Supplemental UV-A and UV-B Affect the Nutritional Quality of Lettuce and Tomato: Health-Promoting Phytochemicals and Essential Nutrients

Abstract

UV radiation plays an important role not only in \nplant growth and development but also in the \naccumulation of essential nutrients and health-promoting phytochemicals \nin plants. The main objective of this study was to examine the effects of \nsupplemental UV-A, UV-B, and UV-AB on the nutritional quality of lettuce (Lactuca sativa, cv. red leaf “New Red Fire” and green leaf “Two Star”) and tomato (Solanum lycopersicum L., cv. BHN-589) grown in a greenhouse. Supplemental UV radiation was \nprovided by UV lamps 5 - 6 days prior to harvest. Supplemental UV-A produced higher accumulation of total phenolic compounds \nand higher antioxidant capacity in red leaf lettuce compared to other \ntreatments. Overall, supplemental UV-A produced a stronger response than other \nUV treatments and control in the accumulation of many phenolic compounds \nincluding luteolin-7-glucoside, quecetin-3-glucoside, \nand apigenin-3-glucoside in red leaf lettuce. However, UV-B and UV-AB had a \nnegative response in the accumulation of many phenolic compounds including \nchlorogenic acid, luteolin-7-glucoside, quercetin-3-glucoside, and \napigenin-3-glucoside in both red and green leaf lettuce varieties. In tomato \nfruits, supplemental UV-A had no effect on \ntheir total phenolic concentration. However, supplemental UV-B radiation for 3 \nh or UV-AB radiation for 9 h exposure produced higher total phenolic \nconcentration in the fruits compared to other supplemental UV treatments. \nSupplemental UV-AB (3 hexposure) was \ngenerally more effective than other UV treatments in increasing the \naccumulation of a number of phenolic \ncompounds including chlorogenic acid, caffeic acid, chicoric acid, luteolin-7-glucoside, and other flavonoids in ripe \ntomato fruits. Supplemental UV-A produced higher \naccumulation of carotenoids including lutein and β-carotene than other supplemental UV treatments, while supplemental UV-AB \nincreased the accumulation of lycopene in fully ripe tomatoes. With regard to the essential nutrients, green \nleaf lettuce was more responsive to the supplemental UV treatments than red \nleaf lettuce. All the supplemental UV treatments produced an increase in \nprotein concentration in the leaves of green leaf lettuce. However, \nsupplemental UV-AB produced a stronger response compared to the control and \nother UV treatments in increasing the accumulation of many nutrients including \nprotein, phosphorus, potassium, sulfur, and zinc in green leaf lettuce “Two Star”. Supplemental UV-treatments \ndid not affect the accumulation of any essential nutrients in fully ripe tomato \nfruits. The results show that supplemental UV enhances the nutritional quality \nof lettuce in relation to both \nhealth-promoting phytochemicals and essential nutrients. Similarly, \nsupplemental UV enhances nutritional quality in tomato fruits with higher accumulation of both phenolic compounds \nand carotenoids than does the control treatment.