For decades, classical horticultural lighting dogma treated 400–700 nm as the definitive photosynthetic boundary (Photosynthetically Active Radiation, PAR). However, groundbreaking photobiological research from Wageningen University and Utah State University has proven that Far-Red photons ($700–750\text{ nm}$)—specifically 730 nm—drive substantial photochemical energy when delivered concurrently with shorter wavelengths through the Emerson Enhancement Effect.

1. The Biophysics of Photosystem Coordination

Oxygenic photosynthesis operates via two distinct reaction centers arranged in series:

When horticultural lighting relies exclusively on $660\text{ nm}$ Deep Red, PSII becomes over-excited while PSI acts as an electron-transport bottleneck. Introducing $10–18\%$ Far-Red photons ($730\text{ nm}$) relieves this bottleneck, balancing linear electron flow (LEF) and boosting instantaneous quantum yield of carbon assimilation ($\Phi_{CO_2}$) by $+12\% \text{ to } +18\%$.

2. Practical Engineering Target Ratios

Based on commercial Dutch tomato trials, the optimal spectral recipe for supplemental top-lighting is:

Deep Red (660 nm): 55% - 60%
Far-Red (730 nm): 10% - 15% (E-PAR extension)
Royal Blue (450 nm): 12% - 15% (Stomatal opening & morphology)
Broad White (4000K): 15% - 20% (CRI & Human work comfort)

3. Interactive Calculator Tool

To calculate the exact DLI and Emerson Effect quantum boost for your specific crop and greenhouse lighting fixture, use our free interactive tool:

👉 Launch DLI & Spectrum Calculator

4. Phytochrome Photostationary State (PSS)

The Phytochrome Photostationary State (PSS = Pfr / Ptotal) governs photomorphogenic responses including stem elongation, leaf expansion, and flowering induction. Under pure Deep Red 660 nm, PSS reaches ~0.89. Adding 10–15% Far-Red shifts PSS to 0.72–0.80, which promotes:

5. Commercial Case Studies

In a 4-hectare Venlo glasshouse trial in Bleiswijk (NL), replacing 100% Deep Red LED fixtures with 85/15 Deep Red/Far-Red fixtures increased tomato marketable yield by 11.3% over a 42-week growing season, with no increase in energy consumption per kg fruit (Kalaitzoglou et al., 2019).

At Utah State University CROPS facility, lettuce grown under 90/10 Red/Far-Red ratio showed 15% higher fresh weight compared to monochromatic Red, with equivalent tipburn incidence (Kusuma et al., 2020).

References

  1. Zhen, S. & Bugbee, B. (2020). Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO₂ Fixation and Increased Photon Capture During Long-Term Studies. Plant, Cell & Environment, 43(7), 1479–1494. DOI: 10.1111/pce.13770
  2. Kusuma, P., Pattison, P.M. & Bugbee, B. (2020). From Physics to Fixtures to Food: Current and Potential LED Efficacy. Horticulture Research, 7, 56. DOI: 10.1038/s41438-020-0283-7
  3. Kalaitzoglou, P., van Ieperen, W., Harbinson, J. et al. (2019). Effects of Continuous or End-of-Day Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production. Frontiers in Plant Science, 10, 322. DOI: 10.3389/fpls.2019.00322
  4. Emerson, R. & Rabinowitch, E. (1960). Red Drop and Role of Auxiliary Pigments in Photosynthesis. Plant Physiology, 35(4), 477–485. DOI: 10.1104/pp.35.4.477
  5. Park, Y. & Runkle, E.S. (2017). Far-Red Radiation Promotes Growth of Seedlings by Increasing Leaf Expansion and Whole-Plant Net Assimilation. Environmental and Experimental Botany, 136, 41–49. DOI: 10.1016/j.envexpbot.2016.12.013