1014 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



tion of the photosensitive complex. According to Franck, this is the re- 

 action that converts the intermediate oxidation products, formed by hght, 

 into free oxygen (or sulfur, or other final oxidation products formed in 

 bacterial photosynthesis). When this reaction fails to keep pace with the 

 primary photochemical process, the intermediate oxidation products 

 ("photoperoxides") accumulate in amounts sufficient to oxidize certain 

 metabolites, thus forming a product of narcotizing properties (perhaps an 

 organic acid). The latter is adsorbed on the photosensitive complex, and 

 this retards or stops altogether the primary process. 



Each partial nonphotochemical process of limited maximum rate im- 

 poses its o^^^l "ceiling" on the over-all rate of photosynthesis; and, since 

 the influence of such a ceiling is felt long before it has actually been reached, 

 the saturation value of photosynthesis in strong light may be affected not 

 by one limiting process, but by several such processes— particularly since 

 the maximum capacities of different parts of the photosynthetic apparatus 

 appear to be of the same order of magnitude (as one would expect of a well- 

 adjusted catalytic system). 



In the general discussion of the kinetic curves of photosynthesis in 

 chapter 26, three types of curve sets, P = f{Fi) with Fg as parameter, were 

 described and designated as the first (or "Blackman") type, the second (or 

 "Bose") type and the third type, respectively (see figures 26.2, 26.3 and 

 26.4) . We recall that curves of the first type must arise when the parameter 

 Fz determines the maximum rate of a partial process that does not depend 

 on the independent variable, Fi. This process then imposes a horizontal 

 ceiling on the curve P = f{Fi), but does not affect its initial slope. In curve 

 sets of the third type the parameter affects the initial slope of the light 

 curve, but not its saturation level; this type results when F2 codetermines 

 the rate of a process that is also a function of the independent variable, Fi. 

 In curve systems of the second type, the parameter F2 affects both the initial 

 slope and the saturation level. Carbon dioxide curves offered examples of 

 all three types, depending on the nature of the parameter {cf. page 868). 

 Since most parameters do not affect the rate of the primary photochemical 

 process, and therefore do not change the initial slope of the light curves, 

 the P = /(/) curve systems usually are of the first type, i. e., the various 

 curves coincide at low light intensities, but diverge at saturation. Such 

 are most of the curve systems observed with carbon dioxide concentration 

 as parameter (figs. 28.1, 28.2, 28.4, 28.5 and 28.5A), the only exception 

 being Harder's Fontinalis curves (fig. 28.3). The two light curves of 

 Chromatium with thiosuJfate concentration as parameter (fig. 28.5B) have 

 the same general appearance, and the curve systems with temperature as 

 parametei-, illustrated by figures 28.6, 28.7 and 28.8, are of the same type. 



The efiect of inhibitors, however, is uneven and some results are contra- 



