CULTURAL TECHNIQUE INFLUENCES ON POST-HARVEST QUALITY OF ROCKET (ERUCA SATIVA MILL.)

Abstract

Soilless culture system is a good technique of plant growth offering guarantees of quality to the current rocket (Eruca sativa Mill.) market. This leafy vegetable is often commercialized fresh, cut, washed, conditioned in packages as ready-to-eat food, giving added-value to the fresh products. They have a high commercial potential deriving from market opportunities based on meals on air crafts, trains and boats and on schooland elderly-meals. However, fresh cut sliced vegetables not only maintain their metabolic activities, but also show a higher respiration rate than whole vegetables. The aim of the research was to obtain rocket plants with good food characteristics and the best post-harvest shelf life optimizing different cultural techniques. The rocket was sown on 29 November 1999 and harvested on 7 February 2000. Trays of 40 and 160 cells were used, filled with two media made by different proportions of peat and perlite. Two irrigation systems were used (Ebb-and-Flow and Flotation) with two nutrient solutions containing 30 and 60 mmol L N, respectively. The statistical design was a split-plot design with randomized blocks, with irrigation as mainplot factor and media x cell-tray x N as sub-plot factor. During post-harvest, leaf shelf life was assessed by fresh weight, and nitrate content was measured. The fresh mass was significantly influenced by the growing medium and by the interaction irrigation x cell number and N x irrigation. The best production was obtained using flotation and 40-cell trays. During post-harvest shelf life, the leaf loss of weight was significantly affected by the interaction growing medium x cell tray x N. The greatest loss of weight was found using 160-cell trays and 30 mmol L N with both the tested media. Soilless culture resulted to be an efficient cultivation system to grow homogeneous, clean, marketable and of high quality rocket plants. INTRODUCTION In recent years, the revaluation of the Mediterranean diet and the development of the consumption of ready-to-eat vegetables led to the expansion of fresh herb market. Rocket (Eruca sativa Mill.) is one of the species whose cultivation has increased in Italy because of the growing interest by consumers (Pimpini and Enzo, 1997). Both open field and protected cultivation are expanding in the Veneto, Campania, Lazio, Puglia, Lombardia, Abruzzo, Sardegna and Piemonte Italian Regions, where rocket production plays an important role. Rocket is an herbaceous species found throughout the entire Mediterranean basin and in Asia, and it is cultivated all year long in Italy where it is eaten mixed with salads (Bianco, 1990). Rocket is commercialized through the supply chains that need continuous productions and constant quality, forcing the farmers to carefully program quantity and quality of their productions. One of the problems in rocket commercialization is the high nitrate content of fresh product, critical for human health. Besides, alternative cultural techniques are required to substitute fumigation by 2005 in the control of soil born diseases, and to control mineral nutrition, with the aim of standardizing the qualitative characteristics of Leafy Vegetable Production (LVP) (Incrocci et al., 2001). The Soilless Culture System Proc. Int. Conf. Quality in Chains Eds. Tijskens & Vollebregt Acta Hort. 604, ISHS 2003 686 (SCS) represents a valid alternative to traditional cultivation techniques due to the full control of the inputs that it provides; it might also contribute to the standardization of vegetable quality. In addition, using mineral and sterile media with a low environmental impact may be an alternative to the practice of soil fumigation. The SCS avoids soil contamination of leaves, allowing the production of clean material at harvest, and consequently reducing the need for washing. The general objective of the work was to set up a SCS to grow rocket comparing different growing practices to obtain a fresh-cut product of high quality in terms of hygiene, marketable standards and shelf life. MATERIALS AND METHODS The experiment was conducted in 1999 at the Experimental Center of the University of Turin, in Carmagnola (TO, Italy; 45°22’ N, 7°65’ E), in a glasshouse provided with environmental control for temperature and shading. Galvanized sheet iron tanks (1350x605x200 mm) laying on a bench allowed to compare the Ebb-and-Flow (EF) and the Flotation (FL) irrigation systems (Fig. 1). a) For the EF treatment, trays were laying on metal wires suspended in the tanks (ebb phase). During the flow irrigation, water was raised to the level of the trays in the tanks, maintained for about 1-2 hours according to plant needs (flow phase), and then released by opening the tank plugs. The trays remained then suspended on metal wires to allow water percolation (ebb phase). Once a week fertigation occurred introducing the nutrient solution for 24 hours, following which the nutrient solution was discharged. b) For the FL treatment, plants floated in water for three days, then water was freed and the tanks stayed empty for two days to allow water percolation. Once a week fertigation occurred introducing the nutrient solution for 24 hours, following which the nutrient solution was discharged and after one day the tanks were filled again with water. Two growing media made of peat and perlite in the ratio 1:1 and 3:1 (v/v) were compared (peat: ‘Rekiva’, 90% organic matter on d.m., 52% organic C of biological origin and 0.2% organic N; perlite: ‘Peralit 25’, expanded perlite with great granulometry). Lime was added to peat (1% in weight) to correct pH. Seeds were sown in containerized 40-cell trays (cell shaped as an upside-down frustum of a cone; cell apical and basal diameters: 55 mm and 40 mm) and 160-cell trays (cell apical and basal diameters: 27 and 10 mm). Eight seeds were planted in the 40-cell trays and two in the 160-cell trays in order to obtain an equal plant density. Two N levels were supplied in the nutrient solution: 30 mmol L N and 60 mmol L N (Table 1). Sowing took place on 29 Nov. 1999 using an unselected line of rocket; the seeded trays were stored in a germination chamber for 4 days at 19°C and then transferred into the greenhouse, where they were arranged in the tanks according to the statistical design. Plants were weekly fertilized pouring 40 L of nutrient solution in each tank, adding Micral® (microelements) at the 2 and the 4 fertigation. Four fertigations were effected, ending 10 days before harvesting, which took place on 7 Feb. 2000. The leaves were cut at 20 mm above the soil level; a 20-g sample of fresh leaves from each tray was oven dried at 70°C for 2 days. Leaf dry weight was measured with an analytical scale and dry mass determined; nitrate content was evaluated using the Dionex® DX 500 ion chromatography system (Dionex Corporation). The remaining harvested plants from each tray were divided into 4 samples of 20 g each; the samples were packaged in plastic boxes, closed with film for food and put in a cool chamber at 4°C to evaluate post-harvest shelf life. Every 2 days after harvesting, cold stored samples were weighed and put back in the cool chamber, except for one sample that was oven dried at 70°C for 2 days. The total number of treatments was 2 (irrigations) x 2 (media) x 2 (trays) x 2 (N) = 16. The statistical design was a split-plot design with 4 randomized blocks, with irrigation as a main-plot factor and media x tray x N as a sub-plot factor. Each replication consisted of a single tray. Analysis of variance was performed on collected data. RESULTS