Enhancing leaf lettuce growth and phytochemical composition under continuous light and elevated NH ₄ ⁺ :NO ₃ ⁻ ratios

Production of leaf lettuce ( Lactuca sativa L.) in plant factories with artificial lighting is becoming more common, but there is a need for technologies that can increase crop productivity, decrease production costs, and add value. We hypothesized that the use of a 24‐h photoperiod (continuous lighting [CL]) and NH 4 ‐N applications could enhance productivity and crop value. In this study, we investigated the effects of the photoperiod and the applied nitrogen form on leaf lettuce growth and quality in a nutrient solution supplemented with calcium carbonate (CaCO 3 ) to suppress the decrease in pH. Under these conditions, the shoot fresh weight was 3.5‐ to 5.8‐fold higher under CL than under the 12‐h photoperiod, and the value did not decrease even when the NH 4 ‐N proportion was increased to 75% under CL. Cultivation under CL substantially increased growth, while also increasing ascorbic acid, chlorogenic acid, and total polyphenol contents. Furthermore, increases in the proportion of NH 4 ‐N affected leaf lettuce, resulting in darker leaf coloration, reduced nitrate content, and increased chlorogenic acid content. These results suggest that leaf lettuce cultivation may be improved while sustaining growth under CL with 50%–75% NH 4 ‐N treatments if the nutrient solution pH is maintained. Furthermore, the leaf Ca content did not decrease following the addition of CaCO 3 , even when the NH 4 ‐N proportion was increased, suggesting that CaCO 3 may be a better option than typical pH up solutions for suppressing the decrease in pH due to the application of NH 4 ‐N.