414 



RADIATION BIOLOGY 



Schmidt (1914) confirmed these results and demonstrated that the law 

 applied to different wave lengths. 



The present authors (Koski and Smith, 1948-1949; Koski, 1949) fol- 

 lowed the time course of the transformation at different light intensities 

 (cf. Fig. 7-14). At the lowest intensity the rate followed a second-order 

 law, but the rate law changed with the intensity ; this indicated that the 

 reaction was complicated by certain factors, which have not yet been 

 determined. Nevertheless in the early stages of the reaction the rate 



was approximately proportional to 



100 



• EXPOSED Ot 5°C 

 o EXPOSED ot I8°C 



240 ft-c 



20 30 40 



EXPOSURE TIME, sec 



60 



Fig. 7-14. The time course of the trans- 

 formation of protochlorophyll to chloro- 

 phyll at different light intensities and at 

 temperatures of 5° and 18°C. (Koski, 

 1949.) 



the intensity. 



Intense light inhibits the forma- 

 tion of chlorophyll. Scharfnagel 

 (1931; cf. Noack, 1934) observed 

 that by illuminating an etiolated 

 corn leaf with very intense light he 

 could suspend the transformation 

 of protochlorophyll to chlorophyll. 

 The suspension was only temporary 

 because transfer of the leaf to lower 

 light intensities permitted the trans- 

 formation to take place. 



Lubimenko (1928; cf. Monte- 

 verde and Lubimenko, 1912) has 

 shown that exposure of etiolated 

 cotyledons to direct sunlight de- 

 creases the rate of chlorophyll 

 accumulation by subsequent illumination with weaker light. Three lots 

 of pumpkin cotyledons were exposed to direct sunlight for 0, 5, and 10 

 min and then placed in diffuse daylight for 7 hr. The relative quantities 

 of chlorophyll produced were, respectively, 100, 72, and 62. Lubimenko 

 concludes that light influences not only the transformation of "chloro- 

 phyllogene " to chlorophyll but also other reactions allied with the produc- 

 tion of pigments; this is why such complex relations are sometimes 

 observed between light intensity and the accumulation of chlorophyll. 



Sargent (1940) observed that Chlorella cells grown at high light intensi- 

 ties contain less chlorophyll than those grown at weaker intensities; how- 

 ever, because of the greater growth in full sunlight, they produce a greater 

 total amount of chlorophyll at the higher intensities. He found a ratio 

 of 0.51 for the chlorophyll content of cells grown at full sunlight intensity 

 versus that at one-seventh this intensity, but a ratio of 2.5 for the rates 

 of production of chlorophyll at the two intensities. 



Myers (1946) has also determined the rate of production of chloro- 

 phyll in C pyrenoidosa as a function of light intensity. The rate increases 

 almost linearly with increase in light intensity at low intensities, reaches a 



