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Influence of Spectral Quality of Light on Plant Reproductive Behavior

Dr. Gerald F. Deitzer, Professor Emeritus
Plant Physiologist
Department of Plant Sciences and Landscape Architecture
University of Maryland
College Park, MD 20742

Throughout my career, interest in my laboratory has centered on the area of photobiology and the regulation of plant development by light. Our primary interest involves how plants absorb and respond to various regions of the electromagnetic spectrum, from the ultra-violet to the near infra-red. Specifically, we have investigated how a plant pigment, known as phytochrome regulates plant growth and development. Phytochrome exists in two photo-interconvertable forms, one that absorbs maximally in the red region of the spectrum (600-700 nm) and one that absorbs maximally in the far-red region (700-800 nm). The red absorbing form, called Pr, when it absorbs red light converts the pigment to the active far-red absorbing form, known as Pfr. All regions of the spectrum create a photoequilibrium between these two forms (Pfr/Ptot). Plants perceive this photoequilibrium to regulate all aspects of their development, from seed germination to flowering and seed set.

Most of our research has centered on how long-day plants such as Arabidopsis, barley and wheat are promoted by the addition of far-red light to the photoperiod. We have found that, when added to the end of a short day, flowering is dramatically enhanced. Fluorescent light sources and high energy discharge lamps (Metal Halide and High Pressure Sodium) lack energy in the far-red region, necessitating the addition of incandescent light which does contain far-red energy. Unfortunately, both commonly used fluorescent light sources and incandescent sources are very energy inefficient and will soon be removed from the market. More energy efficient fluorescent sources are becoming available, but none of them contain any far-red energy. There is currently no technology to replace incandescent sources. The iGROW induction lighting, being developed by FSS Hydro, LLC and Sure to Grow, has the potential to replace less efficient fluorescent sources and we have agreed to work with them to compare their effectiveness in controlling plant growth and development to ordinary fluorescent sources and high energy discharge lamps.

We have used two spectroradiometers to analyze the spectra from 250 nm to 1100nm for both a 200 W iGROW Bloom and a 200 W iGROW Bloom induction lamp and compared them to 400 W of F48/T8/TL841/HO fluorescent sources installed in four Conviron BDR8 chambers at the University of Maryland. An analysis of these spectra has also been provided to calculate the biological effectiveness of these sources. So far, we have only run one experiment with the 200 W induction light source examining the growth and flowering of barley. We see no difference in plant physiology and growth when compared to the ordinary fluorescent sources and high discharge lamps. Further analysis is underway and we will soon install a 400 W iGROW Bloom induction light source for comparison.