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Xin Zhao, Edward E. Carey and Takeo Iwamoto

An experiment was conducted at Olathe, Kan., in Spring 2004 to investigate the influence of organic and conventional fertilizer sources and application rates on antioxidant levels of pac choi (Brassica rapa L. cv. Mei Qing) in open fields and poly-covered high-tunnel plots. Organic plots received pre-plant application of composted cattle manure and alfalfa (Hu-More 1–1–1) at 0 kg/ha N, 156 kg/ha N, or 314 kg/ha N, and conventional plots received preplant application of 13N–13P–13K at 0 kg/ha N, 78 kg/ha N, or 156 kg/ha N. Antioxidant levels were measured using the oxygen radical absorbance capacity (ORAC) assay. There were significant effects of fertilizer source and high-tunnel environment on the antioxidant capacity of pac choi. Organic fertilization significantly increased hydrophilic ORAC of pac choi in open field plots, but not in high tunnels. Regardless of the fertilizer source, pac choi grown in the open field had significantly higher hydrophilic ORAC than that grown in tunnels. Lipophilic ORAC was significantly increased by organic fertilization but was not affected by high-tunnel production. Total ORAC (hydrophilic + lipophilic) was significantly higher in pac choi from organic or open-field plots, compared to conventional and high-tunnel plots, respectively. Although fertilizer rate did not show significant impact on antioxidant level of pac choi, hydrophilic and total ORAC seemed to decrease as the fertilizer rate increased, especially under conventional fertilization, while lipophilic ORAC reached the highest level at the medium fertilizer rate. Differences in antioxidant levels were likely associated with the enhanced phytochemical content of pac choi from organically fertilized and open-field plots.

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Xin Zhao, Edward E. Carey, Janice E. Young, Weiqun Wang and Takeo Iwamoto

As the largest group of phytochemicals, dietary phenolics play an important role in human health and disease prevention. Cultural practices have been shown to have the potential for affecting phenolic compounds in food crops. Spring and summer trials were conducted in 2003 to examine the effects of organic fertilization and high tunnel environments on phenolic constituents of lettuce (Lactuca sativa L.) cultivars Red Sails and Kalura. Effects of postharvest storage at 4 °C for 16 days on total phenolics of lettuce harvested from the summer trial were also evaluated. Total phenolics, excluding anthocyanins, were measured spectrophotometrically, and major phenolic constituents were identified and quantified by high-performance liquid chromatography. Chlorogenic acid and quercetin glycosides were found to be predominant in lettuce. ‘Red Sails’ consistently exhibited significantly higher phenolic concentrations than ‘Kalura’. Organic (compost + fish emulsion) and conventional (N–P–K + CaNO3) fertilization did not consistently differentially affect lettuce phenolics in our recently established organic and conventional plots. The high tunnel environment generally reduced phenolic levels in lettuce relative to the open field. However, differences between high tunnel and open field varied with cultivar and season. Effects of production factors on lettuce phenolics were maintained during cold storage. There was a substantial increase in total phenolics during storage, likely correlated with declining lettuce quality. Further studies are warranted to more fully assess the impact of cultivar and production management, including organic fertilization, on lettuce phenolics.