Nitrogen Fixation Does Not Alter the Effects of Salinity on Soybean Resistance to a Generalist Caterpillar

Abstract

Premise of research. 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 test whether plant resistance to a generalist herbivore was altered by salt stress or symbiotic nitrogen fixation, or both.Methodology. We grew Bradyrhizobium-inoculated and noninoculated soybean plants under two salinity levels, subjected half of them to herbivory by cabbage looper larvae, and measured plant performance and resistance to herbivory.Pivotal results. Prior to herbivory, NaCl-treated plants had lower biomass, chlorophyll content, leaf trichome density, whole-plant fixed nitrogen, and biomass allocation to roots, while the levels of N, K, Ca, Mn, B, and Cl in shoot tissues were higher than in control plants. Most salt-stressed plants produced no seeds. Without salt stress, herbivory reduced plant growth by 25% and seed production by 13%. Constitutive and induced resistance decreased with salt stress but were not affected by inoculation.Conclusions. Nodule formation did not alter the negative effects of salinity on seed production or resistance to herbivory, suggesting that the benefits of biological nitrogen fixation are modest compared with the overwhelming negative effects of salt stress on soybean growth, reproduction, and ability to defend from herbivores.