Far-red (FR; 700–750 nm) light induces shade-avoidance responses such as stem and leaf elongation and an increase in specific leaf area (SLA). Previous studies have reported that a high photon flux density (PFD) can mitigate the effects of FR light. However, limited research has explored the impacts of individual waveband PFDs on the effects of high total PFD (TPFD) in regulating FR-light responses. To address this gap, we conducted an experiment hypothesizing that the effects of a high FR fraction [FR-PFD divided by the sum of red (R; 600–699 nm) and FR PFD] on shade-avoidance responses would persist when the TPFD increases were solely from increases in R and FR PFDs. We grew kale (Brassica oleracea var. sabellica) ‘Red Russian’ and lettuce (Lactuca sativa) ‘Rex’ and ‘Rouxai’ under 12 lighting treatments with a 24 h∙d−1 photoperiod, TPFDs of 85, 170, 255, or 340 µmol∙m−2∙s−1, and FR fractions of 0.00, 0.17, or 0.33. The blue (400-499 nm) PFD was constant in all treatments and the alterations in the TPFDs were solely due to R and FR PFDs. Based on preliminary results, high FR fractions increased the leaf length of kale to a similar degree at all TPFDs except for no increase at the TPFD of 85 µmol∙m−2∙s−1. High FR fractions increased the leaf length of lettuce ‘Rex’ and ‘Rouxai’ to a similar degree at all TPFDs. In contrast, the SLA of kale did not respond to the FR fraction at any of the TPFDs. The SLA of lettuce ‘Rex’ and ‘Rouxai’ was increased by high FR fractions to a similar degree at all TPFDs except for no increase at the TPFD of 85 µmol∙m−2∙s−1. Contrary to the paradigm, our results suggest that FR-fraction effects can persist under a high TPFD when R and FR PFDs are elevated. Moreover, the lack of response of kale leaf length and lettuce SLA to the FR fraction at the lowest TPFD implies that a low R and FR PFD attenuates the effect of the FR fraction in eliciting shade-avoidance responses.
