1428 INDUCTION PHENOMENA CHAP. 33 



essential photosynthetic enzyme, the consequent blanketing of chlorophyll 

 by a "narcotic," and the removal of this narcotic by respiration. 



6. Other Theories of Induction 



It is impossible to discuss here in detail the theories of induction suggested by other 

 authors, particularly since they were mostly invented ad hoc, without relation to the 

 totality of our knowledge of the kinetics of photosynthesis. The most elaborate specula- 

 tions concerning the origins of the induction curves have been presented by Kautsky. 

 They were based exclusively on the observation of fluorescence, in disregard of other as- 

 pects of the induction phenomena, not to speak of general kinetics of photosynthesis in 

 constant or intermittent light. 



In his first papers, Kautsky attributed induction to the interaction of excited chloro- 

 phyll with molecular oxygen (which he considered — cf. chapter 18, page 514 — as the 

 universal energy acceptor in dyestufif-sensitized reactions). After he himself had found 

 that the fluorescence of living plants is insensitive to changes in oxygen concentration 

 between 0.5 and 100%, Kautsky substituted, in the role of energy acceptor, an oxygen- 

 acceptor compound, supposed to dissociate only below 1% O2 in the atmosphere. He 

 attributed the first rise of fluorescence to the transfer of excitation energy from chloro- 

 phyll to this compound (ilO-z), which was assumed to be converted into an activated 

 form, fiOo* (perhaps a peroxide). ^Vhile this form accumulates, the fluorescence rises 

 to a peak (because only nOo, and not UO,*, can act as a chemical quencher). In the 

 second stage, fi02* reacts (thermally) with the substrates of photosynthesis, and is con- 

 verted back into QO2; this leads to the renewed decline of fluorescence. In absence of 

 oxygen, ilO^ is dissociated and the fluorescence wave disappears. 



Kautsky later added various additional assumptions intended to explain the de- 

 tails of the fluorescence curves. An elaborate development of the theory was given by 

 Kautsky and U. Franck (1943), who postulated, in addition to the photochemical acti- 

 vation and thermal deactivation of the first energy acceptor, ^62, a sequence of three 

 photochemical activations and thermal deactivations of another energy acceptor, which 

 they made responsible for those features of the fluorescence curves that do not disappear, 

 but, to the contrary, are accentuated in the absence of oxygen. They thought that the 

 four successive photochemical reactions between excited chlorophyll and the two energy 

 acceptors, which they beUeved detectable in the ups and downs of the fluorescence curves, 

 must correspond to as many primary photochemical processes of photosynthesis, and 

 thus explain why the latter may require 4 quanta. 



This explanation fails to deal with the basic problems. While attributing the varia- 

 tions of fluorescence to accumulation and disappearance of various intermediates of 

 photosynthesis, Kautsky does not even ask why these intermediates accumulate to a 

 maximum and then disappear again (instead of assuming a constant level), or why the 

 first peak is reached after an illumination period so short that only a few per cent of 

 chlorophyll molecules can be excited. 



Later, Kautsky (1951) tried to bring his theory of induction in relation to 

 Warburg and Burk's "new theory" of photosynthesis, which also ascribes to molecular 

 oxygen an active role in the photosynthesis cycle. 



Smith (1937) suggested that the induction curves he obtained with Cabomha, as 

 well as those found by Briggs for Mnium and by van der Paauw for Hormidium, can be 

 explained quantitatively by the assumption that chlorophyll has to be activated photo- 

 chemically, to Chi*, by a reaction the yield of which is proportional to (const. — [Chi* P), 



