Why Some Cacti Turn Purple

The goddamn mystery's been plaguing me for years: Why some prickly pear (genus Opuntia) are redish purple. They never seem to be different species from their neighbors.

Why? What the hell makes 'em like that? Is it some genetic thang, like red hair in humans. No. But now I got an answer, at least a partial.

Using my Google magic, I found this thread on the topic, "cactus turning purple."

The short answer is that some cacti turn purples and reds when they're stressed. That could be from drought, root rot, cold, and all kindsa things. A few species go purply red a bit normally, like Opuntia santarita, but even more when they're stressed.

The longer answer is that plants in the order Cactalus, the cacti, have chemicals in them, betalains. That includes the prickly pears, genus Opuntia. Most plants ain't got 'em.

The betalains act like litmus paper, turnin' toward red when the cactus's juices turn acidic and blueish when they go base. In other words, when stress fucks up a prickly pear's chemical balance, the betalains over-color the green chlorophyll in the pads' cell juices.

So now you know. Cool shit.