By Yves Pichon, Plantlife Technologies’ team leader
Horticulture luminaires have been created to bring energy to plants. That seems obvious but when I look at what some luminaire manufacturers tend to promote, I have a growing feeling that this is getting forgotten.
So, let’s start by some fundamentals to get the topic of plants and light clear to everyone:
1/ What is light role for plants?
- Life: Plants cannot live without light.
Plants use light energy (photons) and water to transform CO2 into carbohydrates that feed them. This is done through photosynthesis. - Grow & develop
Light triggers multiple bio-chemical processes that drive the plants grow (size, leaves shape, orientation, color…) but also regulate its life such as the control of gas exchange between the atmosphere and the plant. - Time management
As for many other living species, time plays a major role in plants’ development. Plants are sensitive to day and night and to seasons (primarily because of the change between short days and long days). Light triggers time related processes in plants. - Protect
Light triggers and regulates the production of substances such as terpenes that plants use against predators or diseases.
2/ How do plants perceive light
Plants perceive light through photosensitive pigments. There are a lot of different photosensitive pigments, each of them with playing a specific role in the plants’ life and development and each of them absorbing light in a different way.
The most important pigments are the following:
Chlorophyll
Chlorophyll is absolutely critical to plant life as it absorbs light and transforms its energy into chemical energy that is consumed by the plant to live and develop. This takes place in a process known as photosynthesis. The most common forms of chlorophyll are Chlorophyll a, that initiates photosynthesis and Chlorophyll b that increases the energy yield of photosynthesis.
In order to properly generate photosynthesis, a horticulture luminaire must provide light within the absorption peaks of both chlorophyll a and b. The closer to the absorption peaks, the better: i.e: Around 430nm or around 665nm for chlorophyll a and around 460nm for chlorophyll b.
Carotenoids
Their role has been documented much more recently than Chlorophyll. They play 2 major roles
- The first one is to absorb photons, transforms them into electrons that are transmitted to chlorophyll.
- The second one is less known, but just as important, regulates multiple plants’ function such as growth.
The 2 roles of carotenoids must be considered when designing a light spectrum. Carotenoids key specific actions are triggered between 440nm(blue) and 540nm (green). Not enough green reduces photosynthesis and limits plants growth and development, but too much green increases the growth control function and also reduces plant growth and development of active substances
The most recent studies have demonstrated that the optimal green level is between 20 & 24%
Phototropin
It is a photoreceptor pigment. Photoreceptors are sensors that trigger biological mechanisms when they sense light at a certain wavelength. Phototropin triggers 2 important mechanisms:
- Stem elongation (plant growth) towards light.
- Stomata opening. Stomata are cells on the leaves surface that open to allow gas exchange between the plant and the atmosphere (a bit like breathing).
Phototropin absorbs blue light (primarily between 440nm and 480nm). Providing enough blue light is therefore critical to initiate plants’ grow between the seedlings and the end of the vegetative phases. However, too much blue will lead the plant to grow tall without developing enough leaves. Therefore, a balanced level of blue is required.
Phytochrome
It is a photoreceptor that plays a critical role in triggering flowering in short day and long day plants, but it also regulates almost all development phases of the plants.
Phytochrome requires both red and far-red light to have an effect on plants. Under red light (620 to 700nm), phytochrome is inactive but is modified to become sensitive to far-red light (700 to 775nm) and become active, and, for example, trigger flowering.
Maximizing the role of phytochrome requires light to include enough far red.
There are many other pigments in plants, their quantity and their roles are very different from one plant specie to another one, so there is not a standard light absorption path across all plants. Also, a lot remains to be discovered and explained about interactions between lights and plants. For example, originally horticulture luminaires were primarily aiming at providing blue & red as chlorophyll was the only pigment properly documented. Then the role of carotenoids and their reactions to light started to be studied which triggered a strong development of “full spectrum” type luminaires with a lot of green. Now, we know about the grow regulation role of carotenoids and the necessity to limit green to about 24% on flowering plants for a balanced development.
3/ The importance of light spectrum in plants’ development
As we saw above, photosensitive pigments play a critical role in plants development. Each of them absorbs light at within specific wavelength ranges. This means 2 important things:
1/ Light provided to plants must cover all relevant pigments absorption spectra
- For Chlorophyll A:
Either around 430nm Blue or around 665nm Red. - For Chlorophyll B:
Around 460nm Blue - For Carotenoids:
Between 440nm Blue and 540nm Green - For Phototropin:
Between 440nm and 480nm - For Phytochrome:
Between 620nm and 700nm to make it active.
Between 700nm and 750nm to trigger actual benefits to the plants.
2/ Light provided to plans must not exceed the absorption capacity of the pigments that are important to the plant.
At the very best, plants will reject that light and the energy generated will be wasted. Sun energy is free of charge so wasting it is just a natural effect. Artificial light costs and every µmol generated must be used by the plants in order not to waste money.
In some cases, an excess of light will trigger counterproductive mechanisms, for example, carotenoids will overregulate the plant’s growth and therefore reduce the impact of photosynthesis. This is a “double waste” in energy.
Therefore, creating a horticulture light spectrum must start by identifying the target plants’ needs by understanding what pigments are key to their development. Once this is done, the spectrum must be designed to meet each of these pigments’ needs for light. But this is far from being an easy task as many other factors influence the design of a spectrum. I will cover these in an upcoming article.
Sources
- Hopkins, W. G., and Hüner, N. P. (1995). Introduction to plant physiology.
- Folta, K. M., and Carvalho, S. D. (2015). Photoreceptors and control of horticultural plant traits.
- Jones, M. A. (2018). Using light to improve commercial value.
- Folta, K. M., and Maruhnich, S. A. (2007). Green light: a signal to slow down or stop.