Impact of end-of-day red and far-red light on plant morphology and hormone physiology of poinsettia

• The ornamental pot plant poinsettia is responsive to red and far-red LED light.
• Red light at the end of the day counteracts far-red light-enhanced elongation growth.
• Hormone profiling showed that the red light reduces gibberellin and auxin contents.
• Time to visible flowering did not differ between the red and far-red treatments.
• Phytochrome manipulation of hormones can be a useful tool in poinsettia culture.

Control of morphology is essential in greenhouse production of poinsettia, which is among the largest and economically most important ornamental pot plant cultures worldwide. Plant growth regulators are used to reduce shoot elongation but due to their potentially negative impacts on human health and the environment, it is highly desirable to replace these. Exploiting responses to light quality as an alternative has been investigated to a limited extent only in poinsettia and information about elongation-controlling hormones is scarce. In natural light an increased far-red (FR) proportion at the end of the day (EOD) is known to enhance shoot elongation in several species through action of phytochrome on hormone metabolism. The aim of this study was to assess the effect of manipulation of the phytochrome system at the EOD on elongation growth and hormone metabolism in poinsettia. Depending on cultivar, shoot elongation and internode lengths were reduced 34–54% by exposure to 30 min EOD-R compared to 30 min EOD-FR, both provided by light emitting diodes. The reduced elongation under EOD-R correlated with 29% and 21% lower levels of gibberellin and indole-3-acetic acid, respectively. Specific leaf and bract area were also significantly lower under EOD-R compared to EOD-FR. Bract formation and time to visible cyathia did not differ between the light treatments. In conclusion, manipulation of the phytochrome system by EOD-R in order to modulate the contents of the growth-controlling hormones can be a useful tool in control of shoot elongation in greenhouse-grown poinsettia in order to counteract the effect of the increased FR proportion at the EOD.