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 abo ut changes in resistance associated with the transition from embryonic tiss ue to autotrophic tissue in seedlings. We studied the consequences of trans itions from cotyledons to juvenile true leaves to adult true leaves in cott on seedlings (Gossypium hirsutum) for their resistance to spider mites (Tet ranychus urticae). Mite populations grew much more rapidly on cotyledons than on true leaves. However, there was no detectable difference in the population growth of mit es on juvenile vs. adult true leaves. We suggest that population growth of mites is positively affected by the high rates of photosynthesis of cotyled ons relative to true leaves, or by some process or attribute correlated wit h photosynthesis. Conditions that caused increased rates of photosynthesis (exposure to light and elevated concentrations of CO2) caused mite populati ons to increase. Greater mite population growth on cotyledons was not assoc iated with stored reserves in the cotyledons, as the mites did poorly on co tyledons 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 he rbivory and so critical to understanding plant population dynamics, a broad er consideration of phase changes associated with seedlings is warranted.