When hiking in winter, one of the mysteries I’ve noted for years is the purple hue of winter Prickly Pear, Opuntia spp. I’m an old biologist who specialized in botany way back in college, so I had some suspicions. And like that loose tooth that niggles one to action, seeing another purple cactus last week finally made me do some digging on the color change.
Most of us have heard the reason for fall leaf color in deciduous trees like our local Aspen, Populous tremuloides. A specialized layer of cells (the abscission layer) grows to isolate the leaf from the tree’s circulatory system and makes a sort of perforated line at the base of the leaf stem. That’s why fall leaves break off cleanly with the stem attached. As the chlorophyll in the leaf degrades, the green color disappears and unmasks the colors underneath–in the case of Aspen, their glorious golden hue.
Lots of color molecules exist in plants. Carotenes give the yellows and oranges of carrot roots and fall Aspen leaves. Anthocyanenes give reds, blues and purples to many fruits like apples and blueberries, and the fall leaves of maples and wild geranium. These “accessory pigments” are present all season as an alternate way for plants to utilize sunlight not captured by chlorophyll. And chlorophyll, while a very efficient molecule for the capture of sunlight energy, is not very chemically stable and breaks down within days. Healthy, growing plants continually replenish their chlorophyll much as we routinely make new blood cells. Anything which inhibits chlorophyll production shows quickly as a loss of “greenness”.
Which brings us back around to Prickly Pears. During the growing season, these cactus with formidable thorns are a healthy green. But come New Mexico’s winter with its sunny days and cold nights, two things change for Prickly Pear. The plant accumulates more sugars in its paddles rather than starches, and uptake of the essential element phosphorus is impeded. These two things inhibit fresh chlorophyll manufacture within the cactus. Depending on the amount of inhibition, the green may fade slightly or disappear almost entirely, revealing the underlying lavender hue.
No long-term harm is done, however, so with the return of warmer temperatures the Prickly Pears once again become green and grow!