Critical nitrogen concentration in processing tomato

Growth, N uptake and light interception were studied in processing tomato grown in the field in Central Italy (Tiber Valley, 438N, 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, Mg ha1) 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 DW0.327 for total-N, and %Nc/3.90 DW0.270 for reduced-N. The critical dilution curves were applied when the above-ground dry biomass varied between 1.2 and 12.4 Mg ha1 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, kg N ha1) and accumulated dry matter in the aerial biomass (DW, Mg ha1) fit very well to the model Nupt/10 a DW1b (Lemaire and Salette, Agronomie, 1984, 7, 381/389) for each fertiliser-N level in all three experiments. In all years the ‘critical uptake’ curve (Nupt/45.3 DW 0.673) was between the uptake curves obtained with the fertiliser-N rates of 100 and 200 kg ha1. Increasing N supply increased the amount of absorbed PAR accumulated throughout the growth cycle and slightly affected the Radiation Use Efficiency. Results are discussed also taking into account the effect of N availability on source-sink relationships.