Exploiting narrow-band UV-LEDs for Sustainable, Innovative, Technology-Enabled Cropping (UV-SINTEC)

Advances in LED technology are having a profound effect on the horticultural industry. LEDs are replacing traditional light sources because they are much more energy efficient. Apart from energy efficiency, LED technology also has the potential to accurately control the light spectrum. This can be used to control plant responses.

By varying the proportions of red, blue and green light, plant development processes like rooting, flowering and fruit set can be influenced. So-called “light recipes” instruct growers in this type of crop manipulation. Precision control of light for crop growth and development is therefore rapidly becoming routine, with the combination of LED technology and knowledge of plant responses supporting this. However, LED technology and application are currently limited to the visible part of the spectrum.

The UV-SINTEC project is focusing on ultraviolet (UV) wavelengths, both in terms of UV LED technology as well as plant responses to narrow band UV irradiation. The main aim of the project is to use the advances in the development of UV-emitting LEDs to develop novel, sustainable and technology-driven solutions for commercial protected cropping. To achieve that, the project focuses on developing integrated UV irradiation systems using commercially available narrow-band UV LEDs. These UV LED systems will then be used to gain better understanding of the influence of UV wavelengths on plant responses contributing to a more sustainable production of crops of high nutritional quality.

State-of-the-art: Innovative UV-emitting LEDs

UV-emitting LEDs are increasingly commercially available. These UV-B and UV-C emitting LEDs are pioneering, and improvements in light output and life expectancy, and a decrease in cost are expected. Therefore, commercial use of UV-emitting LEDs is rapidly becoming realistic. The use of UV wavelengths in horticulture is particularly attractive since it improves plant pest tolerance, increases beneficial bioactive phytochemicals and encourages compact plant architecture. Such UV-responses are highly desirable.

Conventional UV sources have many disadvantages including a broad spectral output, the requirement for high voltage supplies and a vacuum based lamp technology with high associated environmental hazards. In contrast, UV-LEDs are compact, produce minimal heat, have a low power consumption, a stable output, and a narrow spectral emission (typically 20-40nm). UV-LED technology is rapidly evolving, but is still in the development phase, with wide variation in output efficiency depending strongly on the device’s output wavelength. For example, 365nm UV-LEDs are typically 5-8% efficient, while at 385nm the efficiency rises to approximately 15%.

Significant advances have been made in the development of LED-based irradiation panels for lighting tasks, ranging from room lighting to machine vision, UV-curing, sterilisation and horticulture. However, there remain significant challenges in the development of UV-LED panels, particularly in the selection and matching of the UV-LEDs, and in the design of the quartz-based focussing optics and the integration of them with the devices. Integration of these systems with a supervisory controller for the output spectrum, system temperature and monitoring of the system performance is essential for successful implementation of effective UV-LED panels.

State-of-the-art: Plant UV-responses

Many studies have investigated the effects of different levels of UV-B radiation on plants, animals, humans and microorganisms, especially following the discovery of ozone layer depletion in the 1980s. UV-B radiation can potentially be damaging to plants, a scenario not unlike sunburn experienced by both humans and some animals. However, under realistic UV-B conditions (i.e. those resembling natural sunlight), UV damage in plants is rare.

Instead, low doses of UV act mostly as an environmental regulator that controls metabolic and stress-protection processes through a dedicated UV sensing molecule named a photoreceptor. UV-B influences expression of hundreds of genes, biochemical composition and morphology of plants. This changes the nutritional value (as well as flavour, taste and colour), pest tolerance, and hardiness of plants. As a consequence of boosting pest tolerance and hardiness, UV-B is a valuable tool for sustainable agriculture and improvement of crop quality. However, little is known about the wavelength dependency of UV responses, the scope for precision manipulation with specific UV wavelengths, and the ability to minimise potential UV stress.

UV increases phytochemicals with proven health benefits

UV can influence levels of plant chemicals (phytochemicals) that play a role in the protection of the plant against stress (for example drought). However, the importance of UV-induced changes in plants’ phytochemical composition goes beyond the plants themselves. Studies have shown a link between increased consumption of vegetables and a lower risk of cancer and cardiovascular diseases in humans. This protective effect is largely attributed to phytochemicals. Understanding how UV regulates levels of these phytochemicals is important because of their pharmaceutical and health-promoting benefits. For example, a diet rich in plant flavonoids is associated with lower risks of osteoporosis, heart disease and cancer. Carotenoid consumption is associated with a lower aggressiveness of prostate cancer. Thus, the UV-induced accumulation of phytochemicals in plant tissues is vital in the context of “food for health”.

An important group of UV-regulated metabolites are flavonols, which are a subgroup of plant flavonoids. They contribute to plant UV protection because of their antioxidant activity and, to a lesser extent, UV absorption (i.e. acting as the plant’s sunscreen). The increased accumulation of flavonols is one of the best documented UV-B acclimation responses. Some flavonols can also be induced by longer UV-A wavelengths, but a lot is still unknown about the exact induction spectrum of many flavonols. Levels of some metabolites increase following UV-B exposure, while those of others change transiently or decrease following exposure to high UV-B doses. Initial studies suggest a complex, highly dynamic, UV-response system, possibly mediated by regulatory interactions between different biosynthesis pathways. This complexity will also impact on the bioavailability of phytochemical compounds for consumers.

This study will generate comprehensive knowledge of UV-induced phytochemicals. Specifically, we will study the wavelength dependency of induction, the identity of the relevant photoreceptor(s), and induction kinetics in leaves of the model plant Arabidopsis thaliana. This understanding can then be used to develop crop specific UV exposure protocols, generating premium produce with enhanced content of one, or multiple, bioactive phytochemicals. We will also determine the bioavailability of induced phytochemicals for human consumers.

UV induces a more compact plant phenotype

Plants exposed to UV-B tend to display a more dwarfed phenotype (appearance). More compact plants are more robust, improving survival of handling and transport, which are features of the modern horticultural trade. Furthermore, the dwarfed UV-B phenotype has also been associated with drought tolerance, an important factor in the transport of plants and a determinant of shelf-life. Given increasing restrictions in the use of chemical plant growth regulators, as well as public aversion to the use of hormones, a more sustainable approach to controlling plant architecture is required. In this context, the UV-B mediated dwarfed phenotype has considerable commercial potential.

It has been hypothesised that UV-B induced dwarfing may decrease exposure of plants to UV-B radiation, especially if dwarfing results in plants being shaded by others. However, major questions remain about this hypothesis. The penetration of UV-B in a canopy remains unclear, with a recent paper arguing that there may be UV enrichment within the plant canopy. Understanding the UV dose within a canopy is important in the context of controlling phytochemical content and pest management (insects shelter below leaves). This study will develop novel optical UV detection technology for use within a canopy, to quantify the relative penetration of UV in a canopy, and measure key morphological parameters.

UV alters plant-pest interactions

UV radiation can enhance plant pest tolerance. Given increasing restrictions in the use of pesticides, increasing resistance problems and consumer preferences, using UV to naturally enhance plant pest tolerance is attractive. However, UV impacts are complex. UV affects the plant and the pest species, as well as their interaction. UV alters accumulation of metabolites (and proteins) with pest-deterring properties. In parallel, UV impacts on plant architecture, including branching and stem elongation. This will affect the microclimate around a plant and therefore pest organisms. Many insects also have vision in UV wavelengths, which leads to UV radiation modifying orientation towards potential hosts, flight activity, feeding behaviour and interaction between sexes.

We hypothesise that longer UV wavelengths (i.e. within the range of insect vision) promote pest orientation and reproduction, while shorter UV wavelengths predominantly boost plant pest tolerance through accumulation of UV-induced phytochemicals. Thus, an intriguing prospect is to accurately manipulate the growing environment by using narrow band UV-emitting LEDs to generate different, but close, wavebands for pest control.

UV and plant stress

High doses of UV-B radiation can cause plant stress. Older literature on UV-B radiation effects is dominated by reports on UV-B damage. Many negative UV effects are due to the use of high UV-doses and/or exposure to short UV wavelengths (< 300 nm). In the last few years, it has become clear that UV damage and stress are rare, and that UV is predominantly a source of information and specific regulation for plants. This conceptual change was driven by technical advances in manipulation of UV-B, and understanding of molecular UV-B perception and physiology. Nevertheless, it is clear that exposure to high doses of short UV wavelengths can cause plant stress. Clearly, the use of high intensity doses of UV-C wavelengths in the horticultural industry (used to control mildew) is risky from a plant damage point of view. The low doses of longer UV wavelengths used for the UV-SINTEC project (295-380 nm) are likely to cause minimal (295 nm) or no (380 nm) plant damage, but this will be routinely monitored using non-invasive, stress-sensitive, imaging.

Novel UV exposure technology can lead to development of novel cropping strategies

Current knowledge of plant UV responses is largely based on the use of older, broad-band UV-B sources, i.e. lamps that emit many different wavelengths of UV simultaneously. There is little knowledge of responses to narrow-band UV wavelengths. Indeed, broad UV spectra simultaneously induce multiple plant responses, and potentially stress. A few groups have used optical filters to narrow the emission spectrum to just a few wavelengths. However, these approaches have been hampered by the availability and cost of optical filters.

The lack of knowledge of plant UV-A responses is particularly striking. Indeed, use of different broad-band UV-A sources has resulted in bewildering, and often contradictory observations, probably due to simultaneous activation of multiple UV sensing photoreceptors. The UV-B photoreceptor UVR8, and the UV-A/blue photoreceptors cryptochrome, phototropin and zeaxanthin are all active in the UV part of the spectrum, but each has a distinct action spectrum. Different photoreceptors can affect different responses. By using narrow wavelength bands, precision control of plant responses will be attempted.

Thus, based on understanding of insect vision and plant flavonol induction, we hypothesise that narrow band UV-B can induce flavonols without facilitating insect flight orienting which depends on longer UV wavelengths. This example highlights the underlying philosophy and the overall aim of UV-SINTEC: Through the development of a better knowledge of wavelength specific biological responses and development of narrow wavelength LED-based UV irradiation units, the project will lay the foundations for novel, technology-driven horticulture that sustainably produces high crop quality.