The quality of flowers and plants is, among other things, determined by their shape and compactness. Many ornamental crops in greenhouses exhibit too much stretch. Therefore, to achieve the desired, uniform shape, growth inhibitors are used.
Several ornamental crops can’t be grown without growth inhibitors. These include: poinsettia, bedding plants, kalanchoe and certain types of (potted) chrysanthemums. However, the use of chemical products is increasingly under discussion based on environmental concerns. In the coming years research will focus on alternative methods to achieve compacter plants with less or even no use of chemical inhibitors.
Many factors influence the compactness of plants such as light intensity, light colours, temperature (regime), movement of plants and breeding. Breeding companies are increasingly able to improve the plant shape. Then the use of inhibitors can be significantly reduced which, besides environmental benefits, also leads to greater cost advantages due to less spraying.
Steering plant shape
During the last few years research has been focusing on the ability to improve plant shape with light. Visible light comprises a spectrum of colours. Over the course of a day and over a year this spectrum is reasonably constant. Nevertheless, during the summer there is, relatively speaking, slightly more blue light. At sunrise and sunset the amount of red light is slightly less than that of blue or far red light. This effect is greater on cloudy days.
By changing the relationship between the light colours it is possible to influence plant shape. Research in climate chambers has shown that relatively more blue light in relation to red light produces a more compact plant. Also the ratio of red:far red is an important factor in the length of a plant. The more far red light the more a plant stretches. So if we want to influence the compactness of plants by means of light colours, we will have to increase the blue:red ratio and the red:far red ratio.
Other relationships between light colours, however, have other effects on the plant. A relatively high amount of blue light produces more branching, less leaf surface area and biomass but thicker leaves so it doesn’t have any effect on photosynthesis. Also, several studies have shown that stomata open further under the influence of blue light so that transpiration increases and leaf temperature may decrease. In addition, changes in colour ratio can affect the number of flowers and the flowering time.
Influence of the light spectrum
We can alter the light spectrum in several ways. These include artificial lighting with specific colours, coloured films, shade screens and photo selective coatings. Lighting is mostly applicable during periods when the radiation is low. It will require a huge amount of lighting capacity to sufficiently alter the colour ratio in the summer months. An option could be to light with extra blue light in the dark-hours but whether this has sufficient effect is not yet known. It is possible to supply extra light during the morning and evening hours by using, for example, SON-T lights to influence the red:far red ratio. SON-T lights radiate a relatively high amount of red light compared with far red light so that stretching is slowed down.
Films are available on the market that change the ratio of red:far red. The film absorbs the far red so that relatively more red light enters the greenhouse that again leads to more compact plants.
Especially in the summer months it is possible to influence the light spectrum by using shading screens and coatings. One coating that alters the ratio of red:far red is ReduHeat. This is a coating that allows the maximum amount of PAR light to enter while shading out comparatively more infrared light. Research has shown that different types of potted plants clearly remain more compact when using ReduHeat (J.O. Stapel et al, 2009). For many plants this effect is still not sufficient to obtain the desired compactness in the plants. The goal is to develop coatings that have a greater impact on compactness. However, it is impossible to achieve this by only using a coating. Mardenkro is working with research centres and suppliers, for example of shade screens and greenhouse films, to develop cultivation strategies in which chemical inhibition is (almost) no longer necessary.
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