BIOSYNTHESIS OF CHLOROPHYLL; THE PROTOCHLOROPHYLL 



1763 



the amount of chlorophyll a begms to rise above the almost stationary- 

 level that was established after the first few minutes of illumination (fig. 

 37. B2). This slow, but steady, increase continues for hours; it must be 

 determined by the rate of replenishment of the pool of chlorophyll pre- 

 cursors by slow thermal reactions. The initial fast pigment formation un- 

 doubtedly is the result of photochemical conversion of pre-existent proto- 

 chlorophyll into chlorophyll. Only the a component is formed in this way. 



400 



700 



500 600 



WAVE LENGTH, m/i 



Fig. 37B.3. Action Bpeetrum of transformation of 

 protochlorophyll to chlorophyll a in etiolated corn 

 leaves: (O) normal, ( + ) albino (Koski, French and 

 Smith 1951). 



The sharp absorption band in the violet, noted by Koski and Smith 

 (1948) in the spectrum of protochlorophyll, has an analogue in the action 

 spectrum of chlorophyll formation, first noted by Frank (1946) in experi- 

 ments on oat seedlings in filtered light. The finding of this band invali- 

 dated a previously used argument against identification of the precursor 

 which produces chlorophyll in light, with protochlorophyll. 



Frank's study left the question of carotenoid contribution to the action 

 spectrum peak in the blue-violet unsettled; an "auto-photocatalysis" 

 by chlorophyll (c/. Vol. I, p. 430) seemed to be indicated, but could not 

 be definitely proved. 



The action spectrum of chlorophyll formation was re- investigated by 



