528 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



between chlorophyll-sensitized photautoxidation in vivo and photo- 

 synthesis will be discussed in more detail on pages 543 et seq. 



2. Photautoxidation in Narcotized or Starved Plants 



Fromageot found, in 1924, that in the presence of 50% or more 

 glycerol aquatic plants change from oxygen production to oxygen con- 

 sumption in light, the rate of which may be three or four times higher than 

 that of respiration in the dark. At approximately the same time, 

 Noack (1925, 1926), while studying the chlorophyll-sensitized benzidine 

 oxidation in vitro, found that this reaction also can be sensitized by 

 living leaves. 



In Noack's experiments, leaves were soaked in aqueous benzidine solution and 

 illuminated for four hours. After this the chloroplasts appeared brown. The brown 

 pigment was extracted and proved to be an oxidation product of benzidine, while the 

 chlorophyll appeared intact and its quantity undiminished. Benzidine oxidation could 

 be observed also in boiled green leaves, but not in white parts of variegated leaves, or 

 in leaves whose chlorophyll was converted into copper pheophytin by treatment with 

 copper sulfate. 



Hardly any experiments on the photautoxidation of external substrates 

 by living plants have been carried out since Noack's work on benzidine, 

 although the oxygen consumption of cells undergoing internal photoxida- 

 tion has often been found stimulated by the addition of glucose or other 

 organic nutrients. Whether these compounds served as direct substrates 

 of autoxidation, or were first converted into metabolites, is unknown. 

 Observations on the photautoxidation of cellular material, although more 

 numerous, have usually been confined to the measurement of oxygen 

 consumption, thus leaving the nature of the oxidation substrates unknown. 



In these experiments, carbon dioxide starvation rather than narcotiza- 

 tion, has been used as the means to suppress photosynthesis. Noack 

 (1925, 1926) thought that carbon dioxide removal is less efficient than 

 urethan poisoning in promoting photoxidation (because carbon dioxide 

 i production by respiration does not allow one to reduce photosynthesis 

 much below the "compensation point," where the net gas exchange is 

 zero.) However, van der Paauw (1932) found that, when Hormidium 

 filaments were exposed to light in an atmosphere which was kept free 

 from carbon dioxide by contact with alkali, oxygen evolution yielded 

 place to oxygen consumption, at a rate which was considerably larger 

 than that of ordinary respiration. In similar, but more detailed experi- 

 ments by Franck and French (1941), leaves of Hydrangea were mounted 

 on a wire gauze rotor a few millimeters above a 10% potassium hydroxide 

 solution, which was stirred by threads suspended from the rotor (Fig. 57). 

 The evolution (or absorption) of oxygen was measured manometrically, 

 in darkness and in the concentrated hght of a 1000-watt lamp. 



