Photothermal ratio (PTR) is defined as the ratio of radiant energy (light) to thermal energy (temperature). The objective of this study was to quantify the effect of PTR during the vegetative (PTRv) and reproductive phase (PTRr) on finished plant quality of `Freedom' poinsettia. In Expt. I, plants were grown under 27 combinations of three temperatures, three daily light integrals (DLI), and three plant spacings from pinch to the onset of short-day flower induction and then moved to a common PTR until anthesis. In Expt. II, plants were grown under a common PTR during the vegetative stage and then assigned to nine combinations of one temperature, three DLIs, and three plant spacings after the onset of short-day flower induction. Both PTRr and PTRv affected final plant dry weight. All components of dry weight (total, stem, green leaf, and bract) responded in a linear way to PTRr and in a quadratic way to PTRv. Stem strength was more dependent on PTRv than PTRr. When PTRv increased from 0.02 to 0.06 mol/degree-day per plant, stem diameter increased about 24% while stem strength increased 75%. The size of bracts and cyathia was linearly correlated to PTRr, but not affected by PTRv. When PTRr increased from 0.02 to 0.06 mol/degree-day per plant, bract area, inflorescence diameter, and cyathia diameter increased 45%, 23%, and 44%, respectively.
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