Nitrate accumulation, yield and leaf quality of turnip greens in response to nitrogen fertilisation

Abstract

Two pot and two open-field experiments were conducted to determine the influence of N fertilisation on nitrate accumulation, yield, and leaf quality of turnip greens (“Nabiças”, Brassica rapa var. rapa). The pot experiments were conducted: (1) in a controlled environment (24 h photoperiod at 250±25 μmol s^-1 m^-2, and 20±1 °C ), and (2) outdoors (315 w m^-2 mean solar radiation, and daily mean air temperature between 19 and 26 °C). The response of plant biomass to N from 0 to 250 g m^-3 of pot-soil, in each of the two experiments, was characterised by a steep increase in weight until the level of N reached 100 g m^-3, with a tendency to stabilisation thereafter. There was a clear turning point around 100 g m^-3 N, beyond which plant biomass did not significantly increase in response to N application. When fertilisation with N was increased from at 100 to 250 g m^-3, plant fresh weight increased by 7.4 and 8.6% and nitrate concentration by 279 and 1315%, respectively, in both growth-room and outdoors pot experiments, showing that turnip greens may easily reach conditions of luxury consumption of N. In the two open-field experiments, turnip plants were grown in a clay soil, in winter and in spring at Lisbon, Portugal. Increasing N fertilisation from 0 to 200 kg ha^-1 resulted in plants with higher fresh weight both in winter and spring. Plant nitrate concentration in winter was higher than in spring at the same applied N. The increase of plant fresh weight in response to N had a clear asymptotic tendency at N exceeding 160 kg ha^-1, but the increase of leaf nitrate had a linear trend showing a clear tendency for nitrate accumulation in the leaves. N induced a darker green leaf colour but had no effect on leaf taste and texture. Increasing N fertilisation up to 200 kg ha^-1 seemed to be an effective means of improving yield and quality of turnip greens and of shortening the cropping cycle.