EFFECT OF NITROGEN AVAILABILITY ON DRY-MATTER PRODUCTION, NITROGEN UPTAKE AND LIGHT INTERCEPTION OF BRUSSELS-SPROUTS AND LEEKS

To analyse differences in nitrogen utilisation, field experiments with Brussels sprouts and leeks were carried out. Dry matter production an d nitrogen uptake during crop growth were studied at different nitroge n application rates. Nitrogen fertilizer application rare stronger aff ected dry matter production, leaf area expansion and nitrogen uptake i n Brussels sprouts than in leeks. When applying all nitrogen before tr ansplanting, Brussels sprouts showed a higher recovery of nitrogen fer tilizer than leeks. This was explained by a higher rate of dry matter production of Brussels sprouts, as a consequence of a faster developme nt of leaf area. A late nitrogen application, whether as a part of a s plit application or not, increased nitrogen uptake stronger than dry m atter production, so that tissue nitrogen concentrations increased. Th e relationship between nitrogen uptake and dry matter production depen ded on nitrogen availability and the crop growth stage, and if all nit rogen was applied before transplanting, the relationship could be desc ribed by an asymptotic function. Plasticity of the plants allowed 'lux ury consumption' of nitrogen taking place when the availability was am ple and 'dilution' of nitrogen when shortage of nitrogen developed dur ing later growth stages. This implies an increasing tissue nitrogen co ncentration with increasing nitrogen application and a decreasing nitr ogen concentration with increasing age. To achieve a near-maximum dry matter production at any time, the nitrogen concentration in the dry m atter should be kept on 2.8-3.1% during the whole growing period for B russels sprouts as well as for leeks. On the other hand a minimum conc entration of 1.2-1.5% in the dry matter was found that still allowed g rowth in Brussels sprouts. In both crops nitrogen uptake increased lin early with leaf area index until maximum leaf area (LAI=4-5) was reach ed and this relationship was neither affected by nitrogen application rate nor by the experimental year. Irrespective of nitrogen applicatio n rate or species 2.3 g above ground biomass per MJ intercepted radiat ion was produced. Therefore measurement of radiation interception by t he canopy can be used as a tool to estimate the nitrogen status of the crop.