Nitrogen deficiency, salinity and herbivory are common stress factors faced by plants, but their simultaneous effects have been overlooked. The goal of our study was to examine whether plant resistance and/or tolerance to a generalist herbivore were altered by salt stress and/or symbiotic nitrogen fixation.
We grew Bradyrhizobium-inoculated and non-inoculated soybean plants under contrasting salinity levels (0 and 100 mM NaCl), and measured their performance along with their resistance and tolerance to herbivory by cabbage looper larvae.
Prior to herbivory, NaCl-treated plants had lower biomass, chlorophyll content, leaf trichome density, whole-plant fixed nitrogen and allocation to roots while the levels of N, K, Ca, Mn, B and Cl in shoot tissues where higher than in control plants. Constitutive and induced resistance decreased with salt stress without being affected by inoculation treatment. In the absence of NaCl, inoculated plants produced twice as many seeds than non-inoculated plants, but this difference disappeared under salinity. Plant growth and seed production decreased by 25% with herbivory, but were unaffected by exposure to salinity or rhizobium symbiosis.
While nitrogen fixation can improve plant performance it had little effect on soybean responses to herbivory, and it was inhibited under salt stress. Resistance to herbivores decreased under salt stress.