The ecological significance of rapid wound-induced changes in plants: insect grazing and plant competition

This paper investigates the hypothesis that a rapidly induced phytochemical response to grazing damage, such as that seen in tomato, serves to deflect insect herbivores away from leaves soon after damaging them (the grazing dispersal hypothesis). As a result, grazing damage is more dispersed than it otherwise would be, and young leaves, which may be of particular importance to a plant in competition for light, are not damaged excessively. In the first experiment, artificial removal of c. 15% of leaf area led to a significant reduction in plant performance compared with undamaged controls, but only when the plants were grown together in competition for light. The second experiment demonstrated that the distribution of grazing damage within the plant was an important factor in the outcome of competition; in those plants in which grazing was applied to the lower leaves there was no effect of damage upon performance compared with undamaged controls, whereas grazing to the upper leaves significantly reduced plant performance. A third experiment provided some insight into how this interaction between damage and competition comes about. It was shown that damage to leaves led to a rapid drop in the rate of extension growth of the main shoot, especially when the upper leaves were damaged, and normal rates of growth were not resumed for at least 3 days. It is argued that in a rapidly growing canopy, such an effect may mean that a damaged plant loses its position in the height hierarchy. The final experiment showed that previous damage to plants can affect the distribution of subsequent grazing by larvae of Spodoptera littoralis, apparently through a wound-induced reduction in leaf palatability. Plants which had been artificially damaged 48 h previously were grazed significantly less than controls, and the avoidance effect was greatest in the young leaves. These results are consistent with the grazing dispersal hypothesis, and suggest that rapid wound-induced responses may be of greatest significance in species characteristic of fertile environments where competition for light is particularly intense.