532 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



the same oxygen concentration but in the absence of carbon dioxide). 

 It is thus impossible to account for the decHne in photosynthesis by 

 assuming that the observed gas exchange is the balance of unchanged 

 photosynthesis and added photautoxidation. Rather, we have to postu- 

 late with Franck and French (1941) that photoxidation inactivates the 

 catalytic mechanism of photosynthesis. 



An effect similar to that of excess oxygen is caused by excess light. 

 The dependence of photosynthesis on light intensity will be the subject 

 of chapter 28 in volume II. The rate increases up to a certain hght 

 intensity — which for plants adapted to direct sunlight is of the order of 

 50,000 lux — and then becomes constant, apparently because one of the 

 enzymes taking part in photosynthesis has a limited capacity and can 

 provide (or utilize) only a certain quantity of intermediates, required for 

 (or supplied by) the primary photochemical process. Photautoxidation 

 has no such limitations, and its rate continues to grow long after photo- 

 synthesis has become "saturated" with light. This explains the phe- 

 nomena known as solarization (dissolution of starch deposits in leaves in 

 intense light, cf. Ursprung 1917), light inhibition and light injury—which. 

 have been known long before their relation to photoxidation became clear. 



The ease with which reversible light inhibition and irreversible light injury can be 

 produced in different plants, depends on their (ontogenetic or phylogenetic) light 

 adaptation. This may be one of the reasons for discrepancies between the findings of 

 different observers. When Reinke discovered the light saturation of photosynthesis in 

 1883, he did not notice any decline in the rate of photosynthesis of aquatic plants until 

 the light intensity was increased to 20 or 50 times that of direct sunlight (that is, to 

 one or two million lux!). Ewart (1896), on the other hand, discovered the inhibition 

 of photosynthesis by excess Ught by exposing the water plant Elodea to direct sunlight. 

 Later, (1897), Ewart observed that the photosynthesis of tropical plants is not inhibited 

 by direct sunUght; still later (1898), he stated that inhibition can be produced in all 

 plants by the use of concentrated sunlight. Light inhibition was again observed by 

 Pantanelli in 1903; but the reality of this phenomenon (which had no place in Blackman's 

 theory of "limiting factors") was doubted by Blackman and Smith (1911), who admitted 

 only the occurrence of light injury in concentrated sunlight, wliich they ascribed to 

 overheating. Gessner (1938) found no inhibition in prolonged experiments with aquatic 

 plants {Elodea and others) in light of 100,000-130,000 lux, even if the near ultraviolet 

 intensity was artificially enhanced by means of a mercury arc in glass. Johansson 

 (1923, 1929) explained the light inhibition by the closure of stomata in strong hght 

 {cf. the theories of the "midday depression" in volume II, chapter 26), while Emerson 

 (1935) concluded, on the basis of experiments with Chlorella in Ught up to 45,000 lux, 

 that light inhibition occurs only if the supply of carbon dioxide is inadequate (thus 

 creating local starvation, with consequences similar to those described on pages 528-531 

 of this chapter). 



Although incidental factors, such as overheating, closure of stomata, 

 or inadequate supply of carbon dioxide, may play an important role in 

 some cases of light inhibition and light injury, it seems certain that 

 these phenomena can occur also when all such factors are eliminated. 



