Plants have evolved a repertoire of defenses that they employ against natural enemies. Lima bean (Phaseolus lunatus L.) plants release hydrogen cyanide (HCN) in response to damage, thereby directly defending plant tissue. However, plants also have multiple indirect defenses: both the release of volatile organic chemicals and the production of extrafloral nectaries (EFNs) serve to attract herbivores’ enemies. Investment in multiple defensive traits requires resources, and is likely to involve trade-offs between defensive traits. Further, the presence of a microbial mutualist, such as nitrogen-fixing Rhizobium bacteria, potentially influences investment in defense because it places additional resource demands on plants. In a series of greenhouse experiments we examined how light and nutrient availability affected the production of HCN and EFNs in P. lunatus seedlings grown in the presence and absence of Rhizobium. In one experiment, defensive traits of lima bean seedlings grown in high and low fertilizer treatments were compared with and without Rhizobium inoculation. In a second experiment, defensive traits of lima bean seedlings grown in full light and reduced light were compared, also with and without Rhizobium inoculation. After six weeks of growth under experimental conditions, EFNs were counted and HCN was quantified using spectrophotometric techniques. 

Results/Conclusions

We found that seedlings grown in the high fertilizer treatment had significantly more EFNs per leaf than did seedlings grown in the low fertilizer treatment. Further, seedlings inoculated with Rhizobium had significantly more EFNs per leaf in both high and low fertilizer treatments. Similarly, seedlings grown in high fertilizer treatments had higher concentrations of HCN in their leaves than seedlings grown in low fertilizer treatments. However, there was no significant effect of Rhizobium inoculation on HCN concentration. Light availability also influenced plant investment in defense. Seedlings grown under high light invested more in EFNs than did seedlings grown under low light. Rhizobium inoculation had no effect on EFN number, regardless of light treatment. HCN concentration in the leaves also increased with increased light availability. While there was no significant effect of Rhizobium inoculation, there was a tendency for seedlings grown with the mutualist to have lower leaf HCN concentrations. These results clearly demonstrate the effects of resource availability on plant investment in defense. However they do not provide clear evidence of trade-offs between multiple defenses as one might expect given the cost that is presumed to be involved in the production of defensive traits.