Why do leaves really track the sun?
This week I'll discuss one of my own papers, "Individual fitness versus whole-crop photosynthesis -- solar tracking tradeoffs in alfalfa", which was recently published in the Evolutionary Applications special issue on Agriculture.
The alfalfa leaf at the right is brightly illuminated because it is facing directly towards the sun, an orientation it maintains by turning slowly over the day. By tracking the sun, this leaf captures more sunlight, so it might be expected to photosynthesize more. On the other hand, the leaf is partly shaded by another leaf, which casts a bigger shadow because it, too, is tracking the sun. This increased shading of lower leaves by upper leaves would tend to reduce overall photosynthesis.
Does increased shading outweigh the photosynthetic benefits of tracking?
Jim Fedders, Barry Harter, and I decided to find out. We measured photosynthesis of a model community of alfalfa plants tracking the sun naturally, then turned them 90 degrees, to temporarily disrupt solar tracking.
(That's me at the left, but these experiments were done 20 years ago, when I was a USDA scientist in West Virginia. By now, I probably look more like my father, William Denison, who was visiting the day the picture was taken. The cloudless days needed for the turning experiment were rare, so I made him come to work and help with the experiment. He was a botanist/mycologist, known for his 1973 Scientific American article, "Life in Tall Trees", so I don't think he minded.)
Here are the results in which I have the most confidence. With a sparse leaf canopy, photosynthesis decreased about 2% when we disrupted solar tracking. (Leaf area index = 2 means there are 2 square centimeters of leaf per square centimeter of ground. Relative photosynthesis of 1.02 means photosynthesis with normal solar tracking is 1.02 times that with tracking disrupted by turning.) Beyond leaf area 4, however, photosynthesis actually increased when we disrupted solar tracking.
Why do leaves track the sun, when doing so decreases photosynthesis? Maybe because the leaves that get shaded are often those of neighboring competitors. We found that tracking reduces light levels near the ground to half what they were without tracking. So seedlings coming up under a solar tracking plant would photosynthesize and grow less, making them less of a competitive threat.
The curved line shows predicted effects of solar tracking on photosynthesis, based on simulations using the computer model, ALFALFA, which I developed around 1984, under the guidance of my postdoctoral mentor, Bob Loomis. The model predicts that solar tracking will have a negative effect on photosynthesis over a wider range of leaf areas than in our experiments. It's only a model, of course. But our artificial plant community is also a model system. One key difference between our small group of plants and an entire field of alfalfa is that lower leaves can get a lot of light from the side, if they're at the edge of a small group of plants. This "edge effect" would tend to reduce the negative effects of tracking on photosynthesis, because shading by upper leaves would have less effect on the total light available to lower leaves. So the computer model might actually be a better predictor of solar tracking effects in the field.
Especially if we believe the computer model, we might be able to increase the photosynthesis and growth of alfalfa by breeding varieties that don't track the sun. They would be less competitive with weeds, though.