INHIBITORY EFFECTS OF AMBIENT LEVELS OF SOLAR UV-A AND UV-B RADIATIONON GROWTH OF CV NEW RED FIRE LETTUCE

The influence of solar UV-A and UV-B radiation at Beltsville, MD, USA, on growth of Lactuca sativa L. (cv. New Red Fire lettuce) was examine d during early summer of 1996 and 1997. Plants were grown from seed in plastic window boxes covered with Llumar to exclude W-A and W-B, poly ester to exclude UV-B, or tefzel (1996) or teflon (1997) to transmit U V-A and UV-B radiation. After 31-34 days, plants grown in the absence of solar W-B radiation (polyester) had 63 and 57% greater fresh weight and dry weight of tops, respectively, and 57, 72 and 47% greater dry weight of leaves, stems and roots, respectively, as compared to those grown under ambient UV-B (tefzel or teflon). Plants protected from UV- A radiation as well (Llumar) showed an additional 43 and 35% increase, respectively, in fresh and dry weight of tops and a 33 and 33% increa se, respectively, in dry weight of leaves and stems, but no difference in root biomass over those grown under polyester. Excluding ambient U V-B (polyester) significantly reduced the UV absorbance of leaf extrac ts at 270, 300 and 330 nm (presumptive flavonoids) and the concentrati on of anthocyanins at 550 nm as compared to those of leaf extracts fro m plants grown under ambient UV-A and UV-B. Additional removal of ambi ent UV-A (Llumar) reduced the concentration of anthocyanins, but had n o further effect on UV absorbance at 270, 300 or 330 nm. These finding s provide evidence that UV-B radiation is more important than UV-A rad iation for flavonoid induction in this red-pigmented lettuce cultivar. Although previous workers have obtained decreases in lettuce yield un der enhanced UV-B, this is the first evidence for inhibitory effects o f solar W-A and UV-B radiation on lettuce growth.