Plant growth can also be influenced by light. Dr. Dirk Ludolph from LVG Ahlem looks at the LED technology from which the horticultural sector expects results with regard to elongation growth, colouration and contained substances. Specialist editor Dr. Heinrich Dressler has summarised the findings.
EFFECTS: PHOTOSYNTHESIS AND PHOTOPERIODS CANNOT BE CLEARLY SEPARATED
According to Ludolph it is not possible to clearly separate the effects between photosynthesis and photoperiods. In experiments in Ahlem with potted plants (poinsettias, begonias) the plants in the LED variants did not display better growth that the variants with high-pressure sodium vapour lamps. Some of the effects in other experiments could be explained by greater light intensity. According to Ludolph there were no specific photosynthetic effects.
As the LED modules radiate little heat it is possible to place them close to stock. The advantages of LED are its energy efficiency and the specific spectral impact, says Ludolph. The interesting lamps from a crop technology point of view according to Ludolph are white (mixed light), blue (440 nm), light red (dark red, 660 nm) and dark red (far red, 730 nm). Blue light promotes elongation growth more, light red light mostly inhibits it and dark red light strongly promotes elongation growth.
LED: DARK RED LIGHT NECESSARY FOR LONG-DAY PLANTS
Dark red is necessary for long-day plants but not for short-day plants (such as petunias, calibrachoa and poinsettias). Petunias display strong elongation growth under dark red light and grow very compactly under light red light. Blue light promoted elongation slightly. In hedera light red light caused shorter internodes and shorter shoots. Bromeliads displayed more pronounced red leaf colouring under blue light and the flower stalks were more elongated on strawberries.
In poinsettias dark red light promoted elongation – this could perhaps be utilised in the production of stems. Longer steps from dark red light could also be advantageous in tulip forcing. Poinsettias were more compact after being provided with light red light between 5 pm and 5.30 pm (the ‘end of day effect’). It may be possible to save some of the inhibitor substances in poinsettias, says Ludolph, as they often grow too wide if they have none. For example inhibitor substances could be given twice at the beginning of cultivation, combined with one hour of red light from sunset onwards.
INFLUENCING CHRYSANTHEMUM GROWTH WITH LED TECHNOLOGY
In chrysanthemums and other summer crops LED had no effect on growth, as there is too much natural radiation in the summer for LED effects. The use of LED is primarily of use in autumn and spring.
According to Ludolph LED does offer many control opportunities overall. The technology is well-developed and the light yield good. However because of the necessary investments this technology is more suited to specialist operations, says Ludolph. Users must pay attention to the spectrum that an LED lamp actually produces because the influence on the plants depends on this. Simple ‘hardware store’ lamps usually have a blue LED with a white filter while more expensive specialist lamps have two or three LED and therefore offer a broader spectrum.
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