With consumers now taking it for granted that they can expect to see summer fruits lining supermarket shelves all year round, it looks as though the use of film technology will play an increasingly important role in the commercial grower’s business. But as we look to a future where use of the right film could give growers a commercial advantage, what research is being done in developing film technology to address our changing needs?
“Choosing the right polyethylene film to meet your needs is all about understanding how the film affects heat and light to encourage plant growth,” explains Dr Richard Henbest, technical consultant at bpi.Visqueen, one of Europe’s leading manufacturers of horticultural polyethylene films. “As we continue to see the effects of climate change, choosing the best film for an individual application and for specific geographical locations will become increasingly important and film development will continue to be heavily focused on light optimisation.”
Seeing Red
While most of a plant’s growth is driven by visible light, plants are also able to respond to light wavelengths beyond the range that we can actually see. In fact only a narrow range of the sun’s radiation is visible to the human eye but all wavelengths in the PAR (Photosynthetically Active Range) are important for plant growth and interrupting any colour in the range can reduce a crop’s yield or affect plant development.
“For example,” explains Dr Henbest, “the absolute intensity of blue light influences plant height and quality. In situations where the level of blue light is reduced, plant growth may be elongated as the plant ‘grows towards the light’. Because the parts of the plant that trigger this response are sensitive to blue light, screening it out can also have a disproportionate effect on plant shape. “The ratio of red light, which is visible to us and far red light which is not, can also have a significant effect on how a plant grows. Increasing the amount of far red light relative to red light causes plants to grow tall and spindly, as part of the plant’s natural shade avoidance response.”
So what does all this mean for the development of polyethylene films? While it may appear that restricting some types of light will help the grower cultivate the shape and height of plant he ideally wants, it is not as simple as that. A small variation in red light can prompt a significant response in the growth pattern of a plant, but the reverse response can only be achieved following a much greater variation in far red light. So, although films that use additives to absorb green or UV light enhance the amount of red light, it is only next generation films that absorb more far red light that can alter the red/far red ratio enough to match the effect of chemical plant growth regulators.
The UV Question
While UV light does not affect photosynthesis, it does seem to play a major role in plant development. Not only have many growers reported that the colour, flavour and fragrance of many crops is less intense when they are grown in protective conditions, it would also seem that plants grown outdoors are more robust than those cultivated under cover.
Dr Henbest continues: “The UV stabiliser systems traditionally used in greenhouse films remove short wavelength UV light from the growing environment. Research has established that this waveband (the UVB) not only encourages the development of phenolics, flavaniods and anthyocyanins in plant tissue, it can also affect plant development by promoting compact growth and thicker, stronger tissue.
“To address this, advances in film technology have produced films that transmit almost all the available UV radiation, allowing the plants access to light which is as close as possible to natural sunlight.”
The new generation of Visqueen UV transmitting films provides various benefits for crops where propagation ‘hardiness’ and/or the levels of secondary metabolites are important for crop colour, taste and extending shelf life. The film also ensures plants continue to receive all the sunlight in the PAR range they need to grow. In commercial trials carried out by Lancaster University and Stockbridge Technology Centre (UK) this film has consistently produced the highest quality propagation salad and vegetable crops with increased leaf thickness, vegetative strength and root:shoot ratio. . It increased final yields of both types of crops by approximately 9 percent over commercially produced glasshouse crops.
Visqueen UV Blocking film can also provide a number of benefits in crops where maximising growth is of prime importance. This film increases yield of all crops tested and has been especially successful in increasing productivity of crops grown for their essential oils such as sage, thyme, peppermint and rosemary. Lancaster University and the Scottish Agricultural College (UK) have reported that blocking wide-spectrum UV increases oil yield by between 119 percent and 641 percent without affecting the quality of concentration of essential oils.
UV Blocking films have also proved to be useful in the control of insects, especially whitefly. Virus diseases which are spread by insects may therefore also be reduced. Models worked up by the team at Lancaster University suggest that UV window would deliver disease control in a warm climate because high UV will stimulate the plants own resistance to disease.
Topics such as insect and disease control are two of the areas for investigation through ongoing research projects being undertaken by leading academics in this field at both the University of Reading and Lancaster University. Such research will no doubt discover other effects and benefits from studying the application and management of UV light.
In a Haze
A clear, colourless greenhouse cladding may transmit the maximum amount of solar radiation, but there can be advantages to healthy plant growth if the film is made cloudy in a controlled way.
Dr Henbest explains: “The ideal is to create a degree of light diffusion so that the amount of light reaching the plant is not significantly less, but it reaches the plants from numerous directions and is spread more evenly around the plant canopy. This is not only advantageous to plant growth, it also helps to control heat build up in the growing environment.”
The degree of diffusion provided by a film is normally measured as a haze factor and the European Standard for light diffusing films sets a minimum haze value of 35 percent, with most commercially produced films offering 40-70 percent. However, there has been a great deal of research in this area and there are now films available offering 90 percent solar radiation diffusion, maximising the crop’s exposure to solar radiation whilst preserving the benefits of growing under cover.
Through the Fog
In addition to managing the light they need for a successful crop, growers also need to pay close attention to managing condensation in a covered growing environment. Damage to plants and increased risk of disease can be caused by water dropping from the film. Furthermore, drops of water on the surface can both increase the risk of scorching by focusing sunlight and reflect light away from the greenhouse, in some cases reducing light availability by up to 15 percent. “So-called ‘anti-fog films’ contain additives that encourage water to condense as a thin layer rather than as droplets,” explains Dr Henbest. “However, close attention needs to be paid to installing these films correctly so that the water can flow without interruption down to the edge of the film.”
This type of film only manages the problem, it does not solve it, as water will always condense if there is moisture in the greenhouse atmosphere and the outside temperature is cold enough. Anti-fog films are therefore most effective when used alongside heating or ventilation solutions to help remove moisture from the growing environment.
Dr Henbest continues: “Anti-fog films will be increasingly popular with growers cultivating crops that respond well to high direct light levels and are ideal for plants that thrive in warmer temperatures. Not only are they made from grades of polyethylene that retain heat well, the increased light transmitted and the water layer on the film surface also prevent heat loss.”
A Hot Topic
While anti-fog films can have advantages for some growers in helping to retain heat, controlling the temperature can be one of the greatest challenges that growers face, particularly in a climate prone to significant variations, like that in the UK.
“The heat requirements of a particular crop must be a major factor in film selection,” cautions Dr Henbest, “as increased heat retention or loss can be a major feature of specialist films, utilised primarily for their light inhibiting or diffusing properties. The greenhouse film can contribute to heat control by both influencing the short wavelength radiation coming into the system, and by retaining the long wavelength radiation losses from the structure.”
White tinted films reduce incoming light evenly across the PAR spectrum, while coloured films reduce visible light which cuts down heat requirements and some highly diffusing films can reduce the effect of incoming heat radiation to help maintain a more even temperature. “As technology advances,” Dr Henbest continues, “we see more and more specialist films that combine variations in light, diffusion, condensation control and heat control to enable growers to select a tailor-made environment for a given crop. The current selection of films demonstrates what can be achieved when the film is matched carefully to the application and growers need to keep pace with developments if they are to continue to benefit from this advanced area of horticultural science.”
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