Milkweed leaf photosynthesis responses to insect herbivory: Factors that influence photosynthetic rate impairment of injured leaves

Common milkweed, Asclepias syriaca (Asclepiadaceae) is a native prairie weed to Nebraska. The first key finding from my studies was a seasonal pattern in the likelihood of A. syriaca leaf photosynthetic rate impairment after defoliation. After leaf defoliation, late season maturing seed-pod stems infrequently experienced photosynthetic impairment, post-flowering A. syriaca stems before seed-pod appearance always experienced photosynthetic impairment that was usually severe, and earlier season pre-flowering and flowering stems usually had photosynthetic impairment that was moderately severe. Photosynthetic rate had a negative relationship with % leaf area removal, the relationship was a continuous (no threshold) response, and was significantly more negative for insect than manual defoliation. Comparisons between herbivores showed no differences except when different amounts or types of tissues were removed. Photosynthetic impairment lasted at least several days after defoliation, impairment only occurred on injured leaves, and some uninjured leaves adjacent to injured leaves had delayed photosynthetic senescence. Photosynthesis was the most sensitive gas exchange parameter to impairment by leaf defoliation, and the most basic causes of photosynthetic impairment were non-stomatal, both with light and dark photosynthetic reactions. Several studies with A. syriaca, other milkweeds, and non-milkweed plants, showed that leaves always had photosynthetic impairment after midrib injury that was a qualitative response. Impairment severity was: midrib injury ≥ insect herbivory ≥ manual defoliation. Some study species had a small amount of variation in the severity of photosynthetic impairment. The position of midrib injury was important because only tissue down-stream (distal) from the injury site had photosynthetic impairment. Also, impairment lasted at least 28 days with only partial recovery. Finally, one experiment showed that the release of A. syriaca stems from interspecific plant interference interacted with water supplementation to affect leaf photosynthesis and photosynthetic tolerance to 50% manual defoliation, but not midrib injury. This study showed that abiotic and biotic variation affected plant photosynthetic response. In general my results showed that studying plant photosynthetic response was a useful assay for studying plant response to stress (e.g., defoliation)