As autumn colors blaze, take a good look at the reds. They reflect a sophisticated leaf security system that, after nearly two centuries of scientific speculation, botanists now are beginning to understand.
As green chlorophyll degrades, it unmasks underlying yellow carotenoid pigments, as textbook botany says it should. But reds are something else. There are no preexisting brown-to-red anthocyanin pigments, which give color to the leaves of 70 percent of mid-latitude forest trees. Aging leaves pump up production of these colorful compounds just in time to replace their fading summer greens with flashy autumn hues.
As recently as a decade ago, many botanists would say, "That's nice, but what's in it for the trees?" Botanist David Lee at Florida International University in Miami says there now is good, if not yet definitive, evidence that the anthocyanins protect the leaves' photosynthesis machinery from destructive influences long enough for the system to shut down in an orderly manner.
Dr. Lee and his New Zealand colleague Kevin Gould at the University of Auckland also suspect that this boosts the efficiency with which plants recover the nitrogen they invested in that machinery during the growing system nitrogen that can be reinvested in new leaves next spring.
In other words, the resources that the trees put into ramping up this late-season protection system enable them to recover an even larger capital investment in nitrogen. Lee says they still need more specific evidence to back up that suspicion.
In any event, Lee explains that the fact that anthocyanin production "is a very widespread phenomenon" shows it is important in the plant world. It has been prominent in plant biology for so long that its development represented what he calls "a key juncture" in plant evolution.
Anthocyanins act as a sunscreen to protect leaves when photosynthesis slows down because of cold air temperature or other factors. Too much light at the wrong time can flood the photosynthesis engine with more energy than it can handle.
That extra energy can cause damage, especially if it induces creation of chemicals that attack biological material. Anthocyanins absorb sunlight in ways that curb excess energy production. They also neutralize the dangerous chemicals.
This kind of security service now is well-documented, although Lee says more research is needed to fully understand it. But plants have to spend significant energy and nutritional resources to make the anthocyanins. Why bother to do it with leaves that soon will be lost anyway?
That's where nitrogen recovery comes in. Lee and Dr. Gould think anthocyanin protection boosts the recovery enough to give the plants that use it at least a small evolutionary advantage. It's like spending $20,000 to salvage $22,000 worth of valuables from a derelict building while the wrecking ball swings. In evolution, every advantage, however small, counts.
Lee and Gould hope to prove there is such an advantage. They'll need the help of other researchers and other projects to do it. Lee acknowledges this is a relatively small research field with few active scientists and poor funding. Nevertheless, he expects that significant progress will be made in this decade. Meanwhile, he says, "we have fun."