Compensation of physiological traits under simulated drought and herbivory has functional consequences for fitness in soybean (Glycine max (L.) Merrill)

Soybean (Glycine max (L.) Merrill), one of the most valuable crops in the world, faces serious challenges due to drought and insect herbivory. Although well studied independently, we lack a comprehensive understanding of interactive effects of drought × herbivory on both soybean and herbivore traits. A holistic examination of soybean morpho-physiology (above and below-ground traits including root morphology) and herbivore performance can help us understand the potential consequences of these two major stressors on soybean yield and fitness. To this end, we imposed simulated-drought and herbivory by soybean looper (SBL) (Chrysodeixis includens Walker) and assessed both host and herbivore performance. Morpho-physiological traits of soybean including shoot height, chlorophyll content, root morphology, photosynthesis, stomatal conductance, and transpiration were measured. Additionally, growth and feeding behavior of SBL were also assessed to analyze the impacts of drought × herbivory on both host and herbivore. Our results show that certain physiological traits were significantly upregulated under drought × herbivory indicating compensation. We also observed that SBL frass weight, and scale of damage was lower on simulated-drought-experienced plants and, in choice assays, SBL preferred well-watered plants. In addition to lower yields observed under simulated-drought and herbivory interaction, soybeans that experienced both drought and herbivory had the highest number of aborted pods. Our study shows that simulated drought and herbivory have synergistic negative impacts on soybean morpho-physiology and support plant vigor hypothesis. Simulated drought negatively impacted SBL performance and made them less attracted to the soybeans that experienced water stress. Ultimately, the interactive effects of these stressors have negative consequences on soybean yield and fitness. This study demonstrates the need to integrate biotic and abiotic stressors for a better understanding of interactive effects on host and herbivores to make informed decisions for breeding and pest management strategies.