Photosynthetic compensation for leaf tissue removal by mechanical damage or painted lady caterpillars (Vanessa cardui: Nymphalidae), in soybean (Glycine max: Fabaceae) plants of different ages

Background/Question/Methods

We explored the effects of herbivory on photosynthetic rates of different aged soybean (Glycine max) plants.  We expected that their primary response would be to compensate for leaf area lost by increased photosynthetic rates, and that plants about to flower would have the highest responses. We planted soybeans on different dates to get four age groups of 40, 45, 58, and 71 days old when damaged.  We used 20 plants of each age for each of three treatments: no damage for the control group, feeding by five-day old painted lady caterpillars (Vanessa cardui) for ten hours, and mechanical damage on ~40% of the center leaflet. Each test leaflet for all of the treatments had a 1.5 cm diameter caterpillar cage with or without caterpillars. We measured photosynthetic rates with an ADC LCi portable infrared gas analyzer on the leaflet adjacent to the damaged one on the third trifoliate and the middle leaflet of the leaf above the damage (4^thtrifoliate) each day for five days following feeding. Statistical analysis included a repeated measures analysis of variance to compare means of rates at different ages, and one-way ANOVA with LSD post hoc tests to compare means of the different treatments. 

Results/Conclusions

The 58 day-old soybean plants showed the greatest compensatory increase in photosynthetic rates after caterpillar herbivory. There were significant differences among the treatments, age of the plants, and day tested. Overall, the photosynthetic rates were significantly higher for the caterpillar and mechanical damage treatments, showing a compensatory increase in photosynthetic rates in the remaining leaf tissue. We found a threefold increase in photosynthetic rates in plants with ~40% leaf area removed as compared to controls. Control photosynthetic rates did not change significantly over time. Leaves fed on by caterpillars had the highest photosynthetic rates at all ages, with mechanically damaged leaves intermediate. Leaflets above the treated leaflet followed the same pattern of rate increases over time, with the newer, higher leaves having significantly higher rates than compared leaflets. We suggest that moderate damage to soybeans by insects increases their photosynthetic rates for several days, and therefore may result in increased yields.