Soilless Culture Technology for High Quality Lettuce

The aim of this research was to evaluate butterhead lettuce quality in a soilless culture system. Butterhead lettuce cultivar “Lores” was cultivated, and two experiments were performed. In the first, floating system (FS) and soil culture (SC) were compared in fall (autumn) prduction [2002 2003], while in the second, substrate culture (PC) (plastic gutter filled with perlite) with three different levels of calcium (0, 3, and 6 me L) in the nutrient solution were compared in spring production [2003]. Growing cycles varied according to the culture system. In general, plants harvested from the soilless culture had a lower dry weight and leaf area, however significant differences were observed in productivity. Nitrate content was significantly affected by the production system. Ascorbic acid content showed no differences resulting from treatments. Floating system results showed some variation between the two years of the experiment. Inorganic cation differences were observed between plant harvest from all systems, especially in K and Mg. Colour intensity changes between production systems were insignificant. No variation in incidence of tip-burn relating to different calcium levels was found in lettuce grown in perlite. INTRODUCTION Increased keeping quality or shelf life is one of the main aims of our research. Combining good shelf life with acceptable quality enables food to be distributed over greater distances and time periods, which is of central importance in a global market. In recent decades much attention has been focused on the packaging and controlled-atmosphere storage of fresh produce. The current importance of supply chain quality and logistics will diminish as research increases our understanding of preharvest factors affecting postharvest quality (Chiesa et al., 2003). All factors affecting postharvest performance must be integrated, including production systems, cultivars, fertilisation, salinity, irrigation, temperature, leaf expansion and maturity stage. Cultivating leafy vegetables in a floating system is the easiest and cheapest means of production, since this system shows high water and fertiliser efficiency and low environmental impact (Gonnella et al., 2003). On the other hand, soilless culture allows direct control of the nutrient solution, making it possible to modify composition and concentration to achieve predictable results in relation to dry matter content, nitrate content or other organoleptic and structural (calcium) features of produce (Elias et al., 1999). Gonnella et al., (2001) found floating cultivation produced acceptable yield and good control quality parameters in baby leaf species (shorter growing cycles, improved uniformity of growth and automation of cultural techniques, in addition to providing high levels of hygiene and quality). Solution composition can have a significant effect on leaf quality. Calcium is one of the most important nutrients and is associated with delayed plant senescence, stabilising influence on cell membranes, and the membrane lipid catabolism which is a characteristic feature of damaged, quiescent plant storage organ and fresh-cut produc. As a result the use of calcium has been suggested as a way to delay senescence in fresh-cut Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. 697 ISHS 2005 44 plant tissue (Picchioni et al., 1996). The aim of this research was to evaluate butterhead lettuce quality in a soilless culture system. The goal was to achieve production of high quality lettuce in terms of commercial standards and shelf life in modified atmospheric conditions (SAGPyA, 2004). MATERIALS AND METHODS The experiment was carried out in an unheated greenhouse in the experimental field of the Horticultural Department of the Faculty of Agronomy, University of Buenos Aires, Buenos Aires, Argentina. Butterhead lettuce cultivar “Lores” was cultivated and two experiments were carried out: Experiment 1 Floating system (FS) (100 cm x 250 cm x 40 cm width x length x height) and soil culture (SC) were compared in fall (autumn) production [2002 2003]. Plants were cultivated in polystyrene containers (1.5 inch thick). The nutrient solution was oxygenated using an air pump to introduce tiny bubbles. Experiment 2 Substrate culture (PC) (plastic gutter filled with perlite-500 cm x 40 cm x 30 cm, length x width x height) with three different levels of calcium (0, 3, and 6 me.L) in the nutrient solution were compared, in spring production (2003). The plants were transplanted at a density of 39 plants.m (FS), 18 plants.m (PC) and 10 plants.m (SC). Two nutrient solutions were used, one for the floating system (Casas Castro, 1999) and the other for the substrate culture in accordance with the conclusions/recommendations of Ki-Young et al., (2001) for butterhead lettuce. The nutrient solution temperature, pH and CE (dS.m) were controlled. Air temperature, relative humidity and solar radiation intensity inside the greenhouse were recorded hourly with a datalogger (Hobo). Treatments were arranged in a randomised complete block design with three replicates. Lettuce were harvested and measured for fresh and dry weight, leaf length and width, leaf length and width ratio, leaf area (Yoshida et al., 1997), leaf number, aerial and root biomass ratio and ascorbic acid, NO, K, Ca, Mg, Na by capillary electrophoresis (Pañak et al., 1998, Ito et al., 2003). A Minolta CR 300 chromameter was used. Colour was quantified in the L*, a*,b* colour space, Chroma [C*= (a*2 + b*2 )0.5 ] and hue angle [ho = tan-1 (b*/a*)]. During the growth cycle the incidence of tip-burn was monitored (Kader et al., 1973). Data were subject to the SAS (Cary, NC) general lineal model procedure. Treatment mean values were compared with the Tukey test (p<0.05). At harvest, according to experiments, intact leaves were packed in multilayer polyolefin bags and stored in a chamber at 1o and 8 °C for 7 days. Samples were taken at 0, 2, 4 and 7 days after harvest and postharvest quality was evaluated. (Data not shown). RESULTS AND DISCUSSION Effect of Soilless Culture (Floating System vs. Soil Culture) Both years showed similar results on growth parameters. At harvest, the soil culture plants had higher aerial fresh and dry weight, leaf area, aerial:root biomass ratio, length and width (p=0.0003) than the floating system plants (Table 1). Siomos et al., (2001) obtained similar results in butterhead lettuce cultivated in perlite. In lettuce from hydroponics culture, lower levels (fresh weight) in K (-38.7 %) (p= 0.03), and ascorbic acid (-72 %) (p= 0.0015) were found in the second year. Table 2.