The Fall Rainbow: Purple

My mom has a Norway maple in her yard that I’d call purple. Or at least purplish, as it’s not so much a purple as it is a reddish brown. Maybe it’s a brownish mauve or a purplish brown (brown being a blend of green and red). Whatever the color may be, it stands out against the green backdrop throughout the summer months. It’s reminiscent of the purples on the spring leaves of poison ivy and the fall leaves of white ash. Here we’ll look at the magic secret behind the Royal Red’s (an ornamental variety of Norway maple) characteristic color and the reasons behind the purple hues on leaves. 

One of the unique features of a plant cell is the giant multipurpose bag within the cell wall called a central vacuole. The membrane-bound sac is large (it can take up 30-90% of the volume of the plant cell) and performs a range of functions, primarily providing structure to the plant body when the organelle swells with water (plants wilt as central vacuoles lose water). It can also store sugars (e.g. sap), salts, metabolic waste, and defensive compounds (e.g. tannins). And in a pinch the central vacuole can suck up, break down, and recycle chloroplasts and mitochondria.

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.

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.

Anthocyanins can also support the leaf’s growth by blocking harmful light from the sun. Here, anthocyanin production is a direct response to sunlight. A string of warm, sunny days in the fall will yield more striking fall foliage, while cooler cloudy days mute anthocyanin production. You can see the effects of sunlight on a single leaf. Take the two images of ash leaves below: the parts of the leaflets that were shaded by other leaflets are bright green while the rest of the leaf is a deep purple brown. This also happens in the spring, particularly with hepatica. You can pull back the deep purple leaves to reveal a layer of bright green leaves below. And the same is true throughout the year on a range of twigs, as in the mountain maple at the bottom or raspberry canes. While anthocyanins reduces the risk of photo-oxidative damage to leaves, this comes at a cost as anthocyanins reduce photosynthetic production of sugars. It is more important, particularly in the fall and spring when the rate of photosynthesis is significantly lower, to protect the leaves from the sun while they construct chlorophyll in the spring or deconstruct chlorophyll in the fall.