Screening lettuce cultivars for low nitrate content during summer and winter production

Abstract

SummaryConcerns about high nitrate levels in vegetable produce have led the European Union to introduce limits on nitrate concentrations in some salad crops including lettuce (Lactuca sativa L.), with the aim of decreasing nitrate intake by consumers. These limits are likely to create problems for lettuce growers in northern European countries such as the UK, where leafy vegetables can accumulate high nitrate concentrations in leaf tissues due to low light levels, especially during the winter. One option to overcome this problem is the use of low nitrate-accumulating genotypes. The objective of this work was to screen a selection of soil-grown glasshouse lettuce cultivars for head weight and nitrate concentration during the summer and the winter seasons. Two pairs of trials were carried out using a Trojan square design, following commercial cropping practices. Eight long-day commercial lettuce cultivars were grown for both early- and late-summer harvest, and eight short-day commercial cultivars for both early- and late-winter harvest. Both pairs of experiments included examples of cultivars belonging to the following types: Butterhead, Cos, Batavia, Curly, Oakleaf, Lollo Rosso and Lollo Bionda. It was found that the commercial fresh weight of the heads depends on season (summer or winter) and cultivar, with significant interaction between cultivar and experiment (i.e. harvest date within season). By comparison, nitrate concentration showed not only great variability between cultivars in general, but also between the main lettuce types and between cultivars within the Butterhead type. The data for the summer crops suggest that ‘Vegus’ (in particular) tended to accumulate less nitrate than the other cultivars of the Butterhead type. For the winter crops, although no single cultivar was found to exhibit consistently lower nitrate concentrations at harvest, the means for the Butterhead group were generally lower than the average of the cultivars for the other types. Several of the long-day cultivars were found to exceed the EU maximum summer nitrate limit of 3500 mg kg–1 fresh weight, whereas relatively few of the short-day cultivars had nitrate concentrations greater than the corresponding 4500.mg kg–1 limit for winter harvested crops.