Light provides a source of external information to plants, which have a series of photo receptors that sensing parameters such as intensity, direction, photoperiod, spectrum and wavelength.

Different parts of the light spectrum can cause different physiological and morphogenetic responses, many of which vary from species to species. This allows growers to use high-tech lighting to tailor the light spectrum towards desirable plant characteristics (while the exact details of the potential of customised wavelength lighting are still being researched and understood, this concept is already well accepted by many indoor gardeners).

Ultraviolet wavelengths

While we are aware of the wavelengths that power photosynthesis (blue—450 to 495 nm—and red—629 to 750 nm—light), other wavelengths are sensed by different receptors in plants and might help control normal plant growth and function. Of particular interest these days is ultraviolet (UV) shortwave light.

Also known as electromagnetic radiation, this light has a wavelength that is shorter than that of visible light but longer than that of X-rays. It is the range 10 to 400nm. Within this range, UV is divided into a number of band spectrums, including UV-C (below 280 nm), UV-B (280 to 320 nm) and UV-A (320 to 400nm). UV is a natural part of sunlight and while humans can’t see UV light, some insects and birds can.

When we think of UV light most of us are reminded of sun burn, skin damage, genetic mutation and other negative affects; however, small doses of UV are also beneficial for humans as it is responsible for vitamin D synthesis. When it comes to plants, UV has been known for some time to have harmful effects plants, but in certain plant species, it seems that small doses can have some very beneficial effects (most of which we are only just starting to understand).

For example, it’s only recently that it was discovered that plant roots can sense UV-B and use it as a signal between cells, which helps young plants with seedling morphogenesis and normal growth patterns (as with all plants, too much UV light can be toxic, and only small doses are required to activate receptors in plant tissue).

Still, this of particular interest for both greenhouse and indoor gardeners—both who have more control of light intensity and wavelength than ever before as they are able to pick and choose from different lamp types and greenhouse films in order to provide varying degrees of UV shortwave light.

UV and protected cultivation

When exposed to UV, plants can produce a range of defence proteins that are similar to those activated when a plant is physically damaged. Shortwave light in the UV bands acts as a stress factor on plant growth and is therefore able to induce a wide range of plant growth and developmental characteristics.

For example, UV-B light has been shown in a number of studies to reduce plant height and cause the development of smaller, thicker and shorter leaves (a typical stress response in plants). While this type of growth effect may be useful in a number of crops where a short, compact plant form is desirable, such as with potted flowering ornamentals, it might not be an advantage for others.

Also, plants can increase their production of these defence proteins as the level of UV light increases up to a point where cell damage starts to become severe.

In greenhouse horticulture, there has been the development of a range of cladding films that have incorporated into the polythene material specific spectral filters designed to block or allow through certain wavebands.

This finding has an obvious and significant benefit to growers of all crops as a reduction in the use of plant protection chemicals and compounds is a major advantage in commercial production. In contrast, some studies have found certain greenhouse pests (such as whitefly) to be significantly reduced under UV-blocking film claddings.

However, at the same time, filtering out all UV might have negative effects on certain aspects of plant growth. Most commercial greenhouse production is still carried out under standard horticultural-grade plastic or glass claddings that block some of the UV wavelengths, allowing other short wavelengths through.

In the future, we can expect to see the development of different greenhouse films for different crops and purposes based on their UV penetration category.

One of the most interesting and commercially important findings regarding UV light is the effect on the production of anthocyanin, which gives the deep red pigmentation in red lettuce cultivars and similar coloured plants.

It has been found that red lettuce (Lollo rosso) grown under greenhouse films that transmit UV light (transparent above 280nm) had eight times more anthocyanin content, and hence a significantly deeper red colour than those grown under UV-blocking greenhouse films. It was also found that total red pigmentation was highest in lettuce when both UV-A and UV-B wavelengths were provided together rather than when just UV-A was provided on its own.

Anthocyanin production in many plants has also been found to be stimulated and increased when UV light was supplemented with use of lamps producing short wavelengths in the UV region. Some studies have reported that other compounds such as beta carotene, which gives the orange colour in fruits and vegetables, is also stimulated by UV light.

Plants produce these antioxidants to protect themselves from the negative effects of shortwave light exposure. For producers of red lettuce and other salad plants where a strong pigmentation is essential for marketability, this is an important finding as the penetration of UV (both as UV-A and UV-B) light down to the crop is likely to be just as essential as the initial selection of highly coloured cultivars to maintain anthocyanin levels.

As well as increased anthocyanin and plant colouration, exposure to shortwave UV light has been found to have a range of other benefits. In many species, UV light is an important contributor to flavour and aromatic compounds.

Overall higher levels of secondary metabolites are produced under UV and it is thought that when both UV-A and UV-B wavelengths are present, plants accumulate secondary products that protect them from damage to the photosynthetic systems.

These secondary metabolites are thought to include UV-protecting or -absorbing compounds that prevent some of the damage to cells and DNA caused by UV radiation reaching the photosynthetic apparatus. This means that the photosynthesis systems are not as damaged as we would expect with exposure to UV radiation.

However, in some studies it has been found that damage can still occur to photosystem II in particular under UV wavelengths. In certain species, this causes a reduction in growth; so, it is possible that under UV wavelengths, the plant might divert some of its photosynthetic energy into producing compounds to protect it, thus reducing overall growth slightly. In yet other studies, no significant effect on growth reduction was seen when UV light was provided, so it’s likely any negative effects are species and environment dependant.

These protective secondary metabolites are likely to include flavonoid and phenolic compounds, many of which are also of interest for human health benefits. Anthocyanins, flavonoids and phenolics provide the anti-oxidant activity in fruits and vegetables, which are linked to a range of health promoting effects and prevention of degenerative diseases. More recently, the possibility of producing commercial crops with increased beneficial compounds under greenhouse films that transmit sufficient UV has been under investigation.

What does this mean for indoor gardeners?

While plants will grow and yield perfectly well under the tried and true red and blue spectrums of photosynthetically active radiation (PAR), there does exist the potential to provide specific supplementary short wavelengths where they provide benefits to certain species. Enhancing the deep red colouration in those plants that require high levels of anthocyanin production—such as red lettuce, ornamentals with highly coloured leaves and others grown indoors—would be one such use.

Another might be to promote short, stocky, compact plant growth, an advantage in the confined space of an indoor garden where height restriction is a bonus. Increased resistance to pest and disease attack with harder, tougher plants and an activation of the plants’ natural immune response by UV wavelengths could be seen as another major benefit of including some shortwave radiation in the plants’ smorgasbord of light.

Many full-spectrum bulbs, fluorescents and other commonly used lamps still provide some UV output, although it pays to check the spectral output to ensure this is in the correct wavelengths of UV-A and UV-B. Despite the potential benefits, the use of UV wavelengths must be considered with some caution: while plants can generate secondary metabolites to protect themselves against low levels of UV, people can’t do the same thing.

UV is still an issue with damaging sensitive human skin, so sunscreen protection might be required when working under UV for prolonged periods of time. Also, we need to remember that research studies have shown that both UV-A and UV-B wavelengths should be used together as they have a synergistic effect on plant response.

Supplying just some UV-A might give no response and no benefit, while UV-A and UV-B together produce a much greater accumulation of flavonoids and other beneficial compounds. So, while shortwave UV won’t necessarily increase growth, it seems to have certain advantages that can’t be ignored when we consider how complex the interaction of plants and wavelengths actually is.