Effects of Time Variation of Light Intensity on the Growth of the Leaf Lettuce “Greenwave”

The growth and morphogenesis of leaf lettuce in response to gradual changes in light intensity over time were investigated. Seedlings of leaf lettuce (Lactuca sativa L. var. crispa "Greenwave") were grown in a blue to red light-emitting diode (LED) ratio of 0.33 and daily light integral (DLI) of 8.7 mol m-2 d-1. The first experiment was conducted with nine treatments, in which light intensity varied every 15 min with its peak during the day and a consistent dark period of 8 h. In the second experiment, three dark period lengths and three irradiation changes were combined based on the irradiations that produced different plant growth responses in the first experiment. Based on the results of the two experiments, most of the irradiation methods with the same DLI and dark period resulted in similar leaf lettuce growth in terms of plant weight and shape. However, plant weight was reduced when a high level of light intensity was employed for a long duration during the light period. Shorter dark periods can accelerate the growth of leaf lettuce. Consequently, the results from this study propose a new irradiation method with a non-constant light intensity in relation to time.