Far-Red Light in Early Growth Stages Boosts Lettuce Biomass and Preserves Anthocyanins

Background and Aims: Light plays a dual role in plants, serving as both an energy source and a regulator of development from seedling to senescence. Recently, far-red (FR) radiation has gained attention in the controlled environment agriculture (CEA) science and grower community for its potential to enhance yield through canopy expansion and improved light capture, contributing positively to photosynthesis. This study explores how supplemental FR light promotes lettuce growth and morphology across weekly intervals as well as analyzing photosynthetic parameters, pigment accumulation, and anthocyanin gene expression. Methods: Red leaf lettuce (Lactuca sativa 'Red Fire') was grown in a commercial plant factory with artificial light for six weeks. 5000K white (W) light was maintained at 300 micromoles per square meter per second, and FR, when supplemented, was added at 100 micromoles per square meter per second in addition to the 300 micromoles per square meter per second of W light. Four lighting treatments were tested under a 16 h photoperiod: (1) W for all 6 weeks (W), (2) 4 weeks of W followed by 2 weeks of supplemental FR (W to W+FR), (3) 4 weeks of FR supplementation followed by 2 weeks of only W (W+FR to W), and (4) W+FR for all 6 weeks (W+FR). Key Results: The shoot dry weight after 6 weeks in W+FR, W+FR to W and W to W+FR was greater than W. Both W+FR and W+FR to W showed a tendency for greater canopy expansion compared to W as well as W to W+FR. There were no significant differences in stomatal conductance among the treatments. On the other hand, in both W and W+FR to W plants, Carbon dioxide assimilation rates were enhanced when FR light was supplemented during measurement, compared to when FR was not provided. Anthocyanin accumulation was greater in both W and W+FR to W, consistent with the expression of key genes involved in the anthocyanin biosynthesis pathway, including anthocyanin synthase (ANS), flavanone 3-hydroxylase (F3H) and dihydroflavonol 4-reductase (DFR). Conclusions: This study demonstrates that FR supplementation during the early growth stages of lettuce promotes biomass accumulation by enhancing both canopy expansion and photosynthetic activity, while maintaining high levels of functional compounds such as anthocyanins.