IS PLANT QUALITY RELATED TO THE RATIO OF RADIANT ENERGY TO THERMAL ENERGY?

Plant growth and development are driven by two of energy: radiant and thermal. Radiant energy drives photosynthesis and therefore dry weight gain, while thermal energy drives development rate. This study was undertaken to determine the effect of the ratio of radiant energy to thermal energy (RRT) on plant quality of Euphorbia pulcherrima 'Freedom'. From pinch to the onset of short-day flower induction, plants were grown under 27 combinations of temperature (thermal energy), light (radiant energy), and spacing, i.e., factorial combinations of three air-temperature settings (19, 23, or 27C), three daily light integrals (5, 10, or 20 mol m^-2.day^-1), and three plant spacings (15 H 15, 22 H 22, or 30 H 30 cm). Plants were treated for 450 degree-days (base temperature of 5C) in Expt. 1 or five weeks in Expt. 2. The results showed that the measured plant quality parameters that can be associated with quality were closely related to RRT. Although modified by spacing, specific leaf weight increased 100% to 150%, stem dry weight increased 80% to 90%, and cross-sectional area of stems increased 35% to 45% as RRT increased from 0.2 to 1.2 molAm^-2 Adegree-day^-1. Plant dry weight (total, leaf, and stem) gain increased with RRT at almost twice the rate for plants at the 30 x 30 cm spacing compared to that at the 15 x 15-cm spacing.