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Chieri Kubota, Cynthia A. Thomson, Min Wu and Jamal Javanmardi

Plants produce various phytochemicals that are of nutritional and medicinal value to humans. Phytochemicals having antioxidant capacity are drawing increased interest from consumers. Population studies among Americans have consistently demonstrated inadequate consumption of fruit and vegetables. Improving intake of fruit and vegetables has been a major public health effort for many years with minimal success. Given this, it seems opportunistic to consider other approaches to enhance the nutritional quality of the American diet. One plausible approach is the development of fresh produce containing a greater concentration of phytochemicals known to improve health, thus while consuming fewer servings of produce, Americans would still have significant exposure to health-promoting food constituents. Controlled environments provide a unique opportunity to modify the concentrations of selected phytochemicals in fruit and vegetables, yet practical information is limited regarding methods effective in optimizing antioxidant capacity. Our research at the University of Arizona Controlled Environment Agriculture Program has shown that application of moderate salt stress to tomato plants can enhance lycopene and potentially other antioxidant concentrations in fruit. The increase in lycopene in response to salt stress in the tomato fruit was shown to be cultivar specific, varying from 34% to 85%. Although the specific biological mechanisms involved in increasing fruit lycopene deposition has not been clearly elucidated, evidence suggests that increasing antioxidant concentrations is a primary physiological response of the plant to the salt stress. Another experiment showed that low temperature during postharvest increased antioxidant capacity of tomato fruit while it maintained the lycopene concentration. More detailed study in this area is needed including accumulation of antioxidant phytochemicals as affected by environmental conditions during the cultivation and the postharvest.

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Chieri Kubota, Mark Kroggel, Mohammad Torabi, Katherine A. Dietrich, Hyun-Jin Kim, Jorge Fonseca and Cynthia A. Thomson

‘Durinta’ tomatoes were grown hydroponically using rockwool substrate in greenhouses to assess the seasonal and postharvest changes of selected quality attributes including lycopene and total soluble solids (TSS, % Brix) concentrations in fruit when grown under varied electrical conductivity (EC) in the nutrient solution. Two levels of EC examined in this study were 2.4 and 4.8 dS·m−1 for standard low EC and high EC treatments, respectively. All fruits at light-red and red ripeness stages were harvested and weighed every week, and nine fruits visually representing the median group of red fruits were selected from each EC treatment and subject to measurements of lycopene and TSS concentrations. Of 53 harvest weeks (Dec. 2005 to Dec. 2006), 45 weeks were subject to fruit quality analyses at harvest and 3 weeks were subject to postharvest quality analyses. Lycopene concentration and TSS showed seasonal differences with larger variation in lycopene, but the high EC treatment induced an overall average of 18% greater lycopene concentration and a 20% greater TSS. The regression analyses indicated that efflux solution EC (EEC) was the most influential factor for both lycopene and TSS concentrations, but secondary influential factors were greenhouse temperature for lycopene and daily light integral for TSS. Postharvest storage test showed that selected fruit quality attributes (lycopene, TSS, ascorbic acid, and total phenolics) changed minimally or not at all for 10 days when stored at 12 °C, a widely recommended tomato postharvest storage temperature. Overall, we consider that producing lycopene-rich tomato by controlling EC of nutrient solution was feasible during year-round greenhouse production using a high-wire rockwool culture system.