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Tanning your plants: The Curious Effects of UV-A and UV-B
UV-A = 320 – 400nm
UV-B = 290 – 320nm
Spending an enjoyable day at the beach is one of our favourite holiday behaviours although we often forget the nasty consequences after sunlight on our skin. Sunlight is the portion of the electromagnetic radiation from the sun which includes infrared, visible and ultraviolet light. It’s these ultraviolet light waves that cause harmful radiation to our dermal tissue with potentially deadly costs. So how have our floral friends evolved to disarm or even utilise this UV light energy?
Modern botanical studies are finding that sunlight regulates different developmental processes over the life of a plant. It can provide certain cues to advance growth and progressive deviations for survival during periods of stress. There is now substantial research on the influence of PAR light (400-700nm) and the photomorphogenic responses to UV B (290-320nm), but limited studies have been conducted on UV-A (320-400 nm), UV-B and PAR interactions.[i]
We understand that sunlight contains UV radiation that can be harmful to plant tissue, but modern research is proving there are several distinctly positive responses to UV radiation if the light fluence, UV/PAR ratios and irradiance is appropriate.
Looking about light energy?
Let’s take a step backwards and look at how ‘light-energy’ influences a plant. Light is a form of electromagnetic radiation where the wavelength determines the energy potential. Humans can see the ‘visible spectrum’ of light between 390-700nm but we understand and utilise the different wavelengths in a huge variety of applications. Microwaves, satellites, x-rays and more all use various forms of electromagnetic energy for their operation.
What is UV light?
Ultraviolet (UV) light is an electromagnetic radiation with a wavelength from 10 nm to 400 nm; shorter than that of visible light but longer than X-rays. Ultraviolet radiation can be broken down into three bands: UVA (400-320nm), UVB (320-290nm), and UVC. Luckily the ozone layer stops the UV-C rays from reaching earth, which could have catastrophic effects.
The potential impacts of an increase in solar UV-B and UV-C radiation reaching the Earth's surface due to stratospheric ozone depletion have been investigated by several research groups during the last 15 years.[ii] This has provided a number of research papers on UV influence on flora.
What to know for plant growth?
As the sunlight shines down upon a crop, the plants expose their leaf surface to transform solar rays into chemical energy via photosynthesis. They’re also using this light energy for a wide range of developmental signals to optimise the photosynthetic processes and detect seasonal changes.[iii]
These photoresponses include reacting to UV radiation. This involves receptors that detect specific wavelengths of light which induces developmental or physiological changes. For scientists to measure the influences of varying light wavelength radiation in plants, they utilise what is known as an ‘Action Spectrum’.
Action spectra can describe the wavelength specificity of a biological response to sunlight and uses a graph that plots the magnitude of a light response (photosynthesis) as a function of wavelength (see graph right).
However it is extremely difficult for scientists to functionally distinguish intrinsic responses to individual photoreceptors. While phytochromes are responsible for absorbing red and far-red light, they also absorb some blue light (300-500nm) and UV A (320-400nm) radiation. These phytochromes mediate many aspects of vegetative and reproductive development.
Cryptochromes, phototropins and Zeitlupe ZTL are the 3 primary photoreceptors that mediate the effects of UV-A and Blue light energy. As in the case of plant hormone signalling, light signalling typically involves interactions between multiple photoreceptors and their signalling intermediates. Photoreceptors are also sensitive to light quantity, quality and duration.
The light responses in plants can also be distinguished by the amount of light required to induce them, however this can also be difficult to measure as the plant canopy itself can have a dramatic effect on both the quantity and quality of incidental light reaching individual plants. Plants growing beneath the canopy use phytochromes to sense the light ratio and regulate such processes as shade-avoidance, competitive interactions and seed germination.
UV-B light is primarily mediated by the UV-R8 Monomer, a 7-bladed B-Propeller protein. In addition to its cytotoxic effects, UV-B radiation can also elicit a wide range of photomorphogenic responses. Our current challenge is to clarify how UV-B exposed plants can balance the damage and adaptation responses in a photobiologically dynamic environment.
It has been proven that UV light influences many photomorphogenic responses including gene regulation, flavonoid biosynthesis, leaf & epidermal cell expansion, stomatal density and increased photosynthetic efficiency. [iv]
We also understand that UV radiation can damage membranes, DNA and proteins however many plants can sense the presence of the radiation and protect themselves. Numerous agricultural crops can synthesize simple phenolic compounds and flavonoids that act as ‘sunscreens’ and remove damaging oxidants and free radicals that are induced by the high-energy UV photons.
So how do we utilise this UV energy without causing damage to our crops?
Modern lamp manufacturers are specialising in horticultural lighting technology that includes precisely calculated amounts of UV-A and UV-B output. Advances in CMH (Ceramic Metal Halide) and MH (Metal Halide) bulbs have allowed for optimised amounts of UV light to encourage essential oil production.
These bulbs are currently used by essential oil and herb farmers who are seeking to increase the flavours and resin production from their crops. Although this is a very recent field of botanical science, there are multiple reports of dramatic increases in essential oils from including higher UV bulbs in flowering crops.
These products are generally recommended for use in the last two weeks of a flowering cycle as the generative development is completely established. This allows for a crop to continually develop in size and growth vigour, while also ‘protecting’ the flowers and canopy with increased resin production.
Negatives and potential damage from UV?
It’s apparent that balance is the key to our effective use of UV; too much or incorrect ratios of PAR/UVA/UVB will not help, but the correct amounts could encourage some incredibly useful results. It also depends on the timing of UV application as your plants adjust to the changing ‘seasons’, we can positively influence and enhance their physiological responses.
Generally, the effectiveness of UV-B varies both among species and among individual strains or genetics of a given species. When given UV-B throughout the entire growth cycle sensitive plants such as leafy greens often display reduced growth (plant height, dry weight, leaf area, etc.) and photosynthetic activity.[v]
If you’re looking to utilise UV in your garden, it’s worth discussing with your local hydroponics store about the best approach for your chosen plant species. But this raises the question, how do we measure the amount of UV radiating from our bulbs or fixtures?!
How to measure UV in your garden?
Most common light testers only provide you with limited information about the lamp spectrum. There are several high-end light testers that will provide spectral output down as far as 320nm, but they can be quite expensive ($1-$2k). It’s worth researching your bulb and bulb manufacturer to see their original published analysis and lighting information from the source.
Overall it’s worth discussing and researching the best applications of UV in your garden, and catering to your specific plants physiological requirements. If we use this technology correctly, we can enjoy the benefits of plant sunscreen! This means your flowers will smell better, your fruit will taste superior and your herbs will have a higher potency in the kitchen!
[iii] Plant Physiology and Development Book page 448
[iv] Plant Physiology and Development Book page 473