As the saying goes, plants reach for the light, but is all light created equal in their eyes?
Scientists have long known that this isn't the case: some photons accelerate photosynthesis, while others can lead to leaf burns and even DNA damage. In collaboration with SFU, let's delve into which materials emit the most beneficial rays for plants and how machine learning can aid in the search for these rays.
This article marks the beginning of N + 1's "Green Projects" series, dedicated to environmental awareness and developments aimed at reducing the impact of human activities on the environment.
Taste the Rainbow
The sunlight that reaches Earth is composed of electromagnetic waves of varying lengths, ranging from 100 nanometers to about 1 millimeter. For photosynthesis, plants primarily utilize the visible part of the spectrum, light with wavelengths ranging from 400 to 700 nanometers. Chlorophyll, the green pigment found in chloroplasts, mainly absorbs red and blue light. Does this mean that only red and blue light are beneficial to plants, and all other wavelengths can be dismissed as useless?
In reality, it's more complex than that. Light for photosynthesis is not only captured by chlorophyll but also by yellow-to-red pigments called carotenoids, which absorb blue and blue-green light. Some scientists also believe that the absorption of light involves not only parts of chloroplasts but also other nearby organelles.
However, the complexities don't end there, as plants require light not only for photosynthesis. Light dictates the daily rhythms of leaf movement, flower and stomatal opening, influences the processes of flowering and fruit set, and more. Plants have photoreceptors—substances that, when exposed to light of specific wavelengths, can influence cellular metabolism and even gene expression.
To determine which light is more beneficial to plants, experimentation is necessary. The first such experiments were conducted independently by botanists Clement Timiriazev and Theodor Engelmann in the second half of the 19th century (for more details, see the "Rays of Support" article). However, complete clarity hasn't been achieved, and scientists continue to experiment by comparing different types of radiation. Plants have various photoreceptors, which means their responses to light may vary even among closely related species. Nevertheless, some general patterns have been identified.
For instance, nearly all studies agree that red light (630–740 nm) is crucial for photosynthesis. About 80% of the energy used in photosynthesis by plants comes from photons with a wavelength of around 650 nm. Additionally, red light accelerates the flowering process.
Some studies specifically highlight far-red light (700–750 nm). Although the energy of these photons is insufficient to excite chlorophyll, they play an important role in photosynthesis. This is where one of the peaks of phytochrome absorption is located—a highly significant regulatory pigment in biosynthesis. In 2017, American scientists discovered that such light helps transfer electrons more efficiently within chloroplasts and reduces the scattering of light as heat. Far-red light also helps plants orient themselves in the daily cycle and prepare for nighttime cold. However, an excess of far-red light can be harmful to plants, causing leaves to become smaller and reducing the amount of chlorophyll they contain.
To initiate the photosynthesis process, light alone is not enough
Chloroplast development and chlorophyll production are influenced by blue light (441–500 nm) and violet light (401–440 nm). Furthermore, photoreceptors that respond to blue light help plants protect themselves from overheating.
Green light (501–565 nm) is less absorbed by leaves. However, both green and close-to-green yellow light (566–600 nm) serve an essential purpose. Such light penetrates deeper into the crown and even into the deeper layers of the leaves, activating a greater number of chloroplasts. This type of radiation is particularly useful for aquatic plants and those often found in the shade.
Only 5% of the solar spectrum consists of highly energetic ultraviolet (UV) radiation. While this part of the spectrum doesn't participate directly in photosynthesis, it serves a regulatory function, and its influence on plants cannot be ignored.