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Bin Liu and Royal D. Heins

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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Bin Liu and Royal D. Heins

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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Genhua Niu, Royal D. Heins, Arthur C. Cameron, and William H. Carlson

Pansy [Viola ×wittrockiana Gams. `Delta Yellow Blotch' (Yellow) and `Delta Primrose Blotch' (Primrose)] plants were grown in a greenhouse under two CO2 concentrations [ambient (≈400 μmol·mol-1) and enriched (≈600 μmol·mol-1)], three daily light integrals (DLI; 4.1, 10.6, and 15.6 mol·m-2·d-1), and nine combinations of day and night temperatures created by moving plants every 12 h among three temperatures (15, 20, and 25 °C). Time to flower decreased and rate of flower development increased as plant average daily temperature (ADT) increased at all DLIs for Yellow or at high and medium DLIs for Primrose. Increasing the DLI from 4.1 to 10.6 mol·m-2·d-1 also decreased time to flower by 4 and 12 days for Yellow and Primrose, respectively. Both cultivars' flower size and Yellow's dry weight [(DW); shoot, flower bud, and total] decreased linearly as plant ADT increased at high and medium DLIs, regardless of how temperature was delivered during day and night. DW in Yellow increased 50% to 100% when DLI increased from 4.1 to 10.6 mol·m-2·d-1 under both CO2 concentrations. Flower size in Yellow and Primrose increased 25% under both CO2 conditions as DLI increased from 4.1 to 10.6 mol·m-2·d-1, but there was no increase between 10.6 and 15.6 mol·m-2·d-1, regardless of CO2 concentration. Plant height and flower peduncle length in Yellow increased linearly as the difference between day and night temperatures (DIF) increased; the increase was larger under lower than higher DLIs. The ratio of leaf length to width (LL/LW) and petiole length in Yellow increased as DIF increased at medium and low DLIs. Carbon dioxide enrichment increased flower size by 4% to 10% and DW by 10% to 30% except for that of the shoot at medium DLI, but did not affect flower developmental rate or morphology. DW of vegetative and reproductive parts of the plant was correlated closely with photothermal ratio, a parameter that describes the combined effect of temperature and light.

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Grete Waaseth, Roar Moe, Royal D. Heins, and Svein O. Grimstad

Varying photothermal ratios (PTR) were supplied to Salvia ×superba Stapf `Blaukönigin' during pre-inductive vegetative development with the exception of a short germination period under uniform conditions. In addition, both unvernalized plants and plants receiving a saturating vernalization treatment of 6 weeks at 5 °C were given two photosynthetic photon flux (PPF) levels (50 or 200 μmol·m-2·s-1) during subsequent inductive 16-hour long days. There were no effects of PTR treatments during vegetative development on subsequent flowering. However, the higher PPF level during inductive long days significantly accelerated floral evocation in unvernalized plants, lowering the leaf number at flowering. The effect was practically negligent after the vernalization requirement was saturated. In a second experiment, varying periods (4, 7, 10, and 14 days or until anthesis) at a PPF of 200 μmol·m-2·s-1 during 20-hour days were given at the beginning of a long-day treatment, either with or without preceding vernalization treatment. Flowering percentage increased considerably as the period at 200 μmol·m-2·s-1 was extended compared with plants grown at a lower PPF of 50 μmol·m-2·s-1. However, the leaf number on flowering plants was not affected, except in unvernalized plants receiving the highest PPF continuously until anthesis, where leaf number was reduced by almost 50%. We propose that the PPF-dependent flowering is facilitated either by the rate of ongoing assimilation or rapid mobilization of stored carbohydrates at the time of evocation. Abortion of floral primordia under the lower PPF (50 μmol·m-2·s-1) irrespective of vernalization treatment indicates that the assimilate requirement for flower bud development is independent of the mechanism for floral evocation.

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Madeline W. Olberg and Roberto G. Lopez

increasing the photothermal ratio, or the ratio of radiant energy to thermal energy (moles per square meter per degree-day). Photothermal ratio has been proposed to balance the ratio of plant growth to plant development, and increased photothermal ratio has

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Tasneem M. Vaid, Erik S. Runkle, and Jonathan M. Frantz

.E. 1977 Oxygen inhibition of photosynthesis. 1. Temperature dependence and relation to O 2 /CO 2 solubility ratio Plant Physiol. 59 986 990 Liu, B. Heins, R.D. 2002 Photothermal ratio affects plant quality in ‘Freedom’ poinsettia J. Amer. Soc. Hort. Sci

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Diane M. Camberato, Roberto G. Lopez, and Brian A. Krug

Liu and Heins (2002) on the moderate-vigor poinsettia ‘Freedom’ indicates that as the photothermal ratio [ratio of daily light integral (DLI) to temperature] decreases after the start of short days (SD), bract size and cyathia diameter decrease

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Christopher J. Currey and Roberto G. Lopez

response to plant growth regulators HortTechnology 16 129 132 Liu, B. Heins, R.D. 2002 Photothermal ratio affects plant quality in ‘Freedom’ poinsettia J. Amer. Soc. Hort. Sci. 127 20 26 Lopez, R. Runkle, E. 2007 Early PGR drench applications on poinsettia

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Joaquin A. Chong, Uttara C. Samarakoon, and James E. Faust

.D. 1998 Modeling poinsettia vegetative growth and development: The response to the ratio of radiant to thermal energy Acta Hort. 456 133 142 Liu, B. Heins, R.D. 2002 Photothermal ratio affects plant quality in ‘Freedom’ poinsettia J. Amer. Soc. Hort. Sci

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James E. Faust and Joanne Logan

environmental parameters has been explored in several studies. The concept of photothermal ratio (PTR) was introduced by Liu and Heins (2002) . This concept suggests that plant quality is directly related to the ratio of DLI to the average daily temperature