Light (radiant energy) and temperature (thermal energy) affect quality of greenhouse crops. Radiant energy drives photosynthesis and, consequently, plant biomass accumulation. Thermal energy is the primary environmental factor driving developmental rate. The concept of a photothermal ratio (PTR), the ratio of radiant energy [moles of photosynthetic (400 to 700 nm) photons/m2] to thermal energy (degree-day), was proposed to describe the balance between plant growth and plant development in greenhouse crops. The objective of this study was to quantify the effect of PTR during vegetative (PTRv) or reproductive (PTRr) phases on finished plant quality of `Freedom' poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch). In Expt. 1, plants were grown under 27 combinations of three constant temperatures (19, 23, or 27 °C), three daily light integrals (DLIs) as measured by the number of photosynthetic (400 to 700 nm) photons (5, 10, or 20 mol·m-2·d-1), and three plant spacings (15 × 15, 22 × 22, or 30 × 30 cm) from pinch to the start of short-day flower induction, and then moved to a common PTR until anthesis. In Expt. 2, plants were grown under a common PTR during the vegetative stage and then moved to combinations of three DLIs (5, 10, or 15 mol·m-2·d-1) and three plant spacings (25 × 25, 30 × 30, or 35 × 35 cm) at a constant 20 °C from the start of short days until anthesis. Both PTRr and PTRv affected final plant dry weight (DW). All components of DW (total, stem, leaf, and bract) increased linearly as PTRr increased, and responded quadratically to PTRv, reaching a maximum when PTRv was 0.04 mol/degree-day per plant. Stem strength depended more on PTRv than PTRr. When PTRv increased from 0.02 to 0.06 mol/degree-day per plant, stem diameter increased ≈24%, while stem strength increased 75%. The size of bracts and cyathia increased linearly as PTRr increased, but was unaffected 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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