Abstract
The alterable light/dark cycle in a plant factory with artificial lighting eliminates the traditional concept of day and night in nature. Adjusting the light/dark cycle to closely align with the inherent circadian rhythm of plants can enhance biomass accumulation. In this study, we examined the effects of different light/dark cycles on the photosynthetic performance, growth, and energy use efficiency of two hydroponic lettuce cultivars (Lactuca sativa L. cv. 'Frillice' and 'Crunchy'). The lettuces were subjected to four light/dark cycle treatments-16 h light/8 h dark (L(16)D(8), as control), 12 h light/6 h dark (L(12)D(6)), 8 h light/4 h dark (L(8)D(4)), and 4 h light/2 h dark (L(4)D(2)), all under LED lamps with white combined red chips at the same light intensity of 250 μmol m(-2) s(-1). Photosynthetic performance and growth index were measured during the slow and rapid growth stages, corresponding to days 9 and 21 after transplanting, respectively. For Frillice, L(12)D(6) achieved the highest shoot dry weight and light and electricity energy use efficiencies on days 9 and 21 after transplanting, primarily due to the largest leaf area, leaf number, and net photosynthetic rate. For Crunchy, L(12)D(6) and L(8)D(4) increased shoot fresh and dry weights due to larger leaf area and leaf number on day 9 after transplanting compared with L(16)D(8). Subsequently, the lettuces in L(16)D(8) exhibited a rapid increase in leaf area and leaf number, along with a high net photosynthetic rate during the rapid growth stage, resulting in fast shoot biomass accumulation. There were no significant differences in the shoot dry weight and energy use efficiency between L(16)D(8) and L(12)D(6) on day 21 after transplanting. Two lettuce cultivars in L(16)D(8) both exhibited the highest water use efficiency on day 21 after transplanting. In conclusion, the light/dark cycle lighting can alter lettuce biomass accumulation by modifying plant morphology and leaf net photosynthetic rate. Additionally, the physiological response to the light/dark cycle was cultivar-dependent. Our findings provide valuable insights for optimizing hydroponic lettuce production to achieve high yield in LED plant factories.