Submitted to: Journal of Food Composition and Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 24, 2006
Publication Date: July 19, 2006
Citation: Caldwell, C.R., Britz, S.J. 2006. Effect of supplemental ultraviolet radiation on the carotenoid and chlorophyll composition of green house-grown leaf lettuce (lactuca sativa l.)cultivars. Journal of Food Composition and Analysis. 19:617-644. Interpretive Summary: Certain fruits and vegetables may provide specific health benefits by supplying dietary antioxidants and antibiotics. Although leaf lettuce may contain significant amounts of these phytonutrients, the levels of these compounds is dependent upon the variety and growth conditions of the lettuce. Sixteen varieties of leaf lettuce were grown in a greenhouse under light conditions likely to modify phytonutrient concentrations. Conditions that increased the levels of flavonoid, anthocyanin and other phenolic phytonutrients decreased the levels of carotenoids such as lutein. Lettuce varieties with the highest carotenoid levels tended to have lower levels of the other phenolic phytonutrients. Therefore, conditions that increased the amount of one type of beneficial phytochemical decreased the levels of a different class of phytonutrient. Since vegetables for local consumption are often commercially grown in greenhouses during the winter, the results of this research provide greenhouse growers and vegetable breeders the information they need to select or develop the best lettuce varieties for winter production.
Technical Abstract: Eight green leaf and 8 red leaf lettuce varieties were grown in a greenhouse under control, supplemental ultraviolet-A (315-400 nm) (UV-A) and supplemental UV-A plus ultraviolet-B (290-320 nm) (UV-B) radiation for the final 9 days before harvest. The entire edible portion of the lettuce was collected, flash frozen, lyophilized and milled to a fine powder. The samples were then extracted in 80% aqueous acetone and the carotenoid and chlorophyll content determined by reverse-phase, high performance liquid chromatography. Regardless of light conditions, lutein was the major carotenoid in the lettuce samples with lesser amounts of carotene, violaxanthin, neoxanthin and possibly zeaxanthin. In general, lettuce growth under supplemental UV-B reduced the lutein concentration by about 25% compared the UV-A and control light treatments. Red leaf lettuce with the highest anthocyanin levels had lower amounts of lutein and certain green leaf lettuce varieties had the highest lutein levels. Pheophytins and other chlorophyll degradation products were identified and appeared to be dependent on both light condition and lettuce variety. Therefore, light conditions known to increase flavonoid, anthocyanin and other phenolic phytonutrients levels in leaf lettuce decreased the amounts of carotenoids. Since lutein levels varied by almost a factor of 10, the relatively large cultivar-specific differences in the levels of the carotenoids suggests that the selection of specific leaf lettuce varieties for greenhouse production may enhance their possible nutritional benefits.