No abstract

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 Ϫ2 d Ϫ1 . 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.

To investigate the effects of different types of irradiation on the growth and morphological characteristics of leaf lettuce (Lactuca sativa L. var. crispa "Greenwave"), a quantitative analysis of the leaf contour using an elliptic Fourier descriptor (EFD) was carried out together with conventional measurement methods. Two experiments were conducted to test the different treatments. Experiment 1 investigated the effects of monochromatic/alternating/simultaneous red/blue irradiation, and Experiment 2 determined the effects of random variations in photosynthetic photon flux density (PPFD) compared with that of constant light. The growth and morphological characteristics in Experiment 1 were consistent with those of previous studies, where the shoot fresh weights in monochromatic and alternating irradiations were larger than those in simultaneous irradiations. EFD showed that the leaf length and petiole under monochromatic and alternating irradiation were longer and more apparent than those under simultaneous irradiation. In Experiment 2, both the conventional measurement methods and EFD could hardly detect any differences in leaf lettuce growth and morphological characteristics under constant PPFD. These results suggested that fluctuating PPFD within certain limits in a PFAL may have little effect on the leaf lettuce growth.