Plant developmental consequences of lighting from above or below in the production of poinsettia

Abstract

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.