Role of light in plant growth - Ep.1 Photosynthesis
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. This first episode focuses on the vital role of photosynthesis.
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. This first episode focuses on the vital role of photosynthesis.
Episode 1: Why is light vital for plants? The role of photosynthesis
How does a plant grow, and where does it get its vital energy from? The answer lies in a key biochemical process: photosynthesis. Through this mechanism, plants convert light into usable energy.
How does photosynthesis work ?
Photosynthesis begins with the capture of light by a green pigment: chlorophyll, found mainly in plant leaves.
This pigment is a photoreceptor that absorbs light energy—primarily red and blue wavelengths—triggering a chain of chemical reactions inside chloroplasts, the "energy factories" of plant cells.
These reactions include:
1- Photolysis of water: Water absorbed by the roots is split into oxygen (O₂), protons (H⁺), and electrons (e⁻).
2- Electron transport chain: The released electrons power the production of ATP and NADPH (two forms of chemical energy).
3- These compounds (ATP and NADPH) fuel the Calvin-Benson cycle, allowing for the conversion of CO₂ into carbohydrates (sugars) used for plant growth.
Energy produced by photosynthesis: the plant's fuel
The carbohydrates produced during photosynthesis are essential: they supply the energy required to build the organic compounds necessary for cell growth, and the formation of leaves, stems, roots, or fruits.
These carbohydrates are mainly simple sugars (hexoses) produced during daylight through photosynthesis.
A portion of them is converted into complex sugars (sucrose and starch):
- Starch acts as an energy reserve that can be broken down into glucose when photosynthesis cannot occur.
- Sucrose is used for transporting energy and nutrients throughout the plant's vascular system, particularly to the roots, fruits, and growing parts.
Tracking starch levels in a leaf over the course of a day shows a cyclical oscillation between production and export1:
- During the day, the plant produces and stores starch.
- At night, the plant exports this starch, a process especially important for tissue growth and fruit sugar loading.Natural light: variability and impact
Natural light: variability and impact
Light received by plants varies naturally with the season, time of day, climate, and even cloud cover. Three key concepts help explain these variations:
Photoperiod
The duration of light exposure. During summer, days are longer than in winter. A 16-hour photoperiod means 16 hours of light followed by 8 hours of darkness.
Light intensity
The amount of light at a given moment, measured in µmol·m⁻²·s⁻¹ (PPFD - Photosynthetic Photon Flux Density). In Southern France during summer, morning light might be 200–400 µmol·m⁻²·s⁻¹, reaching 1800–2000 at noon. These values are much lower in winter.
Light intensity is not constant: it varies with the weather and even with movements of the canopy. These fluctuations likely play an important role in plant physiology, particularly in its photosynthetic capacity.
Quality of light
Beyond quantity, the quality of light — that is, its composition in wavelengths — also changes throughout the day.
The quality of light refers to the composition of wavelengths (Red, Green, Blue, Far-Red, Ultraviolet).
Light quality changes throughout the day. At sunrise and sunset, the sun is low on the horizon. Its light passes through a thick layer of atmosphere, which strongly scatters short wavelengths (blue and violet). As a result, direct light becomes rich in red and far-red wavelengths, creating the characteristic orange hue of dawn and dusk. At the same time, diffuse light from the sky is richer in blue wavelengths.
This spectral variation has a significant impact on plants and involves specialized photoreceptors, which will be discussed in Article 2: Light as a signal triggering key mechanisms in plants.
Wavelengths visible to humans range from violet to red. Plants, however, are sensitive not only to wavelengths within the human visible spectrum, but also to those in the adjacent ultraviolet and infrared regions.
Correlation between light and photosynthesis activity
There is a direct correlation between light intensity and photosynthetic activity2. Activity increases with light intensity up to a saturation point (Is) where it maxes out.
Below a certain compensation point (Ic), respiration consumes more carbon than the plant can fix, limiting growth.
Different wavelengths have different effects.
Chlorophyll, the photoreceptor involved in photosynthesis, mainly absorbs blue and red light, while reflecting green light3—which is why most plants appear green.
In the graph comparing photosynthetic4 activity under various wavelengths and intensities, a red/blue (RB) ratio of 1:1 (equal red and blue light) yields results close to those under white light (which includes all wavelengths).
This confirms that blue and red light are the most effective for photosynthesis.
Pulsed Light: more efficient photosynthesis?
In nature, light can also fluctuate — over longer periods (such as during cloud cover) or shorter ones (like canopy movement). Pulsed light is used by researchers to try to figure out what happens to plant growth when the light and dark change quickly.
Some studies show that the use of pulsed light can positively stimulate photosynthesis and plant growth:
- on tomato plants, Tennessen et al. (1995)5 observed a higher photosynthetic rate with pulses of 198 ms of light followed by 2 ms of darkness.
- on potato seedlings, Jao and Fang (2004)6 reported improved growth using LEDs at 720 Hz with a 50% duty cycle.
- on lettuce plants, Yoneda and Mori (2004)7 achieved increased fresh weight and photosynthetic activity with pulses at 10 kHz and a 50% duty cycle.
However, Hashimoto et al. (1988)8 reported negative results in lettuce production when using pulsed fluorescent lamps at 37 kHz (37,000 oscillations per second), compared to lamps at 60 Hz (60 oscillations per second).
These differences highlight the need for further research to better understand the effects of pulsed light at different wavelengths, frequencies, and duty cycles.
Conclusion
Photosynthesis is the primary metabolic source for plants and the core of plant activity. Its efficiency is influenced by several factors:
- Light intensity: directly affects photosynthetic output.
- Light quality: red and blue wavelengths are most efficiently absorbed by chlorophyll.
- Duration and consistency of exposure: regulates the photosynthesis rhythm.
- Day/night cycles: essential for managing energy reserves and supporting plant growth.
Photosynthesis is tightly linked to the environmental conditions in which the plant grows.
From an agronomic perspective, mastering these parameters opens up exciting possibilities: adjusting photoperiod, optimizing intensity and spectrum, and using innovative lighting technologies. These tools can boost growth, plant health, and crop quality—while also reducing energy costs.
However, there is no one-size-fits-all solution. Each plant species responds differently to light. Lighting strategies must be tailored to the specific needs, cultivation goals, and quality criteria of each crop.
At Orius, our mission is to maximize energy-to-biomass conversion by ensuring an optimal plant production in our cultivation units. We carefully avoid surpassing the photosynthesis saturation point and monitor limiting factors like CO₂ concentration. Yet our approach goes beyond maximizing yield: we also aim to enhance the intrinsic qualities of plants, focusing on their phytochemical profiles and the active compounds they produce.
Coming next in our articles dedicated to light:
Episode 2: Light as a signal triggering key plant mechanisms
Episode 3 : When light shapes plant chemistry
References
7. Yoneda, K. & Mori, Y. Method of cultivating plant and illuminator for cultivating plant. (2004).