EVALUATION OF LIGHT SOURCES FOR PLANT GROWTH 1 2 3

Abstract

Selection of light sources for installation in environmental control chambers for plants has generally been by empirical means. Early workers grew plants under various types of lamps and then selected the lamps providing the most normal looking plants (1). Later experiments established that combinations of fluorescent and tungsten lamps resulted in superior plant performance, and these lamp combinations have been installed in many growth chambers (8). More recently, comparisons of various spectral regions at uniform illumination intensities have been made to provide a more precise means of selection of light sources (6, 4). Faced with the problem of selecting a light source for ourselves, we have developed a simple method of making a comparative evaluation, and have employed it for combinations of some currently available lamps. The basis of the method is to measure the growth rates of plants under a series of light intensities for each light source, and then by plotting a growth/light intensity curve for each source, to compare the light sources by the position and slope of the response curve for each. The total radiant power (watts) in the visible spectrum has been used in reporting the growth responses. The simplest means of measuring light intensity or illumination, however, is by means of a foot-candle meter. While this method does not correct for differences in spectral distribution, it offers great convenience. Hence, the foot-candles produced with the several light sources are also given for easy comparison. Some recently developed lamps were evaluated. Certain combinations of these lamps gave initial intensities as high as 5,000 ft-c at the plant level. High output fluorescent lamps (General Electric Power Groove F48PG17/CW) were used alone and in combination with tungsten and deluxe (color-improved) mercury vapor lamps (General Electric H400-RC1). As a tungsten source, 150 watt reflector floods were used alone, or 75 watt inside-frosted lamps were used in combinations with the other bulbs. The number and kind of lamps in each combination together with the three intensities obtained and total watts radiated for each are in table I. The intensities were measured with a Weston model 603 light meter at the end of each experiment. About 10 to 15 % reduction from initial intensities had occurred when a water filter was used. The approximate spectral distribution of the light provided by these various lamps and combinations is given in figure 1. The experimental chamber was equipped with a refrigeration unit and an electric heater for temperature control. Thermostats held the air temperature at 780 + 20 F. The light plenum was separated from the growing chamber by either a glass or a two mil polyethylene plastic baffle. In order to increase intensities the fluorescent lamps were staggered allowing overlapping of tubes. Additional lamps were also placed at right angles to the main axis on the sides of the lamp plenum to obtain more even illumination. Tungsten and mercury vapor lamps were hung between and below the fluorescent tubes. The growing area (4 ft X 4 ft) was divided into three blocks, each receiving a different intensitv of light. The reductions from full light were obtained with two neutral density screens (Saran-Lumite No. 1804-002 and No. 1808-002) rated as producing 51 % and 72 % actual shade respectively. Differences in orientation of emission resulted in considerable variation from these ratings (table I). A monocotyledenous plant (Knox wheat) and a dicotyledenous plant (red kidney bean) were used as indicator plants. These species were selected on the evidence that they would show minimum morphological aberrations when exposed to various types of light (3). Seeds were sown in plastic pots in a standardized mix of peat moss and sand and fertilized with high analysis soluble fertilizer. Germination and early growth were in a greenhouse at 600 F night temperature. The experiments were started when the third leaf of the wheat was developing and the first internode above the seed leaves of the bean was just starting expansion. This was about 13 + 3 days after sowing. Each pot contained either three wheat plants or one bean plant. The plants of each species were divided into five groups of 12 pots each. One group was used to determine initial dry weights, one group remained in the greenhouse (control), and one group Received revised manuscript August 31, 1959. 2 Journal Paper No. 1417, Purdue University, Agricultural Experiment Station, Lafayette, Ind. 3 This reasearch was supported in part by a grant from the Purdue Research Foundation. Contributions of equipment by the General Electric Co., Large Lamp Division, Nela Park, Cleveland, Ohio, and Ballast Division, Danville, Ill., are gratefully acknowledged.

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