Effect of light quality on Poinsettia (Euphorbia pulcherrima Wild.) morphology

Abstract

Use of chemical growth regulators is the main tool used for making dwarf and compact plant in pot plant industry, although there are ever increasing concerns about the negative consequences of such chemicals to the environment. Therefore, the aim of this study was to investigate the light quality as an alternative tool to reduce stem elongation in pot plants. Similarly, the effectiveness of end-of-the-day red (EOD-R) was also evaluated in terms of reducing plant height. Three experiments, i.e. long day, short day and growth chamber were conducted in controlled growing conditions in SKP (Senter for Klimaregulert Planteforskning), Norwegian University of Life Science (UMB). Two cultivars i.e. ‘Advent Red’ and ‘Christmas Eve’ were taken as plant materials. The three different light sources used in the greenhouse experiments were high pressure sodium (HPS), light emitting diodes (LED) and mixture of high pressure sodium light and light emitting diodes on equal proportion. The LED light consisted of 80% red light and 20% blue light. In the EOD-R treatment, plants were exposed to low intensity red light (5 µ mol/m2/s) for 30 minutes at the end of the day. Similarly, in growth chamber experiment, only high pressure sodium light and light emitting diodes were used to compare the effects on plant height. The cultivar ‘Christmas Eve’ was found less sensitive to the light qualities than the cultivar ‘Advent Red’. Plants in LD experiment did not produce visible cyathia in three light qualities. Plant height in LED was significantly shorter than HPS light both in growth chamber and greenhouse. Similarly, EOD-R was effective to reduce the stem length in HPS, which reduced around 13% plant height than the plants without EOD-R. The results suggest that LED can be used to shorten the plant height in cultivar ‘Advent Red’ and EOD-R in HPS can be equally effective as drop treatment in terms of plant height reduction.