The saguaro pictured above is dying. I was curious about where the colors were coming from. I learned the green color in the photo above is, of course, from chlorophyll; the orange is carotenoids and the purple in the rainbow is betacyanins.
Plants live and die like all living organisms. Some die of old age, but some die when they are young, like this saguaro. A rainbow of colors is revealed as the cactus dies. This is a similar process to what happens to the deciduous tree leaves that change color with the coming of winter, but I wondered what pigments are found in cacti.
The green is chlorophyll, present in the stem for photosynthesis. Photosynthesis is the process which converts light energy into chemical energy that is used to fuel the plant’s activities. As the chlorophyll fades away, the other pigments are revealed.
Associated with the chlorophyll are the yellow to orange carotenoids. As the green chlorophyll degrades, the carotenoids become visible. These pigments act as photoprotective agents as well as additional light-harvesting pigments, enhancing the light collection for photosynthesis.
The nice purplish colors seen here are the betacyanins, reddish to violet betalain pigments. These pigments may act as a screen, protecting the plant tissue, and may also serve an an antioxidant. Fungicidal properties have also been suggested. Betalains are most obvious in the cactus flowers and fruits, such as the yellow-orange betaxanthins and red-violet betacyanins.
So, the rainbow-in-death colors seen in this saguaro are produced by the pigments revealed as the green chlorophyll is reduced.
Book Note: If you’d like to learn about another fascinating Sonoran Desert plant, I wrote Queen of the Night: Night-Blooming Cereus, about the beautiful plants that bloom once a year, all on the same night, usually in July. The blooming is a big deal here in Southern Arizona and something fun and mysterious to learn about. I love to write fun science books (I believe the learning stays when it’s fun), so I wrote Cereus in rhyme. Check it out here.