The Evaluation of Growth Performance, Photosynthetic Capacity, and Primary and Secondary Metabolite Content of Leaf Lettuce Grown under Limited Irradiation of Blue and Red LED Light in an Urban Plant Factory

Plant production in urban areas is receiving much attention due to its potential role in feeding the rapidly growing population of city dwellers. However, higher energy demands in urban plant factories are among the key challenges that need to be addressed. Artificial lighting is responsible for the most significant levels of energy consumption in plant factories; therefore, lighting systems must be modulated in consideration of the sustainable food–energy nexus. In this context, low light irradiation using blue (B) and red (R) LED was applied in a plant factory for the growth of red leaf lettuce (Lactuca sativa L. var Lollo rosso) to evaluate the growth performance and functional quality. The tested B (450 nm) and R (660 nm) light ratios were B/R = 5:1; 3:1; 1:1; 1:3, and 1:5, with a photosynthetic photon flux density (PPFD) of 90 ± 3 µmol m−2 s−1. In the plant factory, the photoperiod, temperature, RH, and CO2 conditions were 16 h d−1, 20 ± 0.5 °C, 65% ± 5%, and 360 ± 10 μL L−1, respectively. The lettuce was harvested 10 and 20 days after the commencement of LED light treatment (DAT). In this study, normal photosynthetic activity and good visual quality of the lettuce were observed. The results show that a higher fraction of R (B/R = 1:5) significantly increased plant growth parameters such as plant height, leaf area, specific leaf area, plant fresh and dry weight, and carbohydrate content. By contrast, a higher fraction of B (B/R = 5:1) significantly increased the photosynthetic parameters and contents of pigment and phenolic compounds. The rate of photosynthetic performance, carbohydrates (except starch), and content of phenolic compounds were highest after 10 DAT, whereas the pigment contents did not significantly differ at the different growth stages. It is concluded that high R fractions favor plant growth and carbohydrate content, while high B fractions favor photosynthetic performance and the accumulation of pigments and phenolic compounds in red leaf lettuce under limited lighting conditions. This study will help in designing artificial lighting conditions for plant factory production to reduce energy demands.