SYNTHESIS OF ANTHOCYANINS IN PLANTS 383 



of them change in color through violet to blue as the H-ion concentration of 

 the medium decreases. Red pigmentation due to anthocyanins is frequently 

 found in flowers, fruits, bud scales, developing leaves, and less commonly 

 in stems, mature leaves (red cabbage, copper beech, red coleus, etc.) and 

 other plant parts. The reds and purplish reds developed in autumn foliage 

 are also due to anthocyanins. Blue and purple pigmentation due to the 

 presence of anthocyanins is restricted largely to flowers and fruits. 



Factors Influencing the Synthesis of Anthocyanins in Plants. — Al- 

 though anthocyanin pigments have been extensively studied in the laboratory 

 from the chemical standpoint, very little is known of their actual synthesis 

 in plants. Certain factors have been recognized, however, which are usually 

 correlated with anthocyanin production in plants. 



1. Genetic Constitution. — The potentiality of producing anthocyanins is 

 controlled by hereditary factors. The red maple, for example, produces an- 

 thocyanins in its flowers, young stems, and developing leaves, while the leaves 

 turn a brilliant red just before leaf fall in the autumn. On the other hand, 

 the sugar maple, a closely related species, produces anthocyanins less abun- 

 dantly, while the black maple, a variety of the sugar maple, only rarely pro- 

 duces anthocyanins in readily observable quantities. Similarly different varie- 

 ties of the same species may differ in the hereditary potentialities for the pro- 

 duction of anthocyanins in the flower parts. Extensive studies of the genetics 

 of anthocyanin flower color production have been made in some species. 



2. Accumulation of Soluble Carbohydrates. — One metabolic condition 

 which seems to be universally correlated with anthocyanin formation is the 

 presence of a relatively high concentration of simple sugars in the cells. 

 The presence of large amounts of these substances does not necessarily pre- 

 dispose the tissues to anthocyanin formation, but this apparently is a necessary 

 prerequisite. The frequently observed anthocyanin formation in the develop- 

 ing leaves and shoots of many species is correlated with their high sugar 

 content at that time. An injury of stems or leaves which interferes with the 

 translocation of foods from those organs often favors anthocyanin produc- 

 tion in such parts. Such injuries may be mechanical in origin, or may be 

 due to insect injuries or fungous infections. Similarly the leaves of many 

 species contain more sugar in the autumn at the time they turn red than 

 earlier in the season. 



3. Temperature. — Lowering of the temperature often favors anthocyanin 

 formation. It seems probable that the effect of this factor on the synthesis 

 of anthocyanins is largely indirect. Reduction in temperature nearly to the 

 freezing point is known to favor the transformation of insoluble carbohy- 

 drates into soluble forms as described later. Reduced temperatures also 



