Acclimatisation of greenhouse crops to differing light quality

Abstract

High-intensity discharge (HID) and light-emitting diode (LED) lights have been widely compared for use in greenhouse plant production but the results are contradictory. In order to obtain more data on the effects of different light sources on plant growth, growth chamber experiments with high pressure sodium (HPS) or LED light and one treatment with alternating HPS and LED light (three days each) were carried out using tomato and rose as model plants. The LED lamps used were composed of blue (B, peak emissions 402, 419, and 445nm) and red/far red (R/FR, peaks in 663 and 737nm) LEDs. Plant growth parameters were recorded, as were photosynthesis, chlorophyll fluorescence, chlorophyll content, leaf temperature, leaf spectral properties and light penetration into the canopy. In roses, stem elongation and leaf area were generally lower for plants grown under LED light while fresh and dry weight was unaffected by the lamp type. For tomato, plants grown in alternating LED and HPS lamps had lower fresh weight as compared with HPS. Specific photosynthetic capacity (Amax) and maximum quantum yield of PSII (Fv/Fm) were higher in leaves developed under LED light than HPS. Leaf transmittance and reflectance were higher for leaves grown in HPS light, which also gave better penetration of light into the canopy. Plants subjected to alternating light regimes generally resembled LED treatment plants more than HPS plants. Leaf temperature was higher under HPS (0.9–1.3°C) favouring plants growing in chambers with HPS light. Leaf temperature and the amount of blue light supplied were concluded to be key factors for plant performance.