Optimum Light 



Macrophyte growth could be affected by the intensity, 

 quality, and photoperiod of the available light. Because 

 they grow submerged in water , macrophytes might respond 

 to these light factors quite differently than do terres- 

 trial species. 



There were only limited opportunities for varying light 

 quality in the artificially lighted growth chambers used 

 in these studies. Some control was possible in the 

 selection of fluorescent bulbs, which provided most of 

 the light, and in the number of incandescent bulbs used 

 to supplement radiation of long wavelengths. In the few 

 variations attempted, there were no indications that any 

 combination was superior to the ratio of fluorescent to 

 incandescent light provided in the Sherer Growth Cabinets, 

 Model CEL 25-7 HL, used in the current studies. Each 

 chamber contained 6, cool-white, 4-foot fluorescent bulbs 

 and 12, 25 watt incandescent bulbs. These are ratios 

 considered suitable for crop plant growth under artificial 

 light. There also was no indication that a light-dark 

 cycle in each 24-hour period was superior to continuous 

 light. 



The effect of variations in light intensity on the growth 

 of Elodea was investigated in an experiment summarized in 

 Figure 4. The experimental setup was similar to that 

 described in Section IV for establishing nutrient critical 

 concentrations by the tray procedure. This procedure was 

 used, because it permitted rapid evaluations of macrophyte 

 response to light. One set of trays was maintained at 

 approximately 1700 ft candles throughout the culture period; 

 another set was exposed to different light intensities 

 every several days during the culture period. 



The results in Figure 4 compare the percent increase in 

 length of Elodea in control trays exposed to continuous 

 light of 1700 ft candles and in trays exposed to different 

 light intensities at various stages of the culture period. 

 The data indicate Elodea was not damaged by light intensities 

 as high as 2600 ft candles; in fact, growth was slightly 

 better at 2600 ft candles than at 1700. When light inten- 

 sity was reduced to 710, 390, and 110 ft candles, growth 

 was less than at 1700 ft candles and progressively less 

 with each decrease in intensity. 



47 



