Balance taste, nutrition, and crop yield with UV light exposure

During the summertime, we are reminded that sunlight can be harmful. We put on sunscreen lotions and wear sunglasses with ultraviolet (UV) blockers in the lenses. We cover up because we know the sun’s UV light can be damaging to living cells; it can even kill some of them.

Sunlight includes wavelengths outside the visible spectrum: light with longer wavelengths [>700 nm, far-red and infrared (IR) light], and light with shorter wavelengths (<400 nm or UV light). Below 400 nm, UV light becomes more energetic as the wavelength decreases and the ability of UV light to break chemical bonds gradually increases as the UV wavelengths get shorter. Because of changes in these chemical effects, UV wavelength ranges are typically broken into UV-A (400‒315 nm), UV-B (315‒280 nm), and UV-C (280‒100 nm) bands. Since Earth’s upper atmosphere absorbs almost all of the UV light at wavelengths shorter than 300 nm, at the surface we typically only consider the biochemical impacts of UV light between 300 and 400 nm with regards to living systems.

Understanding the impacts of terrestrial UV radiation on plants and animals can be complicated. While UV-B light (315 nm and shorter) can cause greater damage to exposed tissue (e.g., sunburn), it is absorbed right at the surface. UV-A radiation, however, gradually becomes less damaging to exposed tissue as the wavelength gets longer, but it also penetrates exposed tissue more deeply as the wavelengths get longer, creating other photochemical related effects. These effects impact all exposed living tissue.

Plants have evolved extensive biochemical means to manage the effects of all terrestrial light from the sun. They are experts in sensing and using different wavelengths of light. The most famous vegetative pigment, chlorophyll, for instance, primarily absorbs the blue and red spectrum and converts the photons into biochemical energy (sugar molecules) via photosynthesis. Other pigment molecules absorb light in the shorter-wavelength part of the spectrum. Plants see light using specialized proteins, and plant photoreceptors are specific to different wavelengths, like phytochromes for red and cryptochromes for blue light detection. One UV receptor has been identified — UVR8 that detects UVB light — but much is yet to be explored regarding the molecular perception of UV light in plants.

UV light sensing is important for plant growth and development. For example, at the seedling stage the orientation of growth has to be directed for optimal light capture, distribution of the wavelengths, and photosynthesis. In controlled environment agriculture (CEA), there are other impacts. Plants grown in greenhouses receive filtered sunlight because UV light shorter than about 350 nm is not transmitted through glass or plastic materials, and the absence of wavelengths between 300 and 350 nm can have different impacts on plants either directly or by causing changes to the development other organisms (e.g., insects, microbes, fungi) in their ecosystem.

UV light can induce the biosynthesis of secondary metabolites such as flavonoids. This biochemical response is primarily a defense mechanism because the colored flavonoids such as anthocyanins capture UV light and therefore act as protectants from the damaging rays. At the same time, the flavor profile of the fruit or leaf is altered but often in a desirable way, such as enhancing the pleasant aroma in tomatoes or reducing the bitterness of lettuce.

Other stress-responsive compounds contributing to a plant’s chemotype are terpenoids, which are also important to the aroma and pigmentation of plants and their fruits. Secondary metabolites in general are believed to improve the nutritional quality of the crop, primarily due to antioxidant and antimicrobial properties of these compounds.

The development of LED lighting in CEA provides the opportunity for adding UV LED light to the grow-light regime, but studies on whether UV supplemental lighting can improve the aroma and flavor of vegetables are still in their infancy. Recent reports such as “Manipulating Sensory and Phytochemical Profiles of Greenhouse Tomatoes Using Environmentally Relevant Doses of Ultraviolet Radiation” (August 2016) and “Light Quality Dependent Changes in Morphology, Antioxidant Capacity, and Volatile Production in Sweet Basil (Ocimum basilicum)” (September 2016), found some herbs (e.g., basil) and tomatoes did achieve an enhanced flavor from supplemental UV.

There may, of course, be additional concerns to keep in mind when supplemental UV light is applied in the grow-facility. Do the plants need continuous exposure in order to obtain improved quality in the final crop? Can the plants achieve the right blend of taste, nutrients, and size? There may also be other safety concerns — how much UV exposure is safe for the facility workers if UV light is required in the CEA process? We already know that too much UV-B exposure may cause significant eye and skin issues.

Future studies at LESA (the Lighting Enabled Systems & Applications Center at Rensselaer Polytechnic Institute) will help dissect the effect of different wavelengths of UV light on produce quality, taste, nutrition, biomass, and crop yield in order to produce higher-value CEA products, which is what LESA’s Plant Science team is working to accomplish.