Light quality for crop flavour, colour and plant shape
Those of us lucky enough to have holidayed in the Mediterranean may recall the strong, fragrant smell of wild rosemary or thyme growing on a sun drenched, rocky coast. Indeed, the link between sun exposure and crop quality is well known and marketed through concepts such as “sun ripened tomatoes”. So, is there any truth in sun ripened crops having a better aroma, colour and overall flavour, and can we, based at more northerly latitudes, improve our crops to capture that Mediterranean feel?
Our project aims to improve crop lighting to better reflect natural sunlight, and using the benefits of light to improve the crop's colour, flavour and plant architecture.
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Light is a key player that both enables and steers the development of crops. Visible light drives photosynthesis, through a process whereby light energy is converted into chemical energy, for example in the forms of sugars. In turn, sugars are used to make a variety of other plant metabolites, thus enabling growth. However, light does much more than drive photosynthesis and growth. Light also controls the development of crops and this relates to plants being able to sense a broad range of properties of light, including day length, direction of light and colour (spectral composition) of light. For example, plants exposed to more far-red (near infrared) than red light tend to elongate more than plants which receive equal portions of far-red and red, while Chrysanthemums will only start to produce flowers when days are shorter than a particular critical daylength.
In outdoor cropping, the light environment depends on the time of day, weather, location and season. However, in protected cropping there are ample opportunities to manipulate the light environment using wavelength-specific cladding and/or lamps. This allows growers to control crop quality. Of particular interest are LED units that allow control of light intensity as well as light spectrum. Such energy-efficient light sources are now routinely used, and enable control of important traits such as plant architecture as well as plant metabolic make-up (i.e. flavour and colour) by selecting appropriate wavelengths. Indeed, a wide range of “light recipes” have been developed to assist growers. Interestingly, while researchers and growers have extensively exploited the different effects of visible and far-red light on crops, the exploitation of UV-A and UV-B effects is still in its early stages. Any outdoor crop is naturally exposed to these UV wavelengths, it is the UV-free environment of a glasshouse that is artificial and lacks these wavelengths present in natural sunlight. Modern LED technology facilitates UV exposure of crops, triggering the question whether this is worthwhile. This is where the SFI-funded UV-SINTEC project comes in.
The UV-SINTEC project has developed LED-based light sources that, as well as visible light, emit specific UV wavelengths. The addition of low intensity UV has two major benefits for crops. It impedes elongation growth of the stem, preventing the development of straggly young plants. In experiments with cucumber done in collaboration with the group of Prof. Strid at Örebro University (Sweden), UV-exposed cucumber seedlings were more compact, facilitating placement in stacked trolleys for transport, and without affecting overall cucumber fruit yield1. In general, UV-exposed plants have shorter stems, but are often more branched, while root development is enhanced. Benefits of UV exposure are not limited to plant structure: UV can also exert strong positive effects on plant flavour. Using dill as an example, the sensory quality (assessed using professional tasting panels) was found to be improved by UV.
Plants, including herbs, grown outdoors in the Mediterranean are naturally exposed to solar UV. The problem for growers at more northerly latitudes is that cladding materials such as plastics and glass filter out UV radiation. Especially in winter, plants grow in protected structures under low light lacking UV wavelengths. This may result in excessive elongation and lack of development of flavour and colour. The UCC-based UV-SINTEC group pioneers the use of UV wavelengths to “trick” such crops in believing they grow on the Mediterranean coast.
1. Qian, M., Rosenqvist, E., Flygare, A.M., Kalbina, I., Teng, Y., Jansen, M.A. and Strid, Å., 2020. UV‑A light induces a robust and dwarfed phenotype in cucumber plants (Cucumis sativus L.) without affecting fruit yield. Scientia Horticulturae, 263, 109110.
2. Castro-Alves, V., Kalbina, I., Nilsen, A., Aronsson, M., Rosenqvist, E., Jansen, M.A.K., Qian, M., Öström, Å., Hyötyläinen, T. and Strid, Å., 2021. Integration of non-target metabolomics and sensory analysis unravels vegetable plant metabolite signatures associated with sensory quality: A case study using dill (Anethum graveolens). Food Chemistry 344, 128714.