The light spectrum plants use for growth is often simplified to the visible range, known as Photosynthetically Active Radiation (PAR), spanning 400 to 700 nanometers (nm). However, light outside this visible range, particularly in the infrared (IR) spectrum, plays a complex role in plant development. Infrared light is a broad spectrum, and its benefit depends entirely on the specific wavelength and the biological response it triggers. This information clarifies which parts of the infrared spectrum influence plant health and how this knowledge can be applied in controlled environments.
Understanding the Infrared Spectrum and Plant Sensors
The infrared spectrum is divided into segments, but for plant biology, the most important distinction is between Far-Red light and Thermal Infrared. Far-Red light (Near-Infrared or NIR) occupies the range of approximately 700 to 800 nm, just outside the visible red spectrum. These photons are not efficient for driving primary photosynthesis; instead, they act as an environmental signal detected by the plant’s sophisticated sensory system.
The mechanism for sensing Far-Red light is the phytochrome photoreceptor system, a protein existing in two convertible forms: Pr (inactive) and Pfr (active). Red light (around 660 nm) converts phytochrome to the Pfr form, while Far-Red light (around 730 nm) reverts it back to the inactive Pr form. The ratio of Red to Far-Red light (R:FR) is what the phytochrome system measures, signaling the plant’s current light environment. By shifting this balance, the plant receives information about potential shade or the approach of dusk, influencing its growth strategy.
How Far-Red Light Controls Plant Morphology
The phytochrome system interprets the Red to Far-Red ratio, controlling developmental and architectural responses. When a plant is shaded, the canopy above absorbs Red light but transmits Far-Red light. This results in a low R:FR ratio reaching the plant below, triggering the Shade Avoidance Response (SAR).
The SAR causes a rapid elongation of stems and petioles as the plant attempts to grow taller to escape perceived shade. This often results in a taller, spindly morphology, which is undesirable in agriculture. Conversely, a high R:FR ratio, typical of open sunlight, promotes compact growth with shorter internodes.
Far-Red light also regulates the plant’s life cycle, influencing the timing of flowering in many species. For plants relying on day length cues, exposure to Far-Red light at the end of the day can accelerate the transition from the vegetative to the reproductive stage. This manipulation of photoperiodism is valuable in controlled environments for ensuring timely harvests.
Supplemental Far-Red light can positively affect overall biomass accumulation and leaf expansion. When combined with visible light, Far-Red can enhance leaf size and improve the overall efficiency of the entire canopy. This synergistic effect increases the total light capture surface.
Managing Thermal Infrared and Plant Heat Stress
While Far-Red light (700–800 nm) is used for signaling, longer wavelengths, known as Thermal Infrared (Far-IR), are perceived by plants simply as heat. These wavelengths are emitted by any warm object, including grow lights, and can cause negative effects if not managed. Older high-intensity discharge (HID) lamps, such as HPS lights, are notorious for producing large amounts of Thermal IR, transferring heat directly to the canopy.
Exposure to excessive Thermal IR leads to heat stress, characterized by reduced metabolic function and visible distress. Symptoms include wilting, leaf rolling, and decreased transpiration as the plant attempts to conserve water. High leaf tissue temperature can cause leaf burn and reduce photosynthetic efficiency.
Managing this thermal energy is distinct from controlling the R:FR ratio. Growers must monitor canopy temperature to ensure the heat emitted by the light source stays within the optimal range. Modern LED grow lights emit significantly less Thermal IR compared to older technologies, simplifying temperature management.
Applying Infrared Light in Grow Environments
In modern controlled environment agriculture, growers manipulate the light spectrum using advanced LED fixtures to steer the crop toward specific outcomes. The practical application of Far-Red light focuses on precisely controlling the Red to Far-Red (R:FR) ratio emitted by the grow lights to manage plant height and flowering time.
Controlling Morphology with R:FR Ratio
To achieve compact, stockier growth, growers maintain a high R:FR ratio, signaling full, open sun without competition. Conversely, to encourage elongation or accelerate flowering, they supplement the light spectrum with Far-Red photons, lowering the R:FR ratio. This supplemental Far-Red light is often applied as a short burst at the end of the day to mimic sunset conditions.
Far-Red light is used as a supplement for signaling, leveraging the phytochrome system to fine-tune plant architecture and development. By strategically adding Far-Red light, growers can achieve larger leaves for better light interception and synchronize the flowering process, leading to a more efficient and predictable crop cycle.