N (constant photosynthetic N use efficiency). At higher tissue N contents, photosynthetic rate responded less to changes in N status. Expressed per unit N, light saturated photosynthetic rate was three times higher in leaves than in silique valves, indicating a more efficient photosynthetic N utilization in leaves than in siliques. Nevertheless, from about two weeks after completion of flowering and onwards total net CO2 fixation in silique valves exceeded that in leaves because siliques received much higher radiation intensities than leaves and because the leaf area declined rapidly during the reproductive phase of growth.Water deficiency in late vegetative and early reproductive growth stages reduced the photosynthetic rate in leaves and, in particular, siliques of medium-and high-N plants, but not of low-N plants.

Photosynthesis and drought adaptation in leaves of field grown rape (Brassica napus L. cv. Global) were investigated in 1992 under temperate climatic conditions in plants grown in lysimeters in a sand and in a loam soil. Light-saturated net photosynthesis (Amax), leaf conductance to water vapour (ge), leaf water potential (Ψe), leaf osmotic potential at full turgor (Ψπ100), specific leaf area (SLA), spectral reflection index (RI) used as a measure of leaf area, and leaf nitrogen content, were determined in irrigated plants and in plants exposed to soil drying. In the early growth stages before flowering, Amax was 35-45 μmol m-2 s-1 and ge was 1-1.5 mol m-2 s-1. Maximum rates of CO2 assimilation greater than 30 μmol m-2 s-1 were obseved for up to 19 days. Stomata partly closed in ageing leaves maintaining a constant CI/Ca ratio. Both photosynthetic nitrogen use efficiency (NUE; Amax per unit of nitrogen) and photosynthetic water use efficiency (WUE; Amax/ge) were high compared with efficiencies of stems and husks and of other C3 plants. In bracts Amax and ge were 10-15 μmol m-2 s-1 and 0.2-0.7 mol mol m-2 s-1, respectively. Both Amax and ge varied linearly with leaf nitrogen content. When soil water was depleted, both Ψπ100 and RI decreased relative to controls on both soil types before any significant decrease in Ψπ occurred. On loam with slow soil drying SLA, ge and Amax decreased before any significant decrease in Ψe occurred. We suggest that these responses might have been triggered by a non-hydraulic signal transmitted from the roots. When water was more depleted, rape maintained positive turgor down to Ψe of -1.6 MPa. Rape had a high TW/DW ratio (9-11) and a 6 limited ability to adjust osmotically, ΔΨe100 being at most 0.3-0.4 MPa.