NITROGEN FERTILIZATION OF NEW CULTIVARS OF LETTUCE

Abstract

The present work aimed to study production and accumulation of nitrate by new cultivars of lettuce in response to nitrogen doses applied to the soil. Two field trials were carried out during 1999-2000 and 2000-2001, the soil had received 30 and 64 kg·ha of N, respectively, for the two trials as a pre-transplanting fertilization. Our main objective was to compare a control unfertilized treatment with two nitrogen doses (75 and 150 kg·ha) applied by fertigation to different cultivars of lettuce belonging to the following types: Lollo Bionda, Canasta, Lollo Rossa, and, in the second trial, Oakleaf. The Lollo Rossa cultivars produced 26 and 56% less than the Canasta type, respectively, in the first and the second year, probably due to the absence of a real head in the Lollo type. The application of nitrogen by fertigation increased yield compared with the unfertilized control, but with no difference between doses and rates of fertilization. On the other hand, nitrogen posttransplanting fertilization increased the nitrate content in leaves. The Lollo cultivars accumulated more nitrate than the other ones. The nitrate levels were greatly below the limit of 4.000 mg kg fw imposed by EU Regulation No. 563/2002. INTRODUCTION High availability of nitrogen fertilizers, not tailored to the crop requirement, can increase the environmental pollution and reduce the quality of the product, as a consequence of accumulation of nitrate in the edible portions, compromising its marketability. The choice of the cultivar represents a crucial decision for a successful crop. This is even more important for lettuce, due to its short cultural cycle and the rapid change of the available cultivars. These factors have important implications for nitrogen fertilization and leaf nitrate accumulation. In fact, lettuce has been shown to be a high nitrate accumulating species, above all when fertilized with nitrogen doses higher than the requirement and/or in low light conditions (Blom-Zandstra and Lampe, 1985). The present research work was carried out with the aim of studying the response of new cultivars of lettuce (Lactuca sativa L. var. capitata L. and var. crispa L.) to nitrogen fertilization, in terms of production and accumulation of nitrate. MATERIALS AND METHODS Two trials were carried out in a commercial farm (“Ripagnola”), at Polignano a Mare (Apulia, southern Italy, latitude 40°59’ N, longitude 17°13’ E) on a loamy soil with a medium content of nitrogen (N) (1.2 mg g) and organic matter (1.9%), during the autumn-winter period in 1999-2000 and 2000-2001. A split plot design with three replications was made, randomizing N levels and the unfertilized control in the main plots (75 and 150 kg·ha, the first dose applied at one or two times) and cultivars into the sub137 Proc. IS Towards Ecologically Sound Fertilisation Strategies for Field Vegetable Production Eds. F. Tei, P. Benincasa and M. Guiducci Acta Hort. 700, ISHS 2006 plots. Cultivars were: ‘Brigida’ (Lollo Bionda), ‘Capora’ (Canasta), and ‘Malibu’ (Lollo Rossa) in the first trial, ‘Camaro’ (Canasta), ‘Cherize’ (Oakleaf), ‘Lobi’ (Lollo Bionda), and ‘Naska’ (Lollo Rossa) in the second trial. Plants (at the 3-5 true leaf stage) were transplanted on 3/11/1999 and 9/11/2000, in rows with a crop density of 11.1 plants m. Before transplanting, an organo-mineral fertilizer (NPK 5-7-9) and a NPK fertilizer with 16% N were applied, in the first and the second trials, respectively, at doses of 0.6 and 0.4 t·ha. The soil was treated with propyzamide and mulched with 0.05 mm thick black polyethylene. Nitrogen fertilizer (calcium nitrate) was applied in a single fertigation for the two levels of N (on 16/12/1999 and 27/12/2000 in the two years), and in two fertigations when the dose of 75 kg·ha was fractioned applying the first half 45 days after transplanting and the remaining part a month later. The plants were harvested on 18/02/2000 and 20/02/01 in the two trials. The following parameters were determined on the shoot portion: leaf area (LICOR 3100 leaf area meter, LI-COR), leaf number, fresh and dry weight of marketable heads (dried to constant mass in a forced-draft oven at 65°C). Nitrate content in dried and finely ground samples was determined by ion chromatography (Dionex model DX500; Dionex Corporation, Sunnyvale, CA) according to Santamaria and Elia (1997). Data were analysed statistically using the SAS general linear model (Cary, NC).