Figure 9.12 Estimated net carbon flux for normal leaves and leaves with simulated gall disease at warm temperatures (38°C). A normal leaf in this environment functions at 90 percent of optimum, and the warmer leaves with galls function at a slightly lower level.
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Something like this may be happening in leaf galls. Plants normally have vigorous defenses against animals that want to eat their leaves, and any herbivore that is too greedy will end up seeing its stream of energy dry up. Plants defend themselves through the so-called injury response. After the initial bite, the plant induces an initial proliferation of cells at the site of injury, called a hyperplasia. This is followed by lignification, essentially the formation of a scar that isolates the site of injury from the rest of the plant. The scar also separates the herbivore from the goodies left behind it. Generally, the more vigorous the attack, the more intense the injury response will be.
Gall inducers probably got their evolutionary start by figuring out, so to speak, how to intervene in this injury response. At its simplest, the intervention involves a simple prolongation of the hyperplastic phase. Among other things, encouraging hyperplasia provides the gall inducer with a steady supply of juicy, succulent plant cells to eat. This bonanza doesn't come free, however. Cell division and cell growth require energy. In a normal leaf, mobilizing this extra energy is an integral part of the injury response;the extra energy must come from uninjured parts of the leaf itself or from other, uninjured leaves on the tree.
By subtly altering the leaf's temperature, gall insects may be manipulating the signals plants use to manage their leaves' energy economy. The signals could be changed in a number of ways (Fig. 9.13). Consider, for example, an ungalled leaf that is operating at a temperature cooler than the optimum. Adding galls to that leaf increases its temperature slightly and, as we have seen, should increase the leaf's net photosynthesis. Net photosynthesis increases in this case because the increase in temperature accelerates photosynthesis proportionally more than it accelerates the leaf's metabolic rate. This results in an excess of sugar—an energy
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