Greenhouse-controlled environment technology was traditionally developed to extend the crop cycle into seasons that are too harsh to produce crops in open fields. The technology has focused on optimizing environmental conditions to maximize yield as well as product quality. Contributions to yield increases were generally made by technological advancements in controlling environment as well as breeding cultivars suitable for greenhouse production. For tomato (Solanum lycopersicum), the annual greenhouse tomato yield has reportedly doubled from 30 kg·m−2 to 60 kg·m−2 in The Netherlands (Higashide and Heuvelink, 2009) and anecdotally reached 100 kg·m−2 in the southwestern United States by combining crop scheduling and advanced environmental control.
Tomato is an economically important horticultural crop cultivated worldwide. The total annual production of tomato in the top 20 producing countries is over 150 Mt per year, of which the United States has the second highest production at an estimated 14,141,900 t per year (FAO, 2011). Tomato is also the predominant food crop produced in North American greenhouses. The successful development of large-scale greenhouse tomato growers with year-round production capacity and consistent high-quality product has resulted in favorable purchasing of greenhouse produce by U.S. retail stores. Nearly 40% of tomatoes available in U.S. retail stores were reportedly from North American greenhouses (Cook and Calvin, 2005).
Compared with the remarkable achievement in increasing yield and market acceptance of greenhouse tomato, it seems that improving organoleptic and/or nutritional quality of the fruit has received less emphasis in North American greenhouse industries, although flavor of nutrient-rich tomatoes has been ranked high in consumer surveys for purchasing decision factors (Weaver et al., 1992). Furthermore, flavor and nutritional quality are considered as “experience-based quality attributes” (Wismer, 2009) that would affect the consumer decision for continued consumption, especially when the quality attribute can be associated with branding (packaging and logos) to identify the specific product and source.
Overall flavor of tomato fruit is commonly evaluated using TSS estimated using a refraction index (Brix) as an indicator. Sugar and organic acids are important flavor components in tomato fruit and measurements of refraction index are affected by sugars predominantly and also by organic acids (Saltveit, 2005). It is therefore generally accepted that tomato fruit with greater TSS is more flavorful. Total soluble solid concentration is affected by water content in fruit and can be increased by limiting water transport to the fruit by growing the plant under lower (more negative) water potential in the root zone. Based on this understanding, growing tomato plants hydroponically using nutrient solution with high EC has been commercially practiced worldwide, yet to a limited extent in North America (Buck et al., 2008).
Lycopene is an antioxidant abundant in tomato that reportedly helps to reduce oxidative stress (Kelkel et al., 2011) and thus, indirectly, oxidative-stress associated disease such as cardiovascular diseases and cancer in the human body (Böhm, 2012; Tanaka et al., 2012). Several environmental factors reportedly can affect lycopene concentration in tomato fruit. Salt stress has been shown to accelerate the lycopene development in fruit (on both a fresh and dry weight basis) when tomato plants were grown hydroponically using nutrient solution containing high EC (4.5 dS·m−1 EC) (Wu and Kubota, 2008). These results suggest that growing tomato plants under high EC could be developed into a commercial technique to improve both flavor and health-promoting attributes of fresh fruit (Kubota et al., 2006).
Validating a new cultivation technique in year-round production is crucial to implement the technology in a commercial setting. Previous studies revealed that there are factors that influence lycopene synthesis, including temperature (e.g., Krumbein et al., 2006) and light intensity (e.g., Dumas et al., 2003), both of which widely change diurnally and seasonally in greenhouse. It is also important to understand the potential change in lycopene concentration and other fruit quality attributes during postharvest storage. Postharvest changes in tomato fruit quality have been extensively studied, including concentrations of carotenoids (Giovanelli et al., 1999; Javanmardi and Kubota, 2006; Liu et al., 2009; Toor and Savage, 2006), TSS (Javanmardi and Kubota, 2006; Liu et al., 2009; Wills and Ku, 2002) and other health-promoting compounds such as ascorbic acid and phenolic compounds (Giovanelli et al., 1999; Toor and Savage, 2006). However, interactions of preharvest conditions (such as EC) and postharvest conditions (such as storage temperature) are not well quantified.
We conducted a year-round production study to evaluate the technique of producing tomato rich in lycopene and flavor under increased EC of the nutrient solution in a high-wire production system similar to commercial greenhouses. The postharvest changes in lycopene and other quality attributes in fruit produced under high and low EC during storage at different air temperatures were quantified. This study was conducted as part of the diet intervention study designed and reported by Thomson et al. (2008) wherein the health-promoting efficacy of tomatoes grown under increased EC was examined. Therefore, some experimental procedures used in this present study were selected to satisfy the requirements of this human health study.
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