Chapter 30 



THE LIGHT FACTOR. HI. PHOTOSYNTHESIS AND LIGHT 

 QUALITY; ROLE OF ACCESSORY PIGMENTS* 



1. Action Spectrum 



The dependence of photosynthesis on the spectral quality of Ught was 

 the subject of much interest, long before the influence of light quantity 

 was first investigated. As early as 1788, Senebier conducted experiments 

 on carbon dioxide assimilation in double-walled vessels, filling the space 

 between the walls with various colored solutions. Since then, the botanical 

 literature has been replete with observations on the behavior of plants in 

 light of different color. (For a review of these investigations, see, for ex- 

 ample, Gabrielsen 1940.) This was to be expected, since everything con- 

 nected with color has always held, and still holds a captivating interest for 

 mankind, even though in scientific photochemistry the qualitative cate- 

 gories of "red," "blue," "yellow" or "green," which were so dear to Goethe 

 (he thought them to be the "primary phenomena" of optics), have been 

 reduced to mere quantitative differences between the energy contents of 

 the light quanta. 



About a hundred years ago the effect of color on photosynthesis be- 

 came a topic of a lively discussion. Its subject was the position of the 

 maximum of photo synthetic efficiency in the solar spectrum. In 1844, Draper 

 found that, when the prismatic spectrum of the sun was thrown upon a 

 plant, the largest amount of oxygen was liberated in the yellow-green re- 

 gion ; this result was confirmed by such authorities in plant physiology as 

 Sachs (1864) and Pfeffer (1871). Sachs pointed out that the yellow is also 

 the region of maximum "luminosity" of light, i. e., of maximum effect on 

 the human retina. He himself saw in this only a coincidence; but other, 

 less cautious authors thought that such a correspondence must be sig- 

 nificant, and attempted to explain it. The belief that photosynthesis pro- 

 ceeds most actively in green light, which is only weakly absorbed by chloro- 

 phyll, led to several peculiar hypotheses, such as that light energy is not 

 used in photosynthesis at all (Pfeffer 1871), or that the role of chlorophyll 

 in plants is merely to protect the carbon dioxide-reducing system from 

 injury by light (Pringsheim 1879, 1881, 1882). 



Timiriazev (1869, 1875, 1877, 1885) vigorously fought these miscon- 



* Bibliography, page 1 188. 



1142 



