After insect leaf defoliation injury, the remaining injured leaf tissue of many plant species either experiences no change in photosynthetic rate, or experiences an increase in photosynthetic rate. A less commonly reported phenomenon is for the remaining injured leaf tissue to experience a reduced photosynthetic rate relative to uninjured leaves. With common milkweed leaves, Asclepias syriaca, we have sought to determine why photosynthetic rate impairment occurs following insect defoliation injury. We have found a seasonal pattern for when photosynthetic rate impairment of injured A. syriaca leaves occurs, based on data collected over 9 summers. In early season pre-flowering and in late season post seed-set plants, photosynthetic rate impairment following defoliation tends not to occur. However, in mid-season plants (flowering to seed-set), photosynthetic rate impairment tends to occur following leaf defoliation. Although artificially simulated injury often results in photosynthetic rate impairment, sometimes only actual insect defoliation (several Lepidopteran species: Euchaetes egle, Danaus plexippus, Cycnia inopinatus, Estigmene acrea) of the same leaf area results in rate impairment. This finding suggests that the pattern and timing of injury also affect whether photosynthetic rate impairment following injury will occur. Seasonal photosynthetic rate impairment also occurs in swamp and butterfly milkweed (A. incarnata and A. tuberosa), honeyvine milkweed (Cynanchum leave) and hemp dogbane (Apocynum cannabinum); however, the patterns are not identical among these milkweeds. Our work indicates that a single study on photosynthetic rate response after leaf defoliation may not be sufficient for understanding how a given plant species photosynthetically responds to leaf injury.
Species 1: Gentianales Asclepiadaceae Asclepias syriaca (common milkweed)
Species 2: Lepidoptera Arctiidae Euchaetes egle (milkweed tiger moth, milkweed tussock moth)
Species 3: Gentianales Apocynaceae Apocynum cannabinum (hemp dogbane, indian dogbane)
Keywords: defoliation, photosynthesis
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