MONOCHROMATIC LIGHT CURVES 1165 



presence, in these conifers, of a nonactive yellow pigment. (It was men- 

 tioned on page 1153 that photosynthesis in these plants declines to zero 

 below 450 or 465 m/x.) 



The same effect should be even more pronounced in leaves of the pur- 

 purea varieties, or other leaves containing large quantities of red antho- 

 cyanine pigments. Engelmann recognized as early as 1887, in an investi- 

 gation entitled ''Leaf Hues and Their Importance for the Decomposition 

 of Carbonic Acid in Light," that the red pigments of land plants do not 

 actively participate in photosynthesis; and Willstatter and Stoll (1918) 

 and Kuilman (1930) confirmed that the presence of these pigments has no 

 influence on the rate of photosynthesis in strong light. Gabrielsen (1940) 

 concluded, from a review of the older work and new experiments with 

 Corylus and Prunus leaves in red-orange, yellow-green and blue-violet 

 light, that for a given amount of incident energy the red varieties have a 

 minimum yield in yellow-green light, where light absorption by the red 

 pigment has its maximum. This minimum was most pronounced in weak 

 light; in strong light, the monochromatic light curves approached the same 

 saturation level, in conformity with the findings of Willstatter and Stoll. 

 Gabrielsen estimated that, in red Corylus leaves, the "screen" absorbed 

 about 37% of incident light in the blue-violet, about 74% in the yellow- 

 green and about 33% in the red-orange region. In Prunus, the absorption 

 was somewhat higher, perhaps because in Corylus the red pigment was pres- 

 ent only in the epidermis cells, while in Prunus it was found also in the 

 mesophyll. These results cause us to give little credence to speculations 

 or observations that relate anthocyanines or flavones to photosynthesis. 



In 1922 Noack observed a photochemical conversion of flavones into anthocyanines 

 in vivo and interpreted this reaction as a chlorophyll-sensitized oxidation-reduction (cf. 

 chapter 19, page 541). He suggested that flavones and anthocyanines form a reversible 

 oxidation-reduction system, which may play a catalytic role in photosynthesis. Sen 

 (1942) asserted that anthocyanine-carrying leaves have a higher photosynlhetic ef- 

 ficiency than ordinary leaves, despite their lower content of chlorophyll. 



We now return to the question, raised on page 1158, concerning the 

 third part of the light curves, after the linear range and the transitional 

 region — the saturation plateau. It was })ostulated there that this plateau 

 should have tlu; same height for all wave lengths. .Vs long as tlu; rate 

 is determined only by the kinetic mechanism of pJKjtosyntliesis, theoretical 

 arguments in favor of this postulate appear conclusive. Whether satura- 

 tion is brought about by a limited supply of reactants, or by limited avail- 

 ability of an enzyme, the maximum rate is determined b}^ the velocity of a 

 dark reaction, and should be independent not only of the quantity, but also 

 of the quality of illumination. 



Light (luality could, however, affect the maxiinuni rote of photosynthe- 



