538 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



The photoxidation of chlorophyll in plants in which photosynthesis 

 has been inhibited by poisoning or carbon dioxide starvation was first 

 studied by Noack (1925, 1926), who used the water moss, Fontinalis 

 antipyretica. After 48 hours of illumination of Fontinalis in C02-free 

 water, the leaves showed a partial bleaching. More effective was the 

 suppression of photosynthesis by poisoning, e. g., by phenylurethan or 

 sulfur dioxide. The bleaching of poisoned chloroplasts set in, not 

 immediately after the addition of the poison, but several hours later 

 (according to page 526, the explanation of this delay must be sought in 

 the preliminary photoxidation of other cellular materials). Once started, 

 the bleaching persisted even after the removal of the poison, particularly 

 in the case of sulfur dioxide. Noack (1926^) suggested that photoxidation 

 of chlorophyll under the influence of sulfur dioxide explains the destruc- 

 tion of vegetation by industrial smoke gases. Continuing Noack's 

 experiments, Wehner (1928) observed the bleaching of leaves in which 

 the photosynthetic apparatus was poisoned by nitrous gases (fuming 



nitric acid). 



The chemistry and kinetics of chlorophyll bleaching in vivo are as 

 yet almost unknown. Noack (1925, 1926) measured the rate of oxygen 

 uptake by killed Fontinalis shoots in light, and found a linear increase in 

 the rate with an increase in [O2], up to 2% oxygen in the air. Prelimi- 

 nary extraction of lipides with petroleum ether did not change the rate 

 of bleaching and of oxygen absorption (Noack considered this as a sign 

 that the stability of chlorophyll in the chloroplasts is not due to an 

 association with lipides). Treatment with copper salts (leading to a 

 substitution of copper for magnesium), slightly increased the rate of 

 oxygen uptake. (This experiment was made because Noack thought 

 that copper-pheophytin, which does not fluoresce and is not efficient as 

 a sensitizer, should also be less subject to photochemical self-oxidation.) 



B. Sensitized Oxidation-Reductions in vivo* 



The replacement of water by hydrogen sulfide, hydrogen, or organic 

 compounds as hydrogen donors in algal and bacterial photosynthesis 

 was discussed in chapters 5 and 6. Sensitized photoxidations, described 

 on pages 526-537 of this chapter, can be attributed to a substitution of 

 oxygen for carbon dioxide as hydrogen acceptor in the photochemical 

 reaction (c/. page 536). We shall now describe attempts to induce 

 plants to use other (inorganic or organic) substitute acceptors. 



1. Chlorophyll-Sensitized Reduction of Nitrate 



Green plants are generally capable of assimilating nitrogen in the 

 form of nitrates, in the dark, reducing them to derivatives of ammonia, 



* Bibliography, page 559. 



