314 THE CHLOROPHYLLS AND THE CAROTINOIDS 



chlorophyll even when illuminated under conditions otherwise favorable for 

 chlorophyll formation. This is in accord with the concept that the chemical 

 transformations leading to the formation of chlorophyll from its precursors 

 involve oxidation processes. 



4. Carbohydrates. — Etiolated leaves which have been depleted of soluble 

 carbohydrates fail to turn green even when all of the other conditions to 

 which they are exposed favor chlorophyll synthesis. When such leaves are 

 floated on a sugar solution, sugar is absorbed and chlorophyll formation occurs 

 rapidly. A supply of carbohydrate foods is therefore essential for the forma- 

 tion of chlorophyll. 



5. Nitrogen. — Since nitrogen is a part of the chlorophyll molecule it is 

 not surprising to find that a deficiency of this element in the plant retards 

 chlorophyll formation. Failure of chlorophyll to develop is one of the com- 

 monly recognized symptoms of nitrogen deficiency in plants. 



6. Magnesiuin. — Like nitrogen this element is also a part of the chlorophyll 

 molecule. Deficiency of magnesium in plants results in the development of a 

 characteristic mottled chlorosis of the older leaves (Chap. XXV). 



y. Iron. — In the absence of iron in an available form green plants are 

 unable to synthesize chlorophyll and the leaves soon becom.e blanched or yellow 

 in color (Chap. XXV). While not a constituent of the chlorophyll molecule, 

 iron is essential for its synthesis. The essential role of this element is believed 

 by some investigators to be an accelerating effect upon the photo-oxidation of 

 protochlorophyll to chlorophyll. 



8. Manganese. — This element, like iron, seems in some way to be essential 

 in the production of chlorophyll. Li the absence of manganese a characteristic 

 mottled chlorosis develops in the younger leaves (Chap. XXV). 



9. Suitable Temperature. — According to Lubimenko and Hubbenet 

 (1932) chlorophyll synthesis in etiolated wheat plants takes place only within 

 a temperature range of about 3 to 48° C. The maximum rate of chlorophyll 

 formation lies between 26 and 30° C. Temperature affects the rate of chloro- 

 phyll synthesis because of its controlling influence upon the rate of transforma- 

 tion of some of the precursors of chlorophyll. The final transformation to 

 chlorophyll from its immediate precursor apparently is a purely photochemical 

 reaction which is uninfluenced by temperature. 



10. Water. — Desiccation of leaf tissues not only inhibits synthesis of 

 chlorophyll, but seems to accelerate disintegration of the chlorophyll already 

 present. A familiar example of this effect is the browning of grass during 

 droughts. 



The mechanism of chlorophyll synthesis is very sensitive to any type of 

 physiological disturbance within the plant. Many other conditions besides 



