FR has long been considered to have a minimal input in photosynthesis and is excluded from the definition of Photosynthetically Active Radiation (PAR; 400 to 700 nm). It is because the photosynthetic efficiency of monochromatic FR light sharply declines with the wavelength. FR is largely reflected and transmitted by plant leaves, and only about 30% is absorbed.
Several recent studies have shown that far-red photons interact with shorter wavelength photons to increase efficiency of photosynthesis. Zhen and Bugbee (2020) have studied effect of FR on whole plant photosynthesis for 14 different crop species. They concluded that adding far-red photons to a spectrum of shorter wavelengths (e.g. broad white spectrum) caused an increase in canopy photosynthesis equal to adding additional light from PAR range (400-700 nm) of the same intensity. The effect was wavelength dependent. The authors postulated that radiation between 700 and 750 nm should be included in the definition of Photosynthetically Active Radiation (PAR).
