By Ellen Reardon and Carl Price, first published in Let’s Talk Plants! October 2008, No. 169. Reprinted October 2022.
Are Your Leaves Turning Red?
In Southern California we enjoy two seasons a year: Wet and Dry, but those of us who have lived in northern latitudes look forward to the spectacular beauty of leaves turning red in the fall. Maples are most notable for their fall colors, but the leaves of some oaks, some sweetgum, and dogwood also turn red, and poison ivy is among the earliest harbingers of fall.
Several questions arise: What are the red pigments of fall leaves? What makes the leaves turn red? And do they have a function other than to delight the human eye?
The red pigments of fall leaves are anthocyanins, a family of red, orange, and blue pigments that are also prominent in flowers and fruits. Anthocyanins are water soluble; they are synthesized in the cytoplasm of the cell and are then transported to the vacuole. Their location also becomes important when we consider function. Anthocyanins are not the only red pigments of plants—most carotenoids are yellow or orange (as in carrots), but some are red.
What makes leaves turn red in the fall?
The average minimum January temperature is 49.7° in San Diego, so that most plants can get along perfectly well through our winters. In most of our country, however, where freezing temperatures are the rule through much of the winter, plants must devise special measures to survive. The strategy of deciduous plants is not simply to shed their leaves but to harvest the valuable nutrients in the leaves and store those nutrients in their stems to fuel the growth of new leaves in the spring. As the days become shorter (or, more specifically, the
nights become longer), the plant’s biological clock sounds an alarm that sets in motion a series of events that tell the leaves to senesce (shut down) and ultimately to abscise (fall off).
Role of anthocyanins in the harvest of leaf nitrogen
Chloroplasts, and especially the chlorophyll in chloroplasts, represent a significant treasure of the nitrogen that will be needed next year. Specifically, 90 percent of the nitrogen that is harvested from leaves and transported to the stems comes from the chloroplasts. But before the nitrogen in chlorophyll can be harvested, it must be cut loose from the membrane proteins to which it has been bound and then enzymatically degraded.
Problem! If free, unbound chlorophyll is exposed to light it will produce singlet oxygen, a form of oxygen that is extremely toxic to senescing leaf cells. In other words, the leaf cells could be killed before they were able to harvest their valuable nutrients and transport them to the stem.
Enter anthocyanins! One of the events triggered by the plant’s night-length-sensitive clock is (for some plants) the production of large amounts of red anthocyanins that are stored in the vacuoles of cells on the upper side of the leaf. These anthocyanins absorb light in the blue and green wavelengths, shading the recently freed chlorophyll from excessive sunlight! What the leaves are saying is . . .
“Never mind whether we can enjoy the highest level of photosynthesis; just keep us from dying!”