424 RADIATION BIOLOGY 



of this review. As an introduction to this phase of the subject, the reader 

 is referred to Bear and Coleman (1949), Pirson (1948), Pirschle (1938, 

 1939), Mandels (1943), and Aronoff (1950). 



Organic Nutrition. Carbohydrates are essential to the accumulation 

 of chlorophyll, according to Palladin (1918). Some etiolated leaves, e.g., 

 wheat, are well supplied with carbohydrate, whereas others, e.g., bean 

 and lupine, are not. When floated on water and illuminated, leaves of 

 the former type become green, whereas those of the latter type remain 

 yellow. If, however, carbohydrate-poor leaves are floated on sugar solu- 

 tions and illuminated, they too become green. The concentration of the 

 sugar solution plays a role: too concentrated solutions are inhibitory to 

 greening (see later). Palladine (1897), by the floating-leaf technique, 

 found several substances to favor chlorophyll formation: sucrose, raffin- 

 ose, glucose, fructose, maltose, glycerin, galactose, lactose, and dextrin; 

 other substances were found to be without effect: inulin and tyrosine; 

 and still other substances completely inhibited the formation of chloro- 

 phyll: mannitol, dulcitol, asparagine, urea, and alcohol. 



Zaitseva (1940) found that removal of the endosperm from germinated 

 wheat seedlings greatly reduced the amount of chlorophyll formed in the 

 illuminated seedling unless sucrose was administered to it. (For the 

 effect of nutrients on the formation of chlorophyll in the dark, see Sect. 7.) 



Inhibition of Chlorophyll Formation. Inhibitors to the formation of 

 chlorophyll have been studied in order to get information concerning the 

 mechanism of the process. The specific effects of several inorganic sub- 

 stances have been determined. 



Boehm (1859) found that etiolated plants, when illuminated in an 

 atmosphere containing ozone, became bleached. Yocum (1946) observed 

 that 0.001 M cyanide stopped chlorophyll accumulation but did not 

 inhibit the transformation of protochlorophyll to chlorophyll in dark- 

 grown bean seedlings. Carbon monoxide-oxygen mixtures (95 per cent 

 CO, 5 per cent O2) reduced chlorophyll formation to about 50 per cent 

 of the values obtained in a nitrogen-oxygen mixture of the same oxygen 

 concentration when etiolated bean leaves were illuminated with red light. 

 But when they were illuminated with blue light, the carbon monoxide 

 inhibition was completely reversed. Respiration of the leaves followed 

 much the same pattern. From these experiments Yocum concluded that 

 "Since protochlorophyll formation is dependent on the functioning of a 

 respiration system which shows a reversible cyanide and a light reversible 

 CO inhibition it is suggested that cytochrome oxidase is essential for 

 protochlorophyll synthesis." According to Euler (1949), seeds soaked in 

 cobalt nitrate solutions, 0.004 M, failed to produce seedlings that greened. 

 He attributed the effect to inactivation of the phosphatases in the 

 mitochondria. 



The action of organic vapors has been studied by various workers. 



