384 CARBOHYDRATE METABOLISM 



result in a diminution in the respiration rate, which favors sugar accumula- 

 tion. The accumulation of the soluble carbohydrates in the cells in turn 

 favors anthocyanin formation. 



4. Light. — The effects of light upon anthocyanin synthesis may also be 

 partly indirect, due to the influence of this factor upon photosynthesis. Light 

 also has a direct effect upon the formation of anthocyanins in some plant 

 organs. Autumnal red coloration, for example, usually develops only in 

 leaves which are directly exposed to the light. If one leaf is covered by an- 

 other during the period of anthocyanin synthesis in the autumn the lower 

 leaf will "photograph" the upper clearly as an area devoid of red pigment. 



Anthocyanin formation is favored principally by the shorter visible and 

 the ultraviolet rays. Arthur (1932, 1936) has found that it is possible to 

 color apples picked late in the summer artificially under light sources suppl}'- 

 ing the shorter wave lengths of the visible spectrum or ultraviolet. The 

 most effective portion of the spectrum was found to be the range from 290- 

 313 7)1^, i.e. in the ultraviolet, although all wave lengths up to 600 mfx. re- 

 sulted in considerable coloring. Areas of the skin of the apples covered with 

 opaque paper before being exposed to these light treatments failed to develop 

 any red pigment. 



While in some plant organs light appears to be necessary for anthocyanin 

 formation, and in others probably favors synthesis of these pigments, there 

 are some plants in which anthocyanins are synthesized in the absence of light. 

 The roots of the beet furnish a familiar illustration of this phenomenon. 

 Similarly the 5'oung leaves of both beet and radish will synthesize anthocy- 

 anins, even if the seeds are germinated in the absence of light. 



5. Available Nitrogen. — Reduction in the available nitrogen supply in the 

 soil favors anthocj^anin formation. Under such conditions a smaller pro- 

 portion of the carbohydrates in the plant is utilized in the synthesis of amino 

 acids (Chap. XXVI) ; the consequent relatively high concentration of car- 

 bohydrates favors anthocyanin formation. Anthocyanin formation is a com- 

 mon symptom of nitrogen deficiency in many species of plants. 



6. Drought. — Drought conditions in general favor anthocyanin forma- 

 tion. Deficiency of water, in many species at least, favors the conversion of 

 insoluble carbohydrates into soluble carbohydrates. Furthermore a deficiency 

 of soil water tends to reduce the absorption of nitrates, which also favors the 

 accumulation of carbohydrates. The formation of red pigments during 

 droughts is therefore principally due to indirect effects of soil water deficiency 

 upon the metabolism of the plant. 



7. Oxygen. — Oxygen is apparently necessary for anthocj'anin synthesis, 

 and seems to be chemically combined in this process. This is seldom if ever 



