PHOTOXIDATION OF CHLOROPHYLL IN VIVO 537 



4. The Photoxidation of Chlorophyll in vivo 



It was stated in chapter 18, that chlorophyll is more photostable 

 in vivo than in vitro. However, chlorophyll in living cells can, too, be 

 bleached by very intense and prolonged illumination, as observed, among 

 others, by Reinke in 1885. This bleaching can be attributed to pho- 

 toxidation. Pringsheim noticed (1881, 1882) that the leaves do not 

 bleach in the absence of oxygen (e. g., in & carbon dioxide atmosphere). 

 Similarly, Funk (1939, 1940) found that dry leaves do not bleach unless 

 air is present in the intercellular spaces. How the photoxidation of 

 cellular reserve substances goes over, upon the exhaustion of these sub- 

 strates, into photoxidation of the pigments was described above. 



The relative light stability of chlorophyll in vivo extends into the 

 ultraviolet, as described, for instance, by Gilles (1939). According to 

 Arnold (1933), the inhibition of photosynthesis by ultraviolet light 

 (253.6 m/i) is not accompanied by a destruction of chlorophyll (cf. 

 Chapter 13, page 346). Montfort (1941, 1942) found that the bleach- 

 ing of algae in intense light is caused mainly by violet and ultraviolet 

 radiations. 



Richter (1932, 1935) observed the bleaching of chlorophyll in vivo by very intense 

 ultraviolet light (< 300 mn). He found two effects: a direct destruction, which requires 

 exposures of several minutes; and an indirect delayed decomposition, which can be 

 induced by an exposure of only 10 or 20 seconds. The latter effect, which Richter 

 attributed to the activation of an enzyme, could be observed (in leaves of Tropaeolum 

 majus) only when the leaves were illuminated from the luiderside. 



According to page 507 et seq., the stability of chlorophyll to light and 

 air in living cells may have a twofold source: it may be due either to a 

 static protection — for instance, to an association with proteins or lipides — 

 or to a functional inhibition (chemical protection), which diverts the 

 energy absorbed by chlorophyll to other reactions and prevents it from 

 being used for self-oxidation. 



The protective effect of proteins (and lipides) on chlorophyll is well 

 known from experiments with chlorophyll colloids; it accounts for the 

 continued — although reduced — stability of chlorophyll in " chloroplastin " 

 preparations obtained by the grinding of leaves under water. The role 

 of chemical inhibition in the stability of chlorophyll in vivo is shown by 

 the fact that bleaching is accelerated by all factors which inhibit photo- 

 synthesis, e. g., excess oxygen, carbon dioxide starvation, and poisons, 

 although the association of chlorophyll with proteins and hpides is not 

 likely to be disturbed by these treatments. To the absence of this inhi- 

 bition we may attribute the fact that " chloroplastin " suspensions or 

 other colloidal complexes in vitro, are not quite as photostable as is 

 chlorophyll in vivo. 



