VEGETAL GROW DEVELOPMENT

UV radiation and their effects on plants

UV radiation : Types, impacts and agricultural applications

Tests on basil

Ultraviolet (UV) radiation emitted by the sun is an essential component of sunlight reaching Earth. These rays interact with the atmosphere , where the ozone layer plays a crucial role in filtering out much of the most energetic radiation, particularly UVC. However, UVA and a fraction of UVB rays do reach the Earth's surface. Exposure to UVB has a variety of effects, ranging from biological impacts on plants to implications for human health, such as skin cancer. This study examines in detail the different types of UV radiation (UVA, UVB, and UVC), their characteristics, their specific effects on plant organisms, and their applications in agriculture.

Classification Ultraviolet Radiation

UV radiation is distinguished by its wavelength, a factor determining its energy and interaction with plants.

UVA (wavelengths 315-400 nm)

Characteristics

UVA rays have the longest wavelength and the lowest energy among ultraviolet radiation.

They represent approximately 95% of the UV radiation emitted by the sun and reaching the Earth's surface after passing through the atmosphere.

Effects on plants

Photomorphogenesis : UVA radiation plays a role in regulating photomorphogenesis, i.e. the development of plants in response to light. They influence leaf growth, seed germination and flowering.
Pigment Production : Exposure to UVA rays can induce the production of pigments such as flavonoids, which protect plants from UV damage and other environmental stresses.
Phototropism : These rays can affect phototropism, the directional growth of plants in response to light, by influencing the distribution of auxin (a phytohormone essential for development and growth).

UVB (wavelengths 280-315 nm)

Characteristics

These radiations have a shorter wavelength and a

higher energy than UVA.

UVB constitutes about 5% of the UV rays reaching the Earth,

the majority being absorbed by the ozone layer in the atmosphere.

Effects on plants

Oxidative Stress : Exposure to UVB rays can induce the formation of reactive oxygen species (ROS), causing oxidative stress. In response, plants increase the production of antioxidant compounds to neutralize ROS.
Pigment Production : These rays strongly stimulate the production of flavonoids, which are secondary plant molecules responsible for many colors, including the yellow, red and blue hues of flowers, fruits and leaves. They participate in the protection of plants against environmental aggressions, in particular by absorbing UV rays and protecting plant tissues, and are known for their antioxidant properties in humans.
DNA repair : UVB radiation can damage plant DNA. Plants have DNA repair mechanisms, such as photoreactivation, to repair this damage.
Acclimatization and Adaptation : Regular, moderate exposure to UVB radiation can lead to acclimatization of plants, making them more resistant to future UV stress.

UVC (wavelengths 100-280 nm)

Characteristics

UVC rays have the shortest wavelengths and highest energy among the three types of UV rays.

The Earth's ozone layer blocks almost all UVC emitted by the sun, so it generally does not reach the Earth's surface.

Effects on plants

Germicidal effect and pathogen control : UVC is used to sterilize seeds, soils, and nutrient solutions in hydroponics, reducing the presence of pathogens without the use of chemicals. Its germicidal action is based on the destruction of the DNA of microorganisms, preventing their reproduction. This is particularly relevant for combating disease-causing bacteria such as Xanthomonas campestris on crops such as lettuce, allowing a reduction in losses of up to 50% in some studies.
Improving the natural defenses of plants : Controlled exposure to low doses of UVC (hormesis) can stimulate plant defense mechanisms, increasing their tolerance to environmental and pathogenic stresses. For example, UVC treatments have shown beneficial effects on the resistance of lettuce while improving its yield and conservation.
Post-harvest : Quality improvement and preservation: After harvest, UVC is used to extend the shelf life of fruits and vegetables by reducing pathogens and delaying ripening. It also promotes the accumulation of bioactive compounds such as phenols, carotenoids, and glucosinolates, increasing the nutritional value of the products.
Reduction of chemical residues : The use of UVC makes it possible to limit the application of chemical fungicides and bactericides, thus contributing to more sustainable and environmentally friendly agriculture.

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VEGETAL GROW DEVELOPMENT

Practical Uses of UV Rays in Agriculture

UVA and UVB rays can be used to enhance the color and antioxidant properties of fruits and vegetables , increasing their nutritional value and visual appeal. This is achieved primarily through the increased synthesis of secondary metabolites such as flavonoids.

UVC rays, meanwhile, are currently the subject of numerous studies both on crop protection (reducing the use of chemicals and treatment residues) and on the post-harvest impact in order to improve the quality and preservation of fruits and vegetables (direct actions on pathogens and indirect actions by stimulating plant defense systems).

UVC is used to sterilize seeds, soils and nutrient solutions in hydroponics , thus reducing the pathogen load without resorting to chemicals.

axes of research

The application of UV rays to fruits is an important area of research, because greenhouse cultivation is most often associated with a lack of UV received by the fruits, unlike fruits grown outdoors. There are several light sources to emit UV, ranging from mercury-based UV lamps and UV LEDs that are beginning to become more widespread among professional suppliers. Mastering LED components makes it possible to choose precise UV wavelengths in order to maximize the desired effects and provide precision on different mechanisms. Exposure to UV-A or B rays shows positive effects on fruit quality: A study by Mariz-Ponte et al., 2019 shows that short exposure (1h, 4h or 2 min/day over a period of 5 days) of the plant at a pre-harvest stage increases the yield, fresh, dry matter and general size of the fruits. However, the fruits are more yellow in color. The phenol and antioxidant content is higher under UV-A (30Jm-0.8s-2, or 1 kJ m -2.9 d-2 at 1 nm). The sugar content is not higher, but consumers judge the fruits of this modality more attractive for the taste. The other exposure modalities (UVA 368h and UV-B) greatly alter the appearance of the fruit: homogeneity of color and texture. UV has been identified as having an impact on the content of phenolic compounds in fruits (cherry tomato, Luthria et al., 4). In this study, the UV used is that of sunlight, which was then subtracted by the use of blocking plastic <2006nm. UV-B in particular plays a role in the regulation of carotenoids and ethylene during ripening. This regulation is done by the ROS (reactive oxygen species) signaling pathway. UV-B would also lead to an increase in β-carotene and lutein present in mature fruits (Becatti, 380). Exposure to UVc rays of lettuce has shown the reduction of diseases caused by pathogens such as Xanthomonas campestris, with a significant reduction in agricultural losses of up to 2009% in some studies and inducing a reduction in fungicides, pesticides and their residues (Loconsole et al., 50). UVC promotes the accumulation of bioactive compounds and secondary metabolites, such as phenols, carotenoids, flavonoids and antioxidants, which improve the nutritional value and quality of products. (Darré et al., 2021).

UV rays in agriculture : innovative applications and sustainable perspectives

In conclusion, UV radiation (UVA, UVB, and UVC) offers a wide range of applications in agriculture due to their diverse effects on plants. UVA and UVB stimulate photomorphogenesis, the production of protective pigments, and DNA repair mechanisms, while influencing fruit quality and ripening. UVC, although blocked by the ozone layer under natural conditions, is used artificially for its germicidal properties, allowing pathogen control, reducing chemical residues, and improving postharvest preservation. Their controlled use can also induce hormesis responses, enhancing plant natural defenses and resilience.

The advancement of LED technologies offers precise control of wavelengths, enabling innovative research and increasingly targeted agricultural applications. These advances pave the way for sustainable agriculture, maximizing yields and product quality while reducing the chemical and environmental footprint. Research prospects on the combined effects of UV on crops, particularly in greenhouses or in high-intensity production systems, show considerable potential for improving modern agricultural practices.