340 



C. S. French 



THE RATES OF UTILIZATION OF THE PHOTOPRODUCTS DURING ILLUMINATION 



The flashing light experiments of Emerson and Arnold^^'\ later 

 followed up with improved methods by Tamiya^l^^, Kok^-*-^^, and oth- 

 ers, were done before it was realized that more than one photo- 

 chemical reaction was involved. The reduction in the yield per 

 flash as the dark period was shortened was interpreted as measur- 

 ing the time-course of a dark reaction initiated by the photochem- 

 ical act. The time-courses for the dark reactions following each 

 photochemical reaction are certainly different, as we have seen 

 with the long flash experiments. The basic idea was to follow the 

 time-course for the utilization of the products of each photochem- 

 ical reaction as the intensity of the background light absorbed by 

 the other pigment system is varied. It has not yet been possible 

 to find conditions giving flash saturation over a range of dark 

 times for one light color in the presence of a continuous back- 

 ground light activating the other pigment system. The range be- 

 tween the rate at flash saturation for one color with background 

 light of another color and the ceiling imposed by the saturation 

 level in continuous light leaves very little scope for significant 

 measurements of the yield per flash with differing dark times. 



Some results with flashing light will be mentioned: In these 

 experiments with Porphyridium at 5° C nearly identical saturation 

 rates V7ith continuous light were found in the two colors. This 

 result differs from those of McLeod^^^^ with other species at room 

 temperature where the rate at saturation was found to vary with 

 wavelength. 



The flash-saturation level at 5° C is identical with red and 

 with green light. With flashes of 9 ms at 95 ms dark intervals 

 the flash-saturation rate in both red and green light was about 

 60 per cent of the rate at saturation with continuous light. With 

 1,5 ms flashes and about the same dark time the available intensi- 

 ty did not produce flash-saturation but at least with red light at 

 several intensities the rate appeared to extrapolate to about the 

 same value found for the longer flashes. However with red flashes 

 of 1,5 ms and dark times of 535 ms a flash-saturation level of 

 about 21 per cent of the continuous saturation level was ap- 

 proached. These flashes were also given with a continuous green 

 background light that produced a rate 25 per cent of saturation, 

 A small enhancement of the rate was seen but flash saturation was 

 not attainable. Evidently greater familiarity with the response 

 of Porphyridium to various regimes of flashing monochromatic light 

 will be necessary before the appropriate experiments leading to 

 the above-outlined objectives can be planned. Various exploratory 

 experiments have, however, turned up some unexpected effects de- 

 scribed in the next section. 



