Growth, N uptake and N critical dilution curve in broccoli cultivars grown under Mediterranean conditions

Adequate nitrogen (N) availability must be ensured in broccoli as it is a high N demand crop, however the N not taken up may cause ground and surface water pollution. Both species-specific growth and N demand predicting models could be useful for rational N fertilization management. Two field experiments were conducted on three broccoli cultivars grown at different N fertilization levels [0 (N0), 75 (N75), and 100 (N100) kg ha−1] and under Southern Italian/Mediterranean conditions (i) to study the growth and the N uptake and (ii) to determine the critical N dilution curve for this crop. Nitrogen fertilization prompted crop growth during the linear phase (growth rate ∼9.0 g m-2 d-1 of dry weight - DW) as a result of the enhancement of the intercepted photosynthetically active radiation (IPAR) and of the efficiency in using IPAR for producing dry biomass (RUE). However, the effect of N fertilization on final aboveground dry biomass production (ADW) (∼4.6 Mg ha-1) was affected by the cultivar specific response to the seasonal variability of the temperature time course. In the ‘Ironman’ cultivar, N nutrition was less effective under suboptimal temperatures, in enhancing ADW and N uptake, because of the reduced leaf growth and IPAR. On the contrary, in ‘Naxos’ and ‘Parthenon’ N fertilization was more effective in limiting the effect on plant growth of the sub-optimal temperatures. Under such conditions, despite a slow reduction in IPAR, both cultivars enhanced RUE, with a concomitant increase in specific leaf N. ‘Naxos’ and ‘Parthenon’ gave higher and more stable yields, ADW, harvest index (HI) and N uptake over the two seasons. Among the N rates, the best N use efficiency in terms of yield (Partial Factor Productivity) and head dry weight gain (Agronomical NUE) per unit of applied N fertilizer, was obtained with the rate of 75 kg ha-1. At the highest N rate, ADW and yield slightly increased, with no effect on HI. Hence, this rate might be considered close to the optimal under the conditions of the experiment. Two cultivar-specific N dilution curves for broccoli have been proposed to account for the difference which emerged between’ Parthenon’ and the other broccoli cultivars mainly for the differences related to the proportion of stems on ADW. Both curves underline that this crop maintains a relatively high fraction of photosynthetically active tissue approaching the harvest compared to other brassicas.