Far red light represents a specific portion of the electromagnetic spectrum. It is located at the extreme red end of what humans can typically see. This type of light plays a distinct role in various natural processes, particularly concerning plant life.
What is Far Red Light?
Far red light is defined by wavelengths ranging from 700 to 800 nanometers (nm), placing it just beyond the visible red light spectrum and before infrared light. Humans can barely perceive it, as it falls largely outside our visual sensitivity.
Far red light is a natural component of sunlight. It is present throughout the day. The proportion of far red light changes, notably increasing during sunrise and sunset as light travels further through the atmosphere, shifting the spectrum towards warmer colors. This natural presence means plants are constantly exposed to and respond to far red light cues in their environment.
Its Role in Plant Life
Plants possess specialized photoreceptors called phytochromes, which are highly sensitive to both red and far red light. These phytochromes exist in two main forms: phytochrome red (Pr), an inactive form that absorbs red light, and phytochrome far-red (Pfr), an active form that absorbs far red light. The ratio of red to far red light determines the conversion between these forms, signaling various physiological processes in the plant.
Far red light influences seed germination. This light-sensitive process is regulated by the relative abundance of red and far red light perceived by phytochromes. The balance between Pr and Pfr dictates whether a seed sprouts or remains dormant.
Far red light influences flowering time, particularly in long-day plants where it can accelerate bloom initiation. Supplemental lighting that includes both red and far red radiation is effective in promoting flowering. Plants use far red signals to determine the appropriate timing for reproduction.
Plants also exhibit a shade avoidance syndrome, a set of responses triggered by changes in the red to far red ratio. When plants are shaded by other vegetation, red light is absorbed by the canopy for photosynthesis, while a higher proportion of far red light is reflected or transmitted. This increased far red signal indicates the presence of competitors, prompting plants to stretch their stems and elongate leaves to reach for more direct light.
The impact of far red light on plant architecture is evident in stem elongation and leaf expansion. An increased proportion of far red light relative to red light promotes extension growth, leading to longer stems and larger leaves. While this can increase light interception and overall biomass, an excess of far red can also cause plants to become tall and stretched, potentially reducing branching.
Practical Applications
Understanding the effects of far red light on plants has led to its targeted application in controlled environments, particularly in horticulture and agriculture. In greenhouses and vertical farms, far red LEDs are integrated into lighting systems to optimize plant growth and yield. These specialized grow lights can manipulate plant responses by adjusting the light spectrum.
For example, far red LEDs are used to promote flowering, especially in long-day plants, by providing specific end-of-day light treatments. This can shorten flowering times for some plants while maintaining yields. Far red light also promotes the elongation of hypocotyls, achieving a desirable length.
Supplementing far red light can increase the fresh and dry mass of plants, largely due to increased leaf area and light interception. While far red can enhance photosynthetic efficiency, its overall impact on plant morphology and yield depends on factors such as intensity, duration of exposure, and the specific plant species. Ongoing research continues to explore the optimal application of far red light in various stages of plant growth and for different crops.