Introduction to Far-Red (FR) Light in Horticulture
Far-Red (FR) light, with wavelengths ranging from 700 to 800nm, lies just outside the Photosynthetically Active Radiation (PAR) spectrum of 400–700nm and is barely visible to the human eye. While FR photons have low quantum efficiency for photosynthesis due to their insufficient energy to drive photochemistry and minimal absorption, they can still play a significant role in plant growth. When combined with shorter-wavelength photons, FR light can effectively complement PAR light, particularly in controlled environments where it can enhance the impact of both sole-source and supplemental artificial lighting.
Characteristics and Impact of Far-Red Light
Under natural solar radiation, FR light is abundant at dawn, dusk, and under cloud cover, providing plants with environmental cues to align their circadian rhythm. Integrating FR light into artificial lighting systems presents certain challenges, as FR light is typically more costly and less efficient to produce with LEDs. However, FR can be a valuable supplement to PAR light, promoting specific growth traits when carefully managed.
The Importance of Red to Far-Red (R:FR) Ratios
The red (R) to far-red (FR) light ratio, or R:FR ratio, plays a crucial role in plant development. This ratio averages around 1.3 in direct sunlight but drops to 0.6 at twilight, which helps plants synchronise their internal clocks with day-night and seasonal cycles. A single leaf layer can reduce the R:FR ratio from 1.3 to 0.2 or lower, signalling plants to adjust their growth in response to shading.
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Plants detect shifts in R:FR ratios through phytochromes, specialised light-sensitive proteins that respond to changes in light conditions. When plants detect lower R:FR ratios, indicating shade or competition, they trigger shade avoidance syndrome (SAS), reallocating resources to stem elongation and canopy expansion to maximise light capture.
Approximately 20-25% of sunlight within the 400–800nm spectrum is far-red light, but conventional artificial light sources emit much less. High-pressure sodium (HPS) lamps have an R:FR ratio around 4.8, metal halide lamps range from 2.6 to 3.4, and various fluorescent lamps have R:FR ratios from 5.5 to 13. With LED technology, however, the R:FR ratio is configurable based on the LED specification and printed circuit board (PCB) composition.
Effects of High R:FR Ratios
High R:FR ratios have been proven to stimulate photosynthesis, boost plant biomass, and encourage the development of larger leaves, elongated stems, and increased plant height. These changes improve canopy structure and air circulation, promoting overall plant health and productivity. For crops like strawberries, combining high R:FR ratios with blue light can accelerate flowering and enhance fruit yields. Ornamental plants often exhibit extension growth, which can impact leaf size, stem length, and overall height, influencing desirability.
In cannabis, elevated R:FR ratios have been linked to higher cannabinoid (CBD, THCVA, CBGA) and terpene concentrations, along with greater inflorescence yields, though conflicting research exists, and results can vary across cultivars. Leafy greens also benefit from some FR treatments, as modifying vegetative morphology can increase harvestable weight, improve crop value, and reduce growth cycles.
Given these benefits, Far-Red (FR) light is a valuable tool in speed breeding programs. By influencing the activity of phytochromes Pfr and Pr, high R:FR ratios can accelerate flowering in certain species, enabling researchers to achieve multiple generations in a shorter timeframe.
Challenges of Low R:FR Ratios and Excessive Far-Red Exposure
Low R:FR ratios can negatively impact plant morphology in young plants, potentially leading to stunted or undesirable growth and reduced fruit production. Excessive exposure to FR light, particularly early in development, can cause overstretched and weakened structures in many plants. In cannabis, this may compromise bud formation and structural support, while in ornamental plants, overuse of FR light can reduce anthocyanin production, altering colour and diminishing perceived quality.
In leafy greens, excessive FR exposure can accelerate elongation beyond biomass accumulation, resulting in pale, structurally weak leaves. Prolonged low R:FR exposure during cultivation has also been linked to shorter shelf life and reduced nutritional quality due to increased oxidative stress and a decline in both enzymatic and non-enzymatic antioxidant activity.
Leveraging Far-Red Light in Controlled Environments
In controlled environment agriculture (CEA), Far-Red (FR) light is a powerful tool for crop steering when thoughtfully integrated with other light spectra. By precisely adjusting the R:FR ratio in relation to PAR light, growers can tailor plant morphology to meet specific objectives. Additionally, end-of-day FR treatments can simulate natural dusk cues, promoting flowering, adjusting harvest timing, and reducing overall energy consumption in some cases.
Optimizing lighting in CEA requires careful management of light quality (spectra and uniformity), quantity (photoperiod, PPFD, DLI), and delivery (control and distribution), all of which must be balanced alongside other environmental and plant factors. As technology advances, growers gain access to increasingly sophisticated control systems, enabling more precise adjustments to light recipes. With modern LED technology and intelligent control platforms, well-calibrated FR light has the potential to enhance crop quality and yield, while also promoting energy-efficient and sustainable cultivation practices.
