Seasonal Patterns of Leaf Photosynthesis after Insect Herbivory on Common Milkweed, Asclepias syriaca: Reflection of a Physiological Cost of Reproduction, not Defense?

—After defoliation occurs leaf photosynthetic rate (Pn) can increase, decrease or remain unchanged relative to control uninjured leaves depending on the plant species. With common milkweed Asclepias syriaca (Asclepiadaceae) we conducted 27 experiments with insect herbivory or mechanical tissue removal to examine whether A. syriaca leaf Pn reductions were correlated with the occurrence of gross cardenolide induction and/or with reproductive phenology. Using spectrophotometry, positive cardenolide induction was detected in only one study when Pn impairment was detected with gas exchange data from injured A. syriaca leaves, while negative or no cardenolide induction was detected in the other five A. syriaca studies with Pn impairment. The occurrence of Pn impairment after partial leaf defoliation did have a seasonal pattern which correlated with A. syriaca reproductive phenology: little or no Pn impairment occurred on leaves of pre-flowering or maturing seed pod plants, while moderate to severe leaf Pn impairment occurred on leaves of flowering and early seed pod formation plants. Our results fail to support either constitutive cardenolide levels or gross cardenolide induction trade-offs to be reflected in injured leaf Pn impairment; however, our results could be explained by a resource or hormone trade-off between investment into reproduction with maintaining leaf photosynthesis after herbivory. Specifically, we suggest that a physiological ‘cost of reproduction’ is increased susceptibility to Pn impairment after herbivory injury on a leaf. Future studies will need to examine whether resource or hormonal regulation trade-offs cause this proposed physiological tradeoff between reproduction and photosynthesis.