|Authors: ||M.G. Blanchard, E.S. Runkle|
|Keywords: ||Euphorbia pulcherrima, paclobutrazol, plant growth retardant|
In the United States, poinsettia (Euphorbia pulcherrima Willd. ex Klotz) is the most valuable ornamental potted plant with over 59 million pots produced in 2005. During commercial production of poinsettia, plant growth retardant (PGR) chemicals are commonly applied to inhibit stem elongation and manage final plant height.
However, growth retardant chemicals applied after first bract color (FBC), or earlier but at an excessive rate, can reduce the size of the colorful bracts.
We performed experiments to quantify how a spray application of a commercial product containing gibberellins A4+A7 and benzyladenine (GA+BA) influenced stem elongation and bract size of poinsettia treated with or without a drench application of the PGR paclobutrazol. ‘Freedom Red’ poinsettia plants were grown in a glass greenhouse at a constant temperature of 20°C and induced to flower under a 9-h photoperiod.
In the first experiment, GA+BA was applied on five different dates relative to FBC (—10, 0, +10, +20, or +30 days) and at 3 or 6 mg∙L-1. In the second experiment, plants received a drench of paclobutrazol at 2 mg∙L-1 at FBC and GA+BA was subsequently applied at 10, 20, or 30 days after FBC and at 3, 5, or 10 mg∙L-1. All GA+BA applications were applied as a foliar spray at a volume of 0.2 L∙m-2. In both experiments, GA+BA applied 10 days after FBC increased stem elongation the most 7 days after application, by 0.9 to 2.4 cm compared to control plants.
Application of GA+BA 20 or 30 days after FBC caused little or no promotion of stem elongation but increased bract area the most.
For example, GA+BA applied 30 days after the paclobutrazol drench increased bract area by 33% compared to drenched plants without a GA+BA application.
We conclude that GA+BA has the greatest promotive effect on poinsettia stem extension when applied near or before FBC, and a later application has the greatest promotive effect on bract expansion.
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