Influences of Organic Fertilization, High Tunnel Environment, and Postharvest Storage on Phenolic Compounds in Lettuce

Abstract

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.