K.-J. Bergstrand, H. Asp, E.H. Larsson Jönsson and H.K. Schüssler
Department of Biosystems and Technology, Swedish University of Agricultural Sciences, Alnarp, Sweden

SUMMARY Artificial light is used for many horticultural crops produced in greenhouses, not least ornamental pot plants. New technologies such as LEDs will possibly replace high intensity discharge lamps as the main technology for lighting in horticulture. However, LEDs are quite different from discharge lamps as regards the handling of waste heat. They have low output of radiant heat, but the waste heat is produced in the fixture and must be cooled away using fans or heat sinks. The low radiant heat will result in lower leaf temperature in the crop, possibly prolonging production time. LED fixtures are often voluminous due to the need for cooling systems, creating large shade effects when installed at the top of the greenhouse. To overcome these issues, this greenhouse study tested placing the light source, in the form of LED bars, under the crop, thus illuminating the abaxial side of the leaves. The results showed that the growth and elongation of the Euphorbia plants was similar irrespective of whether the light was supplied at the abaxial or adaxial side of the leaves. The air temperature within the canopy increased when the light source was placed within the canopy and fresh weight, dry weight and bract length of the bract also increased compared with when the same light was supplied from above, probably due to the higher temperature. Placing LED light sources below the canopy of potted ornamentals was found to be a feasible solution for supplying supplementary light. Keywords artificial lighting, chlorophyll fluorescence, Euphorbia pulcherrima, greenhouse horticulture, light emitting diode, photosynthesis

Significance of this study What is already known on this subject? Using LEDs for assimilation light is different from using high intensity discharge lamps, as very little infrared heat is emitted from the LEDs. It is also known that increasing the Red:Far red ratio within the canopy will reduce stem elongation. What are the new findings? Stem elongation was not affected with respect to the direction of the supplementary light. However, plant fresh- and dry weight was increased if light was given from below, as was the air temperature within the canopy. What is the expected impact on horticulture? LED lighting systems provide possibilities for precision lighting. Placing luminaires inside or below the canopy might be an efficient way of providing light with high precision, and be able to heat the crop inserted of the greenhouse air with the waste heat. Placing fixtures inside canopy also reduces shadow effects.

E-mail: Karl-Johan.Bergstrand@slu.se  

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Received: 6 June 2014 | Revised: 4 November 2014 | Accepted: 9 December 2014 | Published: 22 April 2015 | Available online: 22 April 2015