Study system
We conducted this experiment on tomato plants var. Castlemart (Solanum lycopersicum ). Tomatoes are a valuable field and greenhouse crop which associate with mycorrhizal fungi and have a range of chemical defenses against herbivores. Protease inhibitors are common defenses in tomato leaf tissue (Broadway 1986), which are induced through the jasmonic acid pathway. Since herbivores are commonly nitrogen limited, the inability to digest proteins can significantly retard growth and result in starvation and death.
The efficacy of this defense strategy has been shown in Castlemart tomatoes (Felton et al. 1989, Farmer and Ryan 1990, Rodriguez-Saona et al. 2010, Shrivastava et al. 2015). We chose to grow the tomatoes in intraspecific competition, as tomato plants grown in agricultural systems will most likely be planted with conspecifics.
Rhizophagus intraradices is a generalist arbuscular mycorrhizal fungus in the sub-phylum Glomeromycotina. It is commercially available and used in organic agricultural systems to increase nutrient uptake and decrease fertilizer use. It has been shown to colonize tomato plants (Caron et al. 1986, Fierro-Coronado et al. 2013, Shrivastava et al. 2015). The mycorrhizal inoculum containing R. intraradicesspores, and non-mycorrhizal inoculum for controls were obtained from the International Culture Collection of (Vesicular) Arbuscular Mycorrhizal Fungi (INVAM) at West Virginia University. Both the mycorrhizal inoculum and non-mycorrhizal control were produced in leek trap pots in sand-turface media. The inoculum itself was comprised of sand, turface, leek root fragments, and in the treatment: mycorrhizal spores and hyphal fragments. While the control media did not contain mycorrhizal propagules neither inoculum was sterile.
We used first instar Trichoplusia ni caterpillars in a bioassay to measure leaf quality. These larvae were obtained from a colony maintained on artificial diet at Cornell University for many years.Trichoplusia ni are generalist Noctuid caterpillars that feed on a wide variety of crop plants including Solanaceous and cruciferous vegetables.