BIOSYNTHESIS OF CHLOROPHYLL) THE PROTOCHLOROPHYLL 1761 



leaves). The concentration of organic magnesium reached saturation 

 after about 40 hours' exposure, after which the amount of chlorophyll began 

 to decline slightly, probably because of photoxidation. 



These experiments, particularly that at 7° C, clearly pointed to a two- 

 stage process of chlorophyll formation — a fast photochemical reaction and a 

 relatively slow thermal reaction, brought almost to a standstill at 0° C. 



The formation of ether-soluble phosphorus compounds was found to 

 follow a course parallel to that of the formation of ether-soluble magnesium 

 compounds. 



The isolation of sizable quantities of pure protochlorophyll by Koski 

 and Smith (1948^), and the absolute determination of its absorption co- 

 efficients (c/. fig. 21.8 in Vol. 11,1, page 618) made it possible to correlate 

 quantitatively the formation of chlorophyll with the disappearance of 

 protochlorophyll. 



Smith (1948) made the first such comparison by determining the 

 amount of chlorophyll formed in the first two hours of illumination at 

 0° C. and the amount of protochlorophyll present at the beginning of the 

 e.xposure. He found an approximate proportionality between the two 

 magnitudes, with the proportionality constant <1. In fact, the amount 

 of synthesized chlorophyll corresponded, under these conditions, to little 

 more than one-half of the available protochlorophyll. 



A more precise study was carried out by Koski (1950), by restricting 

 the measurements to the first few minutes of illumination. This allowed 

 working at room temperature, without the slow thermal reactions inter- 

 fering significantly with the fast photochemical transformation. 



Corn seedlings were used. They were germinated for 15 days in the 

 dark and then exposed to fluorescent light (150 foot-candles) at room tem- 

 perature. Protochlorophyll, chlorophyll a and chlorophyll h were deter- 

 mined spectrophotometrically at different times after the beginning of 

 illumination, using the following specific absorption coefficients: 



" Contrary to the observations of Seybold and Egle on pumpkin seeds (Vol. I, p. 

 404 and Vol. II. 1, p. 619), no evidence of the presence of a protociilorophyll b was 

 obtained by Smith and Koski. 



Fig. 37. Bl shows the rapid formation of chlorophyll a in the first minute 

 of exposure, and an exactly equivalent decrease in protochlorophyll con- 

 tent. After the first minute, the amount of chlorophyll formed begins 

 to trail somewhat, but not significantly, behind the amount of proto- 



