COLORATION AND GROWTH OF RED LETTUCE GROWN UNDER UV-RADIATION TRANSMITTING AND NON-TRANSMITTING COVERS

Production of red lettuce for salad mixes in Hawaii is primarily out-of-doors in the drier, leeward sides of the islands. Because the newer acrylic-polycarbonate hard covers transmit only small amounts of UV-radiation (which is essential for anthocyanin production), we experimented with a new ethylene-tetrafluoroethylene copolymer film (F-Clean), which transmits more than 90% of visible light and UV-radiation, for possible use in red lettuce production in high rainfall areas. Four cultivars of red lettuce, ‘Natividad’ - Red Lollo Rossa, ‘Dark’ - Lollo Rossa, ‘Aruba’ - Red Oak Leaf, and ‘New Red Fire’ - Red Grand Rapids, were grown in static hydroponic solution under each of the two levels of light and three UV-radiation environments. Lettuce plants with the most coloration and lowest head weight were produced out-of-doors with 100% UV-radiation. All of the lettuce produced under the non-UV-radiation transmitting, acrylic-polycarbonate cover had the least coloration and largest head weight. All were mostly green. Lettuce grown under the UV-radiation transmitting, F-Clean film with an additional layer of 50% aluminized shade screen were intermediate in color and head weight. The coloration of all the red lettuce studied appeared similar to ‘Dark’ - Lollo Rossa, where the anthocyanin, cyanidin 3-(6-malonyl) glucoside is present. UV-radiation appears to be the most important factor in coloration of red lettuce. Even with the additional layer of 50% shade, daytime temperatures were 4 to 6°C higher in the F-Clean house than in the acrylic-polycarbonate house. Yet, the cooler acrylic-polycarbonate house, which had little UV-radiation, produced green “red” lettuce with little coloration.