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Growth, N uptake and light interception were studied in processing tomato grown in the field in Central Italy (Tiber Valley, 43°N, elev. 165 m) at different levels of fertiliser-N in 1996–97–99. Plant N concentration declined during the entire crop cycle. The relationship between the total above-ground dry biomass (DW, Mgha⁻¹) and critical N concentration (%Nc, the minimum N concentration required for maximum plant growth, calculated according to Justes et al., Ann. Bot., 1994, 74, 397–407) was %Nc=4.53 DW^−0.327 for total-N, and %Nc=3.90 DW^−0.270 for reduced-N. The critical dilution curves were applied when the above-ground dry biomass varied between 1.2 and 12.4 Mgha⁻¹ and for stages of development ranging from the onset of fruit growth (∼40 days after transplanting) to maturity. Increasing N availability increased dry matter accumulation. Relationships between crop total-N uptake (Nupt, kgNha⁻¹ …