412 RADIATION BIOLOGY 



pation of a minor pigment, such as protochlorophyll, in the chlorophyll- 

 forming process, even though such a pigment is known to be present in 

 normal green leaves (Smith and Koski, 1947-1948). This arises from 

 several causes: the ettectiveness curve obtained would be seriously dis- 

 torted by the inner-filter action of chlorophyll; the additional chlorophyll 

 formed would be quantitatively insignificant; and the photosynthetic 

 action spectrum of chlorophyll itself would play a dominating role. 



Rate of Formation of Chlorophyll. There is no question but that the 

 first trace of chlorophyll appears in etiolated leaves after only a few 

 seconds' exposure to light of moderate intensity. Monteverde (1893- 

 1894) placed this time at 5 sec in diffuse daylight. Liro (1908) found 

 that chlorophyll was formed in spectroscopically determinable amounts 

 in 5-10 sec by irradiation with diffuse daylight. In some plants 30 sec 

 was sufficient to transform all the " leukophyll " (protochlorophyll) . The 

 time depended on the thickness of the plant part being illuminated. 

 Inman (1935) exposed etiolated corn leaves to radiation from a 100- w 

 Mazda lamp at 8-in. distance and detected chlorophyll spectroscopically 

 after 10 sec and a typical chlorophyll spectrum after 150 min. The 

 present authors (Fig. 7-14; cf. Koski, 1949) found that in corn leaves 

 10 sec of exposure converted 10.3 per cent of the protochlorophyll to 

 chlorophyll at 30 ft-c intensity, 43.8 per cent at 120 ft-c, and 59.3 per 

 cent at 240 ft-c. The transformation of protochlorophyll to chlorophyll 

 in etiolated corn leaves brought about by irradiation with weak mono- 

 chromatic light, wave length 650 m/x, conforms to the second-order 

 reaction-rate law (cf. Table 7-4). 



From these results it is clear that, at moderate light intensities, chloro- 

 phyll can be detected after 5 sec or less of irradiation (cf. Liro, 1908, 

 pp. 28-29). The reaction is a photochemical process in which the photo- 

 chemically active component is used up. 



The rate of accumulation of chlorophyll is a slow reaction as compared 

 with the transformation of protochlorophyll (cf. Figs. 7-12 and 13; see 

 also Liro, 1908, pp. 90 92). After the initial transformation of proto- 

 chlorophyll there is a lag period in which httle additional chlorophyll is 

 formed ; then chlorophyll increases at an accelerated rate suggestive of an 

 autocatalytic process in which the chlorophyll accumulated contributes 

 to its own formation. This probably happens through the accelerated 

 photosynthesis of substances contributory to chlorophyll production. 

 Once this autocatalytic phase is over, the process slows down until a 

 steady state is reached. The steady state is considered by some to be a 

 balance between the production and destruction of chlorophyll (Zava- 

 lishina, 1951). 



Dependence of Chlorophyll Formation on Light hitensity. The minimum 

 intensities of light which bring about chlorophyll formation are very low. 

 Tessier (1783) observed that the light of the moon was sufficient to green 



