PLANT PHASE CHANGE AND RESISTANCE TO HERBIVORY

All plants pass through a series of predictable developmental stages during their lives, called phase changes. The phase change from juvenile to adult leaves is known to be associated with changes in resistance against plant pathogens and herbivores in several species. Virtually nothing is known about changes in resistance associated with the transition from embryonic tissue to autotrophic tissue in seedlings. We studied the consequences of transitions from cotyledons to juvenile true leaves to adult true leaves in cotton seedlings (Gossypium hirsutum) for their resistance to spider mites (Tetranychus urticae). Mite populations grew much more rapidly on cotyledons than on true leaves. However, there was no detectable difference in the population growth of mites on juvenile vs. adult true leaves. We suggest that population growth of mites is positively affected by the high rates of photosynthesis of cotyledons relative to true leaves, or by some process or attribute correlated with photosynthesis. Conditions that caused increased rates of photosynthesis (exposure to light and elevated concentrations of CO2) caused mite populations to increase. Greater mite population growth on cotyledons was not associated with stored reserves in the cotyledons, as the mites did poorly on cotyledons kept in the dark. This study indicates that phase changes can have profound effects on plant resistance to herbivores. Because the seedling stage is so vulnerable to herbivory and so critical to understanding plant population dynamics, a broader consideration of phase changes associated with seedlings is warranted.