Maintaining optimal daily light integrals (DLI) and nutrient solution concentrations is essential for enhancing growth traits of leafy greens such as lettuce (Lactuca sativa L.) in greenhouse production systems. Research is needed to better understand the synergistic effect of supplemental lighting and nitrogen (N) concentrations on hydroponic lettuce production during seasons of low natural light. The objective of this study was to determine the effects of narrowband supplemental lighting treatments and nitrogen concentrations on the growth, yield, and physiological traits of lettuce cultivars in a nutrient film technique (NFT) hydroponic system. Three lettuce cultivars (cvs. Nancy, Salvius, and Thurinus) were grown in the fall of 2024 (Nov 13. to Dec 18.) and the winter of 2025 (Jan 29. to March 03.) under three light treatments including supplemental 80% red and 20% blue light (80R/20B); 80R, 10B, and 10% far red light (80R/10B/10FR); non-supplemental natural light (control) and two nitrogen concentrations (100 and 200 ppm N). Both light treatments supplemented a DLI of 6 mol.m-2.d-1. Supplemental lighting treatments had limited effects on the weekly net assimilation rate, averaging 11.124 and 10.130 µmol.m-2.s-1 versus 12.046 and 10.564 µmol.m-2.s-1 in the control treatments in fall and winter cycles, respectively. In fall with 200 ppm N, the 80R/20B and 80R/10B/10FR light treatments significantly increased the fresh yield by 90% and 45% respectively when compared to the control treatment. While in winter with 200 ppm N, these increases were only 10% and 1%, suggesting a relatively limited benefit of supplemental lighting during this season. With 100 ppm N during the winter cycle, compared with the non-supplemented light, 80R/20B and 80R/10B/10FR lights increased the yield by 58% and 30% respectively while contrasting results were observed with the light treatments with 100 ppm N in the fall cycle. Supplemental light in the fall cycle increased the average leaf area index (LAI) by 27% only when combined with 200 ppm N but not with 100 ppm. In contrast, in winter, supplemental light increased LAI by 11% only with 100 ppm N but not with 200 ppm N. Overall, the beneficial effect of supplemental lighting on the growth and yield traits was greater in fall when combined with 200 ppm N but was greater in winter when combined with 100 ppm N.
