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 plant growth and development but also in the accumulation of essential nutrients and health-promoting phytochemicals in plants. The main objective of this study was to examine the effects of supplemental 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 provided by UV lamps 5 - 6 days prior to harvest. Supplemental UV-A produced higher accumulation of total phenolic compounds and higher antioxidant capacity in red leaf lettuce compared to other treatments. Overall, supplemental UV-A produced a stronger response than other UV treatments and control in the accumulation of many phenolic compounds including luteolin-7-glucoside, quecetin-3-glucoside, and apigenin-3-glucoside in red leaf lettuce. However, UV-B and UV-AB had a negative response in the accumulation of many phenolic compounds including chlorogenic acid, luteolin-7-glucoside, quercetin-3-glucoside, and apigenin-3-glucoside in both red and green leaf lettuce varieties. In tomato fruits, supplemental UV-A had no effect on their total phenolic concentration. However, supplemental UV-B radiation for 3 h or UV-AB radiation for 9 h exposure produced higher total phenolic concentration in the fruits compared to other supplemental UV treatments. Supplemental UV-AB (3 hexposure) was generally more effective than other UV treatments in increasing the accumulation of a number of phenolic compounds including chlorogenic acid, caffeic acid, chicoric acid, luteolin-7-glucoside, and other flavonoids in ripe tomato fruits. Supplemental UV-A produced higher accumulation of carotenoids including lutein and β-carotene than other supplemental UV treatments, while supplemental UV-AB increased the accumulation of lycopene in fully ripe tomatoes. With regard to the essential nutrients, green leaf lettuce was more responsive to the supplemental UV treatments than red leaf lettuce. All the supplemental UV treatments produced an increase in protein concentration in the leaves of green leaf lettuce. However, supplemental UV-AB produced a stronger response compared to the control and other UV treatments in increasing the accumulation of many nutrients including protein, phosphorus, potassium, sulfur, and zinc in green leaf lettuce “Two Star”. Supplemental UV-treatments did not affect the accumulation of any essential nutrients in fully ripe tomato fruits. The results show that supplemental UV enhances the nutritional quality of lettuce in relation to both health-promoting phytochemicals and essential nutrients. Similarly, supplemental UV enhances nutritional quality in tomato fruits with higher accumulation of both phenolic compounds and carotenoids than does the control treatment.