1918 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



violet light; the thermodynamic isotopic equilibrium at —55° C, corre- 

 sponds to an enrichment of 0(18) in O2, which is somewhat short, but not 

 too much short, of that actually found in atmospheric oxygen. 



Dole (1952) found that the percentage of 0(18) in the photosynthetically 

 produced oxygen in the sea is 0.2005%. On the other hand, measurement 

 of the isotopic composition of dissolved oxygen in different depths in the 

 sea (Rakestraw, Rudd and Dole 1951), were interpreted by Dole, Lane, 

 Rudd and Zaukelies (1954) as indicating a fractionation factor of 1.009 in 

 favor of 0(16) in the reverse process of oxygen consumption in the ocean 

 (presumably by plankton respiration). This could account for a photo- 

 stationary 0(18), concentration of 0.2005 X 1.009 = 0.2023% which is 

 substantially less than the actual concentration of 0(18) in the atmosphere 

 (which is 0.2039%). It thus again appeared that the "photostationary 

 state" concept was insufficient to account for the 0(18)-accumulation in 

 the atmosphere; at the same time, however, no positive support for the 

 "stratosphere equilibration" hypothesis could be derived from measure- 

 ments of the 0(18)-content in air at high altitudes. 



More recent analyses of the isotope discrimination in respiration, by an 

 improved technique (Dole 1954) gave, for certain objects (such as a species 

 of fungus), discrimination factors as high as 1.02. This could account for 

 a photostationary 0(18) concentration of 0.2005 X 1.02 = 0.2045 — more 

 than enough to explain the isotopic composition of atmospheric oxygen. 

 However, it remains to be seen whether these new figures will prove appli- 

 cable to respiration in general, including that of bacteria. 



(c) Water Factor 



Dehydration. Brilliant and co-workers have continued the study of re- 

 lation between water content and photosynthesis (of. p. 333), investigating 

 in more detail the influence of the rate of dehydration (19430, its duration 

 (I943-) and its repetition (1943^). A summary of these studies, and of 

 numerous other — to a large proportion, Russian — investigations concerning 

 the influence of water content and osmotic state on the yield of photo- 

 synthesis, in land plants and aquatics, can be found in the monograph 

 Photosynthesis as Life Process of the Plant by Brilliant (1949). Particular 

 attention is devoted there to the occurrence, under certain conditions, of a 

 yield maximum at a certain subnormal Avater content (cf. p. 333), to the de- 

 pendence of the dehydration effect on the method of dehydration {cf. p. 335) 

 and to "adaptation" of plants by repeated dehydration (Brilliant 1943^). 

 Phenomena of this type are considered as evidence that water affects photo- 

 synthesis not as a reactant, in accordance with the law of mass action, but 

 indirectly, by influencing the over-all state of the living protoplasm. This 

 is undoubtedly correct (cf. p. 333) and is reflected in our treatment of hy- 



