There are contrasting effects of far-red (FR; 700–750 nm) light on leaf area and biomass in plants. These differences have been attributed to photon flux density (PFD) and species/cultivar differences. In a previous experiment, total PFD (TPFD) did not mediate the influence of FR light on leaf area and shoot mass when the TPFD alterations were only of red (R; 600–699 nm) and FR light. Therefore, we hypothesized that blue (B; 400–499 nm) light controls the influence of TPFD in regulating the effects of FR light on leaf area and shoot mass. We cultivated kale (Brassica oleracea var. sabellica) ‘White Russian’ and lettuce (Lactuca sativa) ‘Rex’ and ‘Rouxai’ under 12 lighting treatments with a 24 h∙d−1 photoperiod and TPFDs of 85, 170, 255, or 340 µmol∙m−2∙s−1 and FR fractions [FR-PFD divided by the sum of R and FR PFD] of 0.00, 0.17, or 0.33. The alterations in the TPFDs were solely due to B-PFD; the sum of R and FR PFD was constant in all treatments. Preliminary results indicate that elevated FR fraction did not increase leaf area and shoot mass of all three crops in the absence of B light, when the TPFD was 85 µmol∙m−2∙s−1. However, a high B-PFD and thus TPFD amplified the effects of a high FR fraction at increasing leaf area and shoot mass of all three cultivars. These high FR-fraction effects were correlated with increased biomass partitioning to leaves at a high B-PFD and thus TPFD. These results imply that the contrasting effects of FR light on leaf area and biomass in previous studies could be attributed to the B-PFD. In addition, the influence of TPFD on FR-fraction effects is primarily influenced by the B-PFD.
