432 RADIATION BIOLOGY 



is derived is not known, but it is unlikely that it comes directly from 

 water, since no oxygen is evolved in the transformation (Smith, 1950-1951). 



In the angiosperms the transformation of protochlorophyll is controlled 

 by photochemical action. This is demonstrated by the low temperature 

 coefficient for the photochemical transformation, by the proportionality 

 of the transformation to the light intensity, and by the nature of the 

 action spectrum. 



Whether the photochemical process is also enzymatically controlled is 

 not known. The photochemical transformation does not take place in 

 organic solvent extracts-or protochlorophyll, in etiolated leaves killed by 

 hot-water immersion, or in a puree of fresh etiolated leaves (Scharfnagel, 

 1931 ; Noack, 1934). It is reported to take place, however, in frozen etio- 

 lated leaves and in dried leaves (Liro, 1908) — about which there is some 

 doubt (Eyster, 1928; Scharfnagel, 1931) — and in etiolated leaves treated 

 with cyanide. These observations are ambiguous concerning the direct 

 involvement of enzymes in the photochemical reaction, but they suggest 

 that the photochemical reaction can occur as long as the holochrome is 

 intact. 



The demonstration that under favorable temperature conditions leaves 

 repeatedly form protochlorophyll and transform it to chlorophyll when 

 placed alternately in the dark and in the light brings convincing evidence 

 that protochlorophyll is the precursor of chlorophyll in the over-all proc- 

 ess of greening. This effect substantiates the assumption that the green- 

 ing process is the result of the continuous thermochemical formation of 

 protochlorophyll and its photochemical transformation to chlorophyll. 



The thermochemical reaction is undoubtedly controlled by enzymic 

 processes, because the over-all greening is hindered by unfavorable tem- 

 peratures both low and high and by the action of enzyme poisons. 



In the greening of higher plants both chlorophylls a and b accumulate. 

 It has been proposed that chlorophyll b is derived from chlorophyll a 

 (Rudolph, 1934). Since chlorophyll b appears subsequently to chloro- 

 phyll a in the course of chlorophyll accumulation, this assumption is 

 plausible. However, once chlorophyll b is formed, the two chlorophylls 

 accumulate in direct proportion to each other, and the ratio of the rates 

 of formation is little affected by temperature or by intermittency of illumi- 

 nation (Fig. 7-18). If chlorophyll b were formed from chlorophyll a by 

 thermochemical action, its proportion should be greatly increased by the 

 use of intermittent illumination; if it were formed from chlorophyll a by 

 photochemical action, it should increase at a rate proportional to the con- 

 centration of chlorophyll a; and if it were in rapid, mobile equilibrium 

 with chlorophyll a, it should be formed at an earlier stage during inter- 

 mittent illumination than during continuous illumination. None of these 

 propositions is true. Therefore it is more reasonable to assume that 

 chlorophylls a and b are formed concurrently from some common pre- 



