Nutrient solution culture of leaf lettuce under artificial light. II. Growth promotion induced by continuous illumination with low light intensity.

Abstract

Extension of the period of illumination from photoperiodic to continuous was examined as a way of producing leaf lettuce (Lactuca sativa L. cv. Grand Rapids) economically under artificial light. Plants were grown under photoperiodic (17.5 hr) and continuous illumination at a total daily radiation (moles of photosynthetic photons) of 18.7 mol⋅m-2⋅day-1. The continuous illumination increased shoot fresh weight, dry weight of whole plant, stem size and chlorophyll content. Subsequently, plants were grown under continuous illumination at a photosynthetic photon flux (PPF) of 250, 350 and 450 μnol⋅m-2⋅s-1. Shoot weight tended to increase when the light intensity was decreased, while root weight had a tendency to decrease under the same conditions. Higher light intensity increased the length of the largest leaf, specific leaf area and dry matter ratio of leaves, and induced the development of tipburn. It was found that continuous illumination under low light intensity promoted the growth of leaf lettuce without causing any abnormality. These results were analysed in relation to the apparent quantum yield in photosynthesis. Since the apparent quantum yield increased with the decrease of the light intensity, growth promotion by continuous illumination under the same total daily radiation is associated with a high quantum yield. The present studies suggest that continuous illumination saves light energy more than photoperiodic illumination under equal total daily radiation, and that continuous illumination with a low light intensity at less than 250 μmol⋅m-2⋅s-1 PPF promotes the growth of leaf lettuce without causing abnormalities and contributes to the saving of lamp fixtures.