Photothermal ratio: Relating plant growth and development to the ratio of light and temperature

Photothermal ratio (PTR) is defined as the ratio of radiant energy (light) to thermal energy (temperature). It describes light energy available for photosynthesis per unit of developmental time. In this project, the relationship between PTR and poinsettia plant quality was studied. Poinsettia ‘Freedom’ plants were grown under 27 treatment combinations of 3 temperatures, 3 daily light integrals and 3 plant densities during the vegetative stage and 9 treatment combinations of one temperature, 3 daily light integrals and 3 plant densities during the reproductive stage. The effects of PTR on plant dry weight, stem diameter and strength, bract and cyathia size were quantified. Plant quality parameters were linearly correlated to PTR. A high PTR during vegetative development enhanced plant stem strength and reduced stem breakage at anthesis. A high PTR during reproductive development improved plant appearance by increasing bract and cyathia size and reduced cyathia abscission. A poinsettia plant growth and development model was also developed to examine the response of plant growth and development to PTR. The simulation results confirmed that PTR is a useful parameter for plant quality control. In separate experiments, light, plant density and temperature were adjusted to increase PTR in an attempt to improve plant quality. Temperature adjustment to regulate PTR and improve poinsettia plant quality did not prove practical. However, the potential for light adjustment to increase PTR and plant quality was substantial. Reducing overhead shading and decreasing plant density improved light interception greatly and increased plant quality. The minimum daily light integral for acceptable poinsettia plant quality was about 10 to 12 mol m−2 day−1 at 20°C with plant spacing of 25 x 25 cm. This minimum daily light integral increased as temperature increased and decreased as plant density decreased.