Critical nitrogen concentration in lettuce

Field trials were carried out in 1996, 1997 and 1999 in Central Italy (Tevere Valley, 43oN 12oE, elev. 165 m) in lettuce [Lactuca sativa L. var. capitata (L.) Janchen, cv. Audran] to determine the critical % N dilution curve and to analyse the effect of N availability on light interception and radiation use efficiency. Lettuce was transplanted in late spring at a density of 10 plants m-2, with rows 0.30 m apart. Increasing fertiliser-N levels were applied as ammonium nitrate at transplanting. Plants were sampled at weekly intervals until the final harvest to determine dry weights (DW) of stems and leaves, LAI, reduced-N and nitrate-N concentrations. Canopy light interception was determined as a function of the crop ground cover. The radiation use efficiency (RUE) was determined as the slope of the linear regression of dry weight on cumulative intercepted light. Critical N concentration was determined as proposed by Justes et al. (Annals of Botany 74: 397-407, 1994). The critical dilution curve for total-N was % total-Ncr = 4.56 DW-0.357 and for reduced-N was % reduced-Ncr = 3.79 DW-0.290. The critical dilution curves were applied when above-ground dry weight ranged between 0.9 and 3.4 t ha-1 (i.e. from about 25 DAT to final harvest). The two curves differed because lettuce accumulated nitrates which do not drive growth. However, nitrate concentration at harvest was always lower than 2500 mg NO3 kg-1 fresh weight (i.e. EU limit). In all years, the critical “uptake” curve was very close to the uptake curves obtained with the fertiliser-N rate of 100 kg ha-1 but the highest marketable yields were obtained with 100 kg N ha-1 in 1996, 200 in 1997 and 150 in 1999 due to the increase of water content related to the osmotic effect of nitrates. Increasing N supply increased light interception throughout the growth cycle and slightly increased RUE.