In field experiments with Brussels sprouts (cv. Kundry) and leeks (cv. Arcona) on a sandy soil, DM production and N uptake during crop growth were studied at different N application rates. N fertilizer application rate affected DM production, leaf area expansion and N uptake more strongly in Brussels sprouts than in leeks. When all N was applied before transplanting, Brussels sprouts showed a higher recovery of N fertilizer than leeks. This was explained by a higher rate of DM production in Brussels sprouts, a consequence of faster leaf area expansion. Late N application, whether as a part of a split application or not, increased N uptake more than DM production, so that tissue N concentrations increased. The relationship between N uptake and DM production depended on N availability and crop growth stage, and if all N was applied before transplanting, the relationship could be described by an asymptotic function. Plant plasticity allowed 'luxury consumption' of N to take place when availability was ample and 'dilution' of N when shortages developed during later growth stages. This implied an increasing tissue N concentration with increasing N application and a decreasing N concentration with increasing age. To achieve near-maximum DM production at any time, tissue N concentration should be kept at 2.8-3.1% DW during the whole growing period for Brussels sprouts as well as for leeks. However, in Brussels sprouts a minimum concentration of 1.2-1.5% DW still allowed growth. In both crops N uptake increased linearly with LAI until maximum leaf area (LAI = 4-5) was reached and this relationship was not affected by N application rate or by experimental year. Irrespective of N application rate or species, 2.3 g above ground biomass per MJ intercepted radiation was produced. Therefore, measurement of radiation interception by the canopy can be used as a tool to estimate the N status of the crop.