Far-red light in early growth stages boosts lettuce biomass and preserves anthocyanins

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 supplementary FR light promotes lettuce growth and morphology across weekly intervals as well as analysing photosynthetic parameters, pigment accumulation and anthocyanin gene expression.

Red leaf lettuce (Lactuca sativa 'Red Fire') was grown in a commercial plant factory with artificial light for 6 weeks. White (W) light, 5000 K, was maintained at 300 μmol m-2 s-1, and FR, when supplemented, was added at 100 μmol m-2 s-1 in addition to the 300 μmol m-2 s-1 of W light. Four lighting treatments were tested under a 16-h photoperiod: (1) W for all 6 weeks (treatment W), (2) 4 weeks of W followed by 2 weeks of supplementary 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).

The shoot dry weight after 6 weeks in W + FR, W + FR to W and W to W + FR was greater than that in W. Both W + FR and W + FR to W showed a tendency for greater canopy expansion compared with 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 the CO2 assimilation rates were enhanced when FR light was supplemented during measurement, compared with 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).

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.