Another class of pigments in tree leaves are anthocyanins. Anthocyanins absorb blue and green light. Light reflected from leaves containing anthocyanins is red in color.
Anthocyanins are dissolved in the cell sap of the tree leaves. The color of anthocyanin pigments depends on the pH of cell sap — redder when cell sap is very acidic and purple when it is less acidic. Chemical reactions between sugars, proteins, and light in the cell sap create anthocyanins. These reactions do not occur until the sugar concentration in the sap is high.
Anthocyanins are responsible for the red skin of ripe apples and the purples of ripe grapes. The side of an apple that faces the sun is red due to sunlight aiding the reactions between sugars and proteins needed to create anthocyanins.
Aspen trees that can turn red do not turn red in all years. It turns out that both genes and seasonal conditions determine whether an aspen tree will turn red. Seasonal conditions that promote the most vibrant red aspens include fall days with warm sunny days and cool nights above freezing.
Scientists hypothesize that red anthocyanin acts as a sunscreen for tree leaves. When photosynthesis stops, the “sunscreen” protects the tree leaves from drying out and allows additional time to absorb leaf nutrients (anthocyanins are associated with excess sugars in the cell sap).
Although most anthocyanin production occurs in the fall, it can be observed in the early spring when a tree turns red first then changes to yellow when the days get longer and chlorophyll production and photosynthesis kicks in.