Induced response of Solanum carolinense to dispersed and aggregated leaf damage

Background/Question/Methods   Plants often experience herbivory from various insects, and respond by increasing chemical or physical defenses (induced resistance, IR). Different insects impose different damage patterns, and past studies have shown that the distribution of damage can affect IR; for example, localized damage may elicit higher levels of induction. Some plants also can produce multiple stems from a single root, which might diffuse the effects of damage if experienced on different stems. To address these issues, we used Solanum carolinense to (1) examine the effects of damage patterns on IR, (2) examine whether plant genotypes vary in their responses and (3) determine whether the influence of damage patterns differs between single-stemmed and multi-stemmed plants. In this study, 150 plants were grown in the greenhouse from root cuttings from five genotypes. Plants were randomly assigned to either an aggregated damage, dispersed damage, or a no damage treatment. Damaged plants received thirty percent total leaf tissue damage by Manduca sexta larvae. For multi-stemmed plants, damage was dispersed equally across stems. Undamaged leaves from all treatments were fed to M. sexta larvae, and herbivore performance (relative growth rate) was measured after three days of feeding. Plant performance was also measured using root biomass.

Results/Conclusions   Undamaged plants differed significantly in constitutive resistance among genotypes. In contrast, induced resistance, as measured by herbivore relative growth rates, depended on the number of stems. Among single-stemmed plants, dispersed damage (DD) led to induced susceptibility (i.e., higher herbivore growth on DD plants relative to control plants). Among multiple-stemmed plants, DD led to induced resistance (lower herbivore growth on DD plants). Aggregated damage (AD) did not trigger an induced plant response. This result contradicted our prediction of increased induction for localized (or aggregated) damage. Damage patterns also affected plant performance. DD plants had significantly lower root biomass than both the control and AD plants. Because DD plants received damage on more leaves than AD plants (although the total amount damaged remained the same), DD plants would have more damaged edges exposed. This could have led to shedding of damaged leaves, leaving fewer leaves for photosynthesis, and ultimately reducing root biomass. This may explain why DD plants were more sensitive to herbivory and had lower root biomass.