But as it concerns us here, that malleable bag can also be a swirling orb of brilliant pigments that paint a cell red, orange, blue, or purple. Most of the pigments that color leaves, petals, and fruits belong to a class of water-soluble flavonoids called anthocyanins (we’ll talk about the oil-soluble carotenoids coming up). They’re primarily responsible for purple and red tones, though their color can change with pH (acidic sap yields reds and orange while alkaline sap yields purples and blues) and in the presence of metal ions.
Anthocyanins as signals to other organisms
Lump a bunch of these colorful cells together into the form of a petal and a plant can attract pollinators, skin a fruit in them and they signal when fruits are ripe, adorn a thorn with them and they highlight the defensive feature to potential herbivores. The key here is that these pigments in the central vacuole signal specific information about the plant to other organisms. Most of the herbivory (60-80%) that happens to leaves takes place in the early spring when the leaves are young. Young leaves put out by plants in the spring often have a red to purplish hue to them, and since these new leaves don’t yet have their chemical defenses up and running, this signals to herbivores that the leaves nutrient deficient. As each leaf ramps up chlorophyll production, they become inedible in other ways (thicker cell walls, thicker waxy cuticle, more chemical defenses, etc.) while anthocyanins get recycled back into the plant cells and the leaves take on a green appearance. In some ornamentals, like the Royal Red or Crimson King Norway maples or the Purple Fountain European beech (see image below), anthocyanin production continues through the summer and mask the green chlorophyll.