The Effect of Growing Conditions On Health-Promoting Phytochemicals In Two Lettuce Cultivars

Genotype along with growing and management conditions can affect the content and the composition of phytochemicals in plants. A field study was conducted to examine the effect of contrasting growing conditions such as open field and high tunnel on the phytochemical content of two lettuce (Lactuca sativa L.) varieties; a green loose-leaf type, Baronet, and a red loose-leaf type, Red Sails. Three-week old seedlings grown in the greenhouse were transplanted to the open field or high tunnels. The total phenolic content and antioxidant capacity of lettuce leaves increased in response to transplanting to both open field and high tunnels. However, the increase was much greater when seedlings were transplanted to the open field and was more than four-fold over the pre-transplant stage. The two major phenolic compounds in lettuce were chicoric acid and chlorogenic acid and their concentrations were higher in both varieties when grown in open field than in high tunnels. In addition, Red Sails accumulated caffeic acid four-times as much in open field as it did in high tunnels. Overall, Red Sails accumulated higher amount of phenolic compounds, especially in open field, than did Baronet. However, there were no clear differences in quercetin-3-O-glucoside and ascorbic acid levels either between the varieties or the growing conditions. Although lettuce plants grown in open field were richer in phytochemicals, they did not do as well in biomass accumulation as those grown in high tunnels. A significant reduction in shoot and root biomass accumulation occurred when plants (both varieties) were grown in open field as compared to high tunnels. These results show that growing conditions, in addition to genotype, can significantly affect the content of many phenolic compounds in lettuce and that growing lettuce under open field conditions can have a positive impact on its health-promoting qualities