Plant Morphology and a Phytochrome B Model Reveal That the Effects of Far‐Red Light on Shade‐Avoidance‐Like Responses Persist Under High Light Intensity

Several studies have concluded that high photon flux density (PFD) attenuates the effects of the red (R; 600-699 nm) to far-red (FR; 700-750 nm) light ratio on morphology. However, the suppressive effects can depend on individual wavebands that modulate photoreceptor activity. We postulated that morphological responses of shade-avoiding plants to the FR fraction (FR-PFD divided by R + FR PFD) act independent of total PFD (TPFD; 400-750 nm) when TPFD increases are only from R and FR light. We grew kale and lettuce under three FR fractions and four TPFDs while maintaining a constant blue (B; 400-499 nm) PFD. An increase in the R + FR PFD reduced leaf elongation and specific leaf area (SLA). However, higher light did not suppress the FR-fraction effects on leaf elongation and SLA. We estimated PHYB activity with a three-state PHYB model to mechanistically explain the suppressive effects of high light on leaf elongation and SLA but not on FR-mediated leaf elongation and SLA increase. PHYB model predictions were in accordance with the morphological responses of kale and lettuce. This study is the first to apply the three-state PHYB model to explain photon-spectrum-induced morphology of light-grown whole plants, demonstrating its potential use to crops and for applications.