Robert B. Withrow 399 



zinc, and copper, appear to participate in certain of the enzymatic steps 

 of chlorophyll synthesis. Phosphorus may very likely enter into energy 

 exchanges associated with the synthesis. The role of the other elements 

 in causing chlorosis is obscure. 



In higher plants, chlorophyll synthesis is a photochemical reaction 

 which proceeds at its maximum rate under relatively low light intensi- 

 ties. It appears to be a photocatalytic type of reaction in which the light 

 energy is absorbed by a pigment which presumably is protochlorophyll. 

 Protochlorophyll is present in the dark-grown leaf in relatively low 

 concentrations as compared with the ultimate maximum concentration 

 attained for chlorophyll in the green leaf (47). During the synthesis of 

 chlorophyll, therefore, there must be a concomitant synthesis of proto- 

 chlorophyll. The synthesis of protochlorophyll appears to be a thermal 

 reaction and not directly dependent upon light. In the algae, such as 

 Chlorella, the synthesis of chlorophyll can be thermally activated and 

 will proceed in the dark in the presence of sugars. Both protochlorophyll 

 and chlorophyll are magnesium porphyrins which may be extracted 

 with ether and other lipoid solvents. 



J. H. C. Smith (48) has recently initiated a program of study of 

 chlorophyll synthesis and has redetermined the absorption spectrum 

 of protochlorophyll. It contains two strong maxima which, in ether 

 extracts, appear at 623 and 432 mu, with the blue maximum much 

 stronger than the red. There is appreciable absorption, however, 

 throughout the ultraviolet and visible range from 250 to 650 mu. The 

 maxima coincide quite closely with the position of the action spectrum 

 maxima of chlorophyll synthesis determined with light filters by S. 

 Frank (//). 



Smith (48) reported that, in the synthesis of chlorophyll, there is a 

 concomitant decrease in protochlorophyll concentration and an increase 

 in ether-soluble magnesium and phosphorus, the time-course patterns 

 of which coincide closely with the appearance of chlorophyll. In the 

 excised leaf, detached from the root system, the total ash, including 

 magnesium and phosphorus, is relatively unchanged, but there is a 

 large increase in ether-soluble fractions of both these elements. For 

 leaves on an intact plant, however, there is an increase in ash corre- 

 sponding with increases in lipoidal magnesium and phosphorus (Figure 



