By Nabiha Ben Sedrine 1, Paulo Mendes 2, Ricardo Santos 3, Miguel Baltazar 3, Sandra Pereira 3, Lia Tânia Rosa Dinis 3, Sarah Bollanti 4, Daniele Murra 4, Antonia Lai 5, Loretta Bacchetta 6, Simona Lucioli 7 and Paolo Di Lazzaro 4
A Note from IUVA: This sub-Task Force, “UV light for the benefit of plants: UV-hormesis from the laboratory to the field,” aims to turn controlled UV light into a pioneering tool that supports agriculture against pests, diseases and drought, among others, by stimulating the plant’s defense system. A careful study of the effects of UV light on the ecosystem (soil, auxiliary fauna and flora) will be carried out. This effort is part of the work of the IUVA Food and Beverage Task Force, chaired by Tatiana Koutchma and Keith Warriner, as a first step in establishing a new IUVA “Farm to Fork initiative.” Thank you to Nabiha Ben Sedrine (Castros SA) and Paulo Mendes (Matglow) for engaging with IUVA to help develop a new model for the exploration of emerging UV applications as part of the IUVA Task Force Farm to Fork initiative. The IUVA welcomes additional participation and ideas for investigating new opportunities.
As climate change, global water shortages and soil salinization are impacting arable lands negatively, there is a vital need to find ecofriendly, innovative solutions for fresh food production to meet the demands of a growing population (expected to reach ~10 billion by 2050). However, it is estimated that over one-third of all food produced (~2.5 billion tons, worth $230 billion) is lost or wasted each year, one-third of which occurs in the food production stage. Indeed, apart from food safety standards, retail and consumer preferences, the causes of food waste are due to agricultural processes in both pre-harvest and post-harvest of crops. Pre-harvest losses are caused by two key factors. The first factor is associated with poor horticultural techniques, including a) mechanical damage to fruit and b) inappropriate supply of nutrients, causing physiological disorders (such as blossom-end rot of tomato). The second factor is associated with biotic (pathogens) and abiotic (water, temperature, light) factors, which not only affect yield through fruit rots (such as grey mold of tomato caused by Botrytis cinerea), but also indirectly can reduce yield due to a reduction in plant vigor.
A Promising Ecofriendly Innovative Solution: “UV-Hormesis”
Ultraviolet (UV) is part of the light spectrum composed of three wavelength regions: UV-A (315-400 nm), UV-B (280-315 nm) and UV-C (200-280 nm). Short-wavelength UV-C is the most damaging type of UV light. UV-C and most UV-B light are absorbed by the stratospheric ozone layer, whereas UV-A and a small part of UV-B reach the earth’s surface and can be absorbed by living beings. Similarly to the fact that sunlight (UV-B light) exposure – for at least 10 minutes/day – triggers the synthesis of vitamin D for humans, plants have evolved responses to cope with natural UV light reaching Earth, inducing the production of secondary metabolites and physiological changes.
UV hormesis is the application of potentially harmful UV light at low doses to living organisms (plants, in this case) in order to induce stress responses and increase their resistance to biotic and/or abiotic factors.
Mitigating Biotic and Abiotic Stress Soon Enough at the Pre-Harvest Stage?
To date, induction of disease resistance with UV light has been focused primarily on post-harvest crop treatment, with most experiments aimed at monitoring disease progression. UV-C hormesis has been known for over three decades and has a broad range of benefits on post-harvest crops. These include increased nutritional content, delayed chlorophyll degradation and disease resistance. The beneficial effects have been observed on many varieties of fresh produce, including climacteric and non-climacteric fruit, tubers, salads and brassicas.
At the pre-harvest stage, several studies have shown that high UV-A doses have a negative effect on Photosystem II (PSII) elaboration, due to a reduction in ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) (C3 species) or PEP carboxylase (C4 species) and an increase in stomatal resistance and reactive oxygen species (ROS) production. UV-B light can induce abiotic stress and photomorphogenic changes. High UV-B exposure induces physiological stress, including ROS production, DNA deterioration and damage, and impairment of cell processes, while low UV-B exposure promotes photomorphogenic changes, such as cotyledon expansion, biosynthesis of anthocyanins and flavonoids, inhibition of hypocotyl growth and stomatal opening.
Disease resistance with UV light, especially UV-C light, acts via two different pathways: disinfection/germicidal action and disease prevention action. While the first action suppresses mainly biotic stress induced by pathogens by damaging DNA/RNA, the prevention action induces the production of secondary metabolites and physiological changes in plants, boosting their defenses, similarly to vaccine effects for humans. Examples include the following:
- UV-C light disinfection/germicidal action has been demonstrated in several crops, such as suppressing fungal powdery mildew in grapes, strawberries and cucumbers and fire blight in apples; decreasing by 50% Botrytis cinerea L. in tomatoes; and decreasing over 80% of kiwi infection.
- UV-C light disease-prevention action has shown stimulated defense response to Mycosphaerella fragariae in strawberries and an increase of antioxidants (phenylpropanoid and phenolic compounds), as well as enzymes involved in the synthesis of phenylpropanoid, in parsley. In grape vines, the treatment has shown stimulated and enhanced plant health through the plant’s own defense mechanism, complementary to organic or conventional phytosanitary protection, achieving a reduction in dependence on fungicide products and saving potentially up to 50% on chemical inputs.
The impact of UV-C light on plant growth and photosynthesis recently was investigated. Beneficial effects were observed on ‘Belladona F1’ tomato (Solanum lycopersicum) plants when exposed to 1.0 kJ/m2 of UV-C, which exhibited an increase of 25% in total fruit number and an increase of 36% in fruit weight, when compared to non-exposed controls.
Other studies demonstrated that pre-harvest UV-C light triggered flowering in certain ornamental plants. Similar findings were observed in lettuce (Lactuca sativa) exposed to UV-C light, showing no reduction in chlorophyll fluorescence (Fv/Fm) at doses up to 1.70 kJ/m2. UV-C priming at 0.85 kJ/m2 improved lettuce development under stress, increasing root and leaf dry weight in saline conditions. Plants treated with a moderate UV-C dose exhibited higher phenol content and increased antiradical activity in the presence of NaCl.
What Next?
UV-hormesis doses are species-specific; in addition, the efficacy of induced defenses in plants has been shown to be affected by harvest date, variety, development stage, exposure levels to visible light after UV-C treatment, and target organ/region (leaf, fruit, etc), so a lot of work has yet to be done.
It is crucial to mitigate food lost or wasted at the pre-harvest stage by boosting plant defenses, while reducing pesticide reliance and avoiding ecosystem (soil, auxiliary fauna and flora) damage or mutation (fauna). The next big steps are a) to bring UV-hormesis from the laboratory to the field and b) to encourage the adoption of rational environmental regulations for the use of this UV technology.
The Portuguese Team acknowledges the financial support provided by the Portuguese Foundation for Science and Technology (FCT) under doctoral grant number 2023.04016.BDANA, and by “la Caixa” and FCT for the project number PD24-00002 (UVineSafe).
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References
- Castros SA, Portugal
- Matglow – Smart Materials, Portugal
- Centre of the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), Department of Agronomy (DAgro), University of Trás-os-Montes and Alto Douro (UTAD), Portugal
- Plasma Applications and Cross-disciplinary Experiments Laboratory, ENEA Frascati Research Center, Frascati, Rome, Italy
- Diagnostics and Metrology Laboratory, ENEA Frascati Research Center, Frascati, Rome, Italy
- Bioproducts and Bio-processes Laboratory, ENEA Casaccia Research Center, Rome, Italy
- Council for Agricultural Research and Economics – CREA, Research Centre for Olive, Fruit and Citrus Crops, Rome, Italy
