2019 ASHS Annual Conference

Researchers have sought to characterize the effect of light quality on photosynthesis for nearly a century. Early studies used prisms to create monochromatic light and concluded that far-red photons (greater than 700 nm) resulted in minimal photosynthesis. Recent studies have found that far-red photons act synergistically with photons of shorter wavelengths to increase leaf photochemical efficiency and photosynthetic rate. However, the effects on whole-plant photosynthesis have not been characterized. Using a whole-plant gas exchange system, we quantified whole-plant photosynthetic responses of 16 C₃ and two C₄ crop species, to the addition of far-red to a cool white LED light. Our results showed that on incident light basis, adding 701-750 nm far-red photons (10 to 30% of total photon flux) caused similar increase in whole canopy photosynthesis as adding the same amount of 400-700 nm photons. Photosynthesis under far-red alone, however, was less than 20% of that under the same photon flux of 400-700 nm photons. This indicates that far-red photons are equally efficient at driving photosynthesis when act synergistically with traditionally defined photosynthetic photons (400-700 nm). To demonstrate proof of concept, we grew lettuce under light spectra with or without 15% of far-red photons, where all light treatments received equal total photon flux within 400-750nm and should elicit equal photosynthetic rate of the same plant canopy. Percent ground cover, which closely correlates with radiation capture, was measured daily in addition to photosynthesis. At harvest, dry biomass of lettuce was 35% higher in treatments with far-red photons, which was almost entirely caused by enhanced radiation capture through leaf expansion. Our data indicate that far-red photons not only directly increase whole canopy photosynthesis, but also induce leaf expansion and promote canopy radiation capture, thus indirectly increasing plant growth during long-term cultivation. In conclusion, far-red photons are far more valuable than previously thought and should be included as photosynthetically active radiation.