Parrish, Judith A. .
Alternative strategies for plant defense: Stand and fight, or grow out of it.
In a series of experiments, we found that some plant species show significant increases in photosynthetic rates after herbivore attack, while other species do not. Four different experiments resulted in our concluding that Glycine max, soybeans, compensate for leaf area removed by herbivores. Whole plants with moderate damage from Japanese beetle herbivory significantly increased photosynthetic rates and marginally increased seed weight compared to plants with no damage. Leaflets adjacent to artificially and beetle damaged leaflets had significantly higher photosynthetic rates than leaflets at similar positions on undamaged plants, and leaflets above beetle damaged leaflets also showed significantly higher photosynthetic rates. We also observed increased photosynthetic rates in soybeans when fed on by painted lady caterpillars (Vanessa cardui). Plants fed on by two caterpillars had a significantly higher increase in photosynthetic rate than those with no caterpillars, and plants with four caterpillars had a significantly higher increase in photosynthetic rate than those with two caterpillars. A separate experiment observed increased photosynthetic rate as a plant response to lost surface area, both by painted lady caterpillar herbivory or manual hole-punch. Herbivory caused a significantly higher increase in photosynthetic rate than manual damage. However, when we performed similar experiments on Helianthus annuus (sunflower), we found that although there was a significant increase in the photosynthetic rates in all leaves over time, there was no significant difference in damage treatment that would suggest compensation. In another experiment on furanocoumarin-defended Pastinaca sativa, (wild parsnips), we found that 4 hours of feeding by Trichoplusia ni (cabbage looper caterpillars) resulted in reduction in photosynthetic rates for up to 72 hours, significantly for 48 hours. There was no evidence that compensation for leaf area occurred in wild parsnip. Crop plants like soybeans have typically had distasteful chemicals bred out - moderate herbivory reduces leaf area, but may mobilize increased sugar production so the plant “combats” herbivory by regrowing. Plants with strong chemical defenses induced by herbivory may show more negative effects of moderate herbivory due to production of defensive compounds - but may be more resistant to long term or more severe feeding damage. These experiments support that there are two conflicting strategies for plants to deal with herbivores – stand and fight with chemical defenses or mobilize to grow faster after the herbivory event.
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1 - Millikin University, 1184 W Main St, Decatur, IL, 62522, USA
Presentation Type: Oral Paper
Candidate for Awards:None