Why does the sunflower turn its head toward the sun? Why do some plants only bloom at night? Why are some plants more fragrant at certain times of the day? Part of the answer lies in one word: light.

More than just lighting, light acts as a signal, an energy source, and a biological clock for plants. It guides their growth, development, and behavior.

In this series of articles, we explore how light influences plants growth and survival. The first episode focused on the vital role of photosynthesis (link to our first article), the primary metabolic source for plants and the core of plant activity. This second article explores how light acts as a signal for plants to trigger many vital functions.

Episode 2: Light, a signal that triggers key mechanisms in plants

While light is essential for photosynthesis, which produces vital energy for the plant, it also serves as information—a language that plants are able to decode using sophisticated receptors.

The role of photoreceptors in light perception

Unlike animals, plants have neither eyes nor nervous systems to see light. However, thanks to photoreceptors, they can perceive a great deal of information about light and adapt their behavior in response to the signals received.

Perception of light quality

The quality of light refers to its composition in various wavelengths: red, green, blue, far-red, and ultraviolet.

The wavelengths visible to humans range from violet to red. Plants, on the other hand, are sensitive to wavelengths within the human-visible range, as well as to adjacent ultraviolet and infrared regions. Wavelengths are measured in nanometers.

Note: The light we perceive as white is actually composed of multiple wavelengths. These colors correspond to different parts of the light spectrum visible to humans. You’ve seen this split before! When a beam of white light passes through a prism or a raindrop (as in a rainbow), it separates into these different colors.

The capture of different wavelengths is made possible by plant light receptors: photoreceptors 1.

Photoreceptors: essential light sensors

• Chlorophyll primarily absorbs blue and red light, reflecting green light, which is why we see plants as green. Chlorophyll is essential for capturing light and producing energy via photosynthesis.

For more details, see our first article on photosynthesis.

• Phytochromes respond to red (R) and far-red (FR) light. They play a crucial role in seed germination, plant growth, and responses to light and shade. They also regulate processes like germination, leaf development, and flower formation.
• Cryptochromes respond to blue and UVA light. They are involved in stomatal function, circadian rhythm regulation, inhibition of stem elongation, and flowering.
• Phototropins capture blue light and are responsible for plant orientation toward light (phototropism) and stomatal opening.
• UVR8 receptors help plants detect UV-B light.

Photomorphogenesis: a light signal behind morphological changes

Photomorphogenesis is a fundamental process where light acts as a trigger, guiding plant development to secure resources essential for survival (light, nutrients, etc.). Unlike photosynthesis, photomorphogenesis involves a low amount of light energy—light acts more as a signal here.

Impact of photomorphogenesis on the plant

• Blue Light shortens the internodes (the space between leaves on the stem), making the plant more compact. This allows the plant to optimize light usage, especially in low-light environments, such as under other plants.

Note: Blue light is particularly important for seedlings and young plants, as it prevents elongation and promotes a strong, compact structure

• Red Light promotes vertical growth, allowing stems to elongate quickly. This is crucial in dense or shaded environments like forests. In response to red light, plants stretch their stems to reach better light sources.

Note : Red light is important for regulating flowering and fruit production

• Shade-avoidance syndrome is an adaptive response to low light caused by competition from other plants. Since red light is absorbed and far-red light is transmitted, shaded plants detect a change in the R/FR ratio. This triggers elongation to help them reach light.

Other impacts of photomorphogenesis

Opening of cotyledons

Cotyledons, the plant's first embryonic leaves, stay closed and colorless without light. When exposed:
- they open and spread out from the plant's central axis.
- they turn green as chlorophyll is activated.
- they begin photosynthesis, providing energy to the seedling. This marks the plant's transition to autotrophy—its ability to produce its own energy via photosynthesis.

Leaf development

Under light, the plant actively develops leaves:

• Leaf primordia (tiny buds atop the stem that will form the plant’s future leaves) grow faster.
• Leaves become broader and flatter, increasing surface area for light capture.
• Internal leaf structure specializes: palisade tissue—tightly packed, elongated cells rich in chloroplasts, and spongy tissue—looser structure aiding gas exchange. This specialization maximizes light capture and gas regulation.

Pigment production

Light stimulates the synthesis of several essential pigments:

• Chlorophyll, green pigments that captures light energy for photosynthesis
• Carotenoids, yellow-orange pigments involved in photoprotection and energy dissipation.
• Anthocyanins, red, violet or blue pigments that may appear under certain light conditions, offering a protection against UV and oxidative stress.

These pigments are functional (photosynthesis, protection) and give a diagnostic, indicating the plant's condition.

Light and plant physiology

Beyond morphology, light acts as a powerful physiological signal, regulating several vital plant functions:

Stomatal Opening

• Blue light stimulates the opening of stomata—tiny pores on leaves.

These allow the plant to:
- Absorb CO₂ for photosynthesis.
- Release water vapor (transpiration). A good management of the stomates help the plant to regulate temperature and water use.

• Red light helps regulate stomatal closure during water stress.

Note on water, energy, and yield efficiency:
Most incident solar energy is converted into latent heat (evapotranspiration). Only 1–5% is used for photosynthesis to fix CO₂ and produce biomass 2. The ratio of evaporated water to biomass produced is called Water Use Efficiency (WUE). Optimizing water and light use is essential for efficient cultivation.

Circadian Rhythms

Thanks to photoreceptors like phytochromes (sensitive to red and far-red light), plants regulate their internal biological clock and detect day length (photoperiod).
The spectral composition of light changes during the day, with more far-red light in the early morning and evening, allowing plants to synchronize activities (photosynthesis, stomatal opening, flowering) with natural 24-hour cycles.

Example of Four o'clock flower and its nocturnal anthesis—flower opening—at night

Mirabilis jalapa, commonly known as the Four o'clock flower, is named for its remarkable trait: its flowers open only in the late afternoon and at night.

This phenomenon is triggered by the plant’s internal biological clock, regulated by ambient light sensed by photoreceptors such as phytochromes.

The nocturnal opening allows the plant to attract its main pollinators: moths.

Hormone Production

Light influences the synthesis and distribution of plant hormones like auxin, gibberellin, and abscisic acid, which control growth, dormancy, flowering, and stress responses.

Example of the sunflower’s race with the sun

So what’s happening with the sunflower? Why and how does it follow the sun throughout the day?

When phototropins detect light, a growth hormone—auxin—is sent to the shaded side of the stem. The shaded side grows faster, causing the sunflower’s head to tilt toward the light. At night, things change! Only the “west-facing” side of the stem grows, reorienting the flower eastward. This process lets the flower absorb more heat from the first rays of sun and become more attractive to bees.

This behavior lasts only during the sunflower’s growth phase. Once the flower opens, it locks into an eastward direction permanently 3.

Stress Responses and Metabolism

For instance, UV-B light—detected via UVR8—triggers protective mechanisms against cell damage, such as antioxidant production or protective pigments.
These responses will be discussed in more detail in Article 3 of our series: When Light Shapes Plant Chemistry.

Light but not only !

Light is a major signal triggering numerous physiological mechanisms in plants. But it never acts alone. Its effects are often modulated by other environmental factors such as temperature and water availability.

• Temperature affects biochemical reaction speed and development pace. Some plants, like winter cereals, need prolonged cold (vernalization) along with light signals to trigger flowering.
• Water availability interacts with light to regulate transpiration, growth, and drought responses. In intense light and heat, stomata may close to limit water loss, though this reduces photosynthesis.
• Other signals like gravity (gravitropism) or biotic interactions (with microorganisms, insects, or other plants) also trigger physiological responses, often in coordination with light signals.

Conclusion

In our first article, we explored photosynthesis—the metabolic core and fundamental engine of plant activity. In this second chapter, we have highlighted another key role of light: as a triggering signal that modulates plant morphology, physiology, and behavior.

From an agronomic perspective, mastering light opens up exciting possibilities: shaping plant form, triggering key phenological phases, or driving plant metabolism. But what plant biology study reveals it’s its complexity. There is no single “optimal light”: every wavelength, intensity, and duration can produce vastly different effects—from growth, to flowering, to plant shape — which in turn affects light capture capacity.

Yet, light never acts alone. It interacts with temperature, water, and many other factors to orchestrate plant development. These combined signals shape the plant's trajectory.

It is with this understanding that we at Orius have developed innovative units like the BiomeboxⓇ and Biomecell. These systems recreate a complex, controlled environment—a biome with the ideal conditions for plant cultivation. Thanks to these high-performance cultivation units, our biology research team can optimize every phase of plant development—from germination to flowering and fruiting.

Coming soon: the third (and final!) article in our series on light: Episode 3: When light shapes plant chemistry.

References

1. Kang, W. H., Kim, J., Yoon, H. I. & Son, J. E. Quantification of Spectral Perception of Plants with Light Absorption of Photoreceptors. Plants 9, 556 (2020).

2. Zhu, X.-G., Long, S. P. & Ort, D. R. Improving photosynthetic efficiency for greater yield. Annu. Rev. Plant Biol. 61, 235–261 (2010).

3. Atamian, H. S. et al. Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science 353, 587–590 (2016).