Author
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Delaney, Kevin |
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Submitted to: Plant Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/19/2011 Publication Date: 2/8/2012 Citation: Delaney, K.J. 2012. Nerium oleander indirect leaf photosynthesis and light harvesting reductions after clipping injury or Spodoptera eridania herbivory: High sensitivity to injury. Plant Science. 185-186: 218-226. Interpretive Summary: We use photosynthetic response curves to insect herbivory intensity (% tissue removal from a single leaf) as a more refined assay of plant physiological tolerance to a stress like injury. We studied N. oleander because it has large constitutive investment in cardiac glycoside defense compounds, so that few herbivores can breach the defenses of this plant. We used clipping injury with two experiments, and actual herbivory with southern armyworm caterpillars in a third experiment, to examine the shape of photosynthetic response curves, and examine other gas exchange and chlorophyll a fluorescence parameters to better understand how photosynthetic activity is reduced on remaining tissue of injury leaves (since not all plant species experience photosynthetic reductions on remaining tissue of injured leaves). We found that plants had a negative linear relationship between photosynthetic parameters and leaf injury intensity in one study (equal drop in photosynthesis for each unit of injury), but in two other studies plants had negative exponential relationships to indicate low photosynthetic tolerance to injury (large decreases at low injury levels, and small or no additional decreases with additional injury). Results with other chlorophyll a fluorescence and gas exchange parameters suggest that injury causes mesophyll limitations to photosynthesis that reduces light harvesting efficiency of photosynthesis and subsequent energy allocation to photosynthetic light reactions. Our results are of interest to those who study a trade-off between photosynthesis and chemical defense, because chemical defense induction after injury would be expected to cause mesophyll limitations to photosynthesis. Thus, oleander and other members of Apocynaceae (e.g., dogbanes) and Asclepiadaceae (milkweeds) may serve as model species for studying chemical defense trade-offs with maintaining photosynthesis after leaf defenses are breached. Technical Abstract: Variable indirect photosynthetic rate (Pn) responses occur on injured leaves after insect herbivory. It is important to understand factors that influence indirect Pn reductions after injury. The current study examines the relationship between gas exchange and chlorophyll a fluorescence parameters with injury intensity (% single leaf tissue removal) from clipping or Spodoptera eridania Stoll (Noctuidae) herbivory on Nerium oleander L. (Apocynaceae). Two experiments showed Ci increases and Pn and gs reductions with increasing injury intensity, suggesting non-stomatal Pn limitation. Also, Pn recovery was incomplete at 3 d post-injury. This is the first report of a negative exponential Pn (and gs) impairment function with leaf injury intensity to suggest high N. oleander leaf sensitivity to indirect Pn impairment. Negative linear functions occurred between most other gas exchange and chlorophyll a fluorescence parameters with injury intensity. The degree of light harvesting impairment increased with injury intensity via lower 1) photochemical efficiency indicated lower energy transfer efficiency from reaction centers to PSII, 2) photochemical quenching indicated reaction center closure, and 3) electron transport rates indicated less energy traveling through PSII. Future studies can examine additional mechanisms (mesophyll conductance, carbon fixation, and cardenolide induction) to cause N. oleander indirect leaf Pn reductions after injury. |
