For over five decades, commercial CEA horticulture strictly adhered to the traditional McCree curve definition of Photosynthetically Active Radiation (PAR, 400–700 nm), treating far-red photons (700–750 nm) as non-photosynthetic light with negligible quantum yield. However, groundbreaking biophysical research by Zhen, Bugbee, and Wageningen University has rewritten plant photobiology. Far-red photons not only drive direct photosynthetic carbon assimilation when combined with shorter wavelengths via the Emerson Enhancement Effect, but they also serve as the primary environmental trigger for shade avoidance, petiole elongation, and canopy photon interception via the Phytochrome Photostationary State (PSS). In modern European Venlo greenhouses—from Niederrhein (Straelen) to Westland—integrating dynamic far-red LED channels enables growers to balance canopy architecture with unprecedented electrical efficacy ($\mu\text{mol/J}$).
1. The Paradigm Shift: From PAR (400–700 nm) to ePAR (400–750 nm)
Under the historical definition established by K.J. McCree (1972), photons with wavelengths longer than 700 nm were excluded from quantum light meters because their solitary quantum yield of $CO_2$ fixation drops sharply. However, inside natural sunlight and mixed LED spectra, Photosystem II (PSII, peak excitation $\sim 680\text{ nm}$) and Photosystem I (PSI, peak excitation $\sim 700-730\text{ nm}$) operate in series.
When leaves are illuminated exclusively with red and blue photons, PSII is over-excited while PSI becomes rate-limiting, causing an electron transport bottleneck at the plastoquinone (PQ) pool. Supplementing with far-red photons (700–750 nm) selectively excites PSI, clearing the PQ pool and restoring full cyclic and non-cyclic electron transport. This synergism is quantified by the Emerson Enhancement Ratio ($EER$):
EER = \frac{A_{combined}(\lambda_{400-700} + \lambda_{700-750})}{A_{PAR}(\lambda_{400-700}) + A_{FR}(\lambda_{700-750})} > 1.0
Where $A$ denotes the net photosynthetic $CO_2$ assimilation rate ($\mu\text{mol } CO_2 \cdot m^{-2} \cdot s^{-1}$). In high-wire fruiting crops such as tomato (Solanum lycopersicum) and sweet pepper (Capsicum annuum), adding up to 15–20% far-red photons to a red/blue or broad-spectrum background yields an equal or greater quantum yield per absorbed photon than adding an identical flux of pure 660 nm red photons.
2. Mathematical Modeling of Phytochrome Photostationary State (PSS)
While far-red photons directly power photosynthesis, they concurrently regulate plant morphology through the phytochrome photoreceptor system. Phytochromes exist in two photo-reversible conformations:
- $P_r$ (Inactive Form): Absorbance peak at 660 nm (Red). Absorbing red light converts $P_r \rightarrow P_{fr}$.
- $P_{fr}$ (Biologically Active Form): Absorbance peak at 730 nm (Far-Red). Absorbing far-red light converts $P_{fr} \rightarrow P_r$.
Under any spectral power distribution $N(\lambda)$ ($\mu\text{mol} \cdot m^{-2} \cdot s^{-1} \cdot nm^{-1}$), the ratio of active phytochrome to total phytochrome reaches a dynamic photostationary state ($PSS$), mathematically formulated using the photochemical cross-sections of Sager et al. (1988) and Gardner (1993):
PSS = \frac{[P_{fr}]}{[P_{total}]} = \frac{\int_{300}^{800} \lambda \cdot N(\lambda) \cdot \sigma_r(\lambda) \, d\lambda}{\int_{300}^{800} \lambda \cdot N(\lambda) \cdot \sigma_r(\lambda) \, d\lambda + \int_{300}^{800} \lambda \cdot N(\lambda) \cdot \sigma_{fr}(\lambda) \, d\lambda}
Where:
- $\sigma_r(\lambda)$: Photochemical extinction coefficient and quantum yield of $P_r \rightarrow P_{fr}$ conversion at wavelength $\lambda$.
- $\sigma_{fr}(\lambda)$: Photochemical extinction coefficient and quantum yield of $P_{fr} \rightarrow P_r$ conversion at wavelength $\lambda$.
In open sunlight, the red-to-far-red ratio ($R:FR \approx 1.2$) establishes a $PSS \approx 0.70 - 0.72$. Beneath dense crop canopies, chlorophyll selectively absorbs 400–700 nm while transmitting >80% of far-red photons, causing $R:FR$ to plummet below 0.2 and driving $PSS$ down to $0.45 - 0.55$. This induces the classical Shade Avoidance Syndrome (SAS).
3. Horticultural Steering: Canopy Expansion vs. Harvest Index
Manipulating the $PSS$ enables precision control over vegetative architecture without chemical plant growth regulators (PGRs):
| PSS Range | Morphological Response | Agronomic Objective | Typical CEA Application |
|---|---|---|---|
| 0.80 – 0.88 | Extreme compact internodes, thick leaves, high specific leaf weight ($SLW$). | Prevent stretching in young transplants; dwarf bedding plants. | Nursery propagation, seedling grafts, vertical indoor leafy greens. |
| 0.68 – 0.74 | Natural open-field phenotype; balanced vegetative/generative partitioning. | Standard vegetative maintenance; maximum harvest index ($HI$). | Commercial Venlo high-wire tomato & pepper production. |
| 0.55 – 0.65 | Rapid leaf lamina expansion, elongated petioles, reduced chlorophyll concentration per unit area. | Accelerate canopy ground closure; maximize fractional light interception ($f_{int}$). | Early transplant establishment; winter low-DLI supplemental steering. |
4. End-of-Day (EOD) vs. Continuous Daytime Far-Red Regimes
Growers face two distinct engineering strategies for applying far-red photons:
Strategy A: Continuous Daytime Far-Red ($ePAR$)
Far-red LEDs (730 nm peak) operate throughout the entire photoperiod alongside broad-spectrum or red/blue top-lights. This simultaneously harnesses the Emerson enhancement effect (boosting net canopy carbon fixation by 8–12%) and maintains a moderate $PSS \approx 0.65$. Because leaf lamina expands more broadly, the canopy absorbs more total daily incident photons ($DLI$), compounding yield gains over 20-week cropping cycles.
Strategy B: End-of-Day (EOD) Far-Red Pulse
To avoid excessive stem stretching during daytime high-DLI conditions while capturing the morphological benefits of shade avoidance, a short pulse of far-red light (15–30 minutes at $20-40 \, \mu\text{mol} \cdot m^{-2} \cdot s^{-1}$) is delivered immediately after the main photoperiod shuts off. This resets phytochrome to the inactive $P_r$ state ($PSS < 0.40$) for the duration of the night. In high-wire cucumber (Cucumis sativus), EOD far-red accelerates petiole elongation and prevents leaf crowding in dense vertical rows without reducing fruit dry matter partition.
5. Energy Efficacy and Venlo Greenhouse Thermal Balance
From an electrical engineering standpoint, modern 730 nm far-red LED packages exhibit exceptionally high electrical-to-optical conversion efficiency ($\eta_{opt} \approx 70-75\%$), achieving fixture efficacies exceeding $3.8 - 4.2 \, \mu\text{mol/J}$—substantially higher than phosphor-converted white or 450 nm royal blue LEDs. Furthermore, because far-red photons are less strongly absorbed by greenhouse structural glass and upper canopy shade screens, radiant dissipation patterns differ markedly from traditional high-pressure sodium (HPS) luminaires.
\Delta H_{canopy} = \int \left[ (1 - \alpha_{PAR}) \cdot PPFD + (1 - \alpha_{FR}) \cdot PFD_{FR} \right] d\lambda + \dot{Q}_{convection}
By shifting 15% of supplemental light fixture power to far-red channels, growers in cool maritime climates like North Rhine-Westphalia can deliver equal photosynthetic carbon driving force while reducing electrical connected load by 7.4% and mitigating midday leaf temperature overheating.
6. Conclusion: Engineering Guidelines for Modern CEA Operators
Integrating ePAR and PSS metrics into commercial climate and lighting computers (e.g., Priva, Hoogendoorn, Ridder) represents the cutting edge of controlled environment agriculture. Key operational rules for growers include:
- Adopt ePAR Light Sensors: Upgrade quantum sensors to wide-band detectors measuring 400–750 nm to account for the full photochemical driving force.
- Target $PSS \approx 0.70$ for High-Wire Fruiting Crops: Maintain $R:FR \approx 1.2-1.5$ during full fruit-load stages to avoid unwanted vegetative sink competition.
- Deploy EOD Pulses for Canopy Opening: Utilize 20-minute EOD far-red treatments during winter low-light weeks to expand light interception without excessive structural stretching.