Effects of supplementary lighting of blue-light during the night on anthocyanin synthesis and growth of red-leaf lettuce (Lactuca sativa L.)
Poor coloration due to low anthocyanin content in leaves has often happened in hydroponic red-leaf lettuce (Lactuca sativa L.). Our previous study suggested that supplementary lighting of blue light improves the coloration by increasing anthocyanin content. In this study, effects of blue light supplementation during the nighttime on the coloration and growth were examined to develop practical methods for improving quality of red-leaf lettuce by hydroponics in a greenhouse. Effect of light intensity on anthocyanin level was examined in red-leaf lettuce. The lettuce was grown under sunlight during the daytime and was exposed to blue light from 18:00 to 6:00 (12 hours) for 7days. The anthocyanin level increased linearly with 60 μmol／㎡／s or more and reached a plateau with 160 μmol／㎡／s. The coloration obtained at 120 μmol／㎡／s was comparable to that produced in the field. The growth of lettuce was also enhanced by the increase of light intensity. The dry matter rates and thickness of a leaf were increased with 60 and 120 μmol／㎡／s of blue light.Effect of lighting duration on anthocyanin level and growth was examined in the lettuce under 120 μmol／㎡／s of blue light. The anthocyanin level increased with 6 hours and the highest value was obtained at 12 hours. Under 120 μmol／㎡／s of blue light condition, at least 6 hours continuous irradiation was necessary to improve the coloration. At that time, the longer the lighting duration was extended, the greater the growth of lettuce could be seen such as the dry weight, dry matter rates, and thickness of a leaf. The anthocyanin content and/or the dry weight were not affected by the irradiation pattern, continuous and 30 and 120 minutes intermittent lighting, under the condition of 120 μmol／㎡／s of blue light for totally 6 hours. It was concluded that 120 μmol／㎡／s of irradiation of blue light for more than 6 hours in nighttime was enough to improve coloration and growth in a greenhouse hydroponic red-leaf lettuce.