Production and nitrate content in lambs lettuce grown in floating system

Small-size leafy vegetables can be profitably cultivated in floating system all year round to obtain ready-to-use vegetables. In the present study two cultivars of lamb's lettuce (Valerianella locusta L. Laterr.) were used: 'Baikal' and 'Valerianella a seme grosso di Olanda', both characterised by long and pale green leaves and suitability for greenhouse growing. The growing set-up consisted of benches containing the nutrient solution and floating boards to support the plants. A density of 1200 plants m-3 was adopted. Approximately 50 days after sowing fresh leaf production was 1900 g m-2, with a dry matter content of 7.5 g 100 g fresh weight (fw)-1. Nitrate content was nearly 4000 mg kg fw-1. The replacement of the nutrient solution with rain water three days before harvesting resulted in nitrate reduction in leaves by one third. Moreover, the obtained product had no discarded parts, substrate or pesticide residues. In the management of the nutrient solution the usual pH increase due to NO3 --N absorption was recorded and continuous correction with acids was needed. The permanence of the nutrient solution in the tanks or benches for a long time proved to diminish oxygen level in the nutrient solution. This aspect is crucial in the high growth phase when solution enrichment with oxygen is needed by recirculating the solution between the benches and the relevant tanks or by insufflating air in the nutrient solution. The daily water consumption increased with increasing growth. Total water consumption was 51 L m-3.