Soil cadmium stress reduced host plant odor selection and oviposition preference of leaf herbivores through the changes in leaf volatile emissions

The Elemental defense hypothesis suggested that metal accumulation in plant tissues could serve as direct defense to reduce herbivore feeding preference as metals are toxic to phytophagous insects. However, the indirectly defensive role of heavy metals on host plant odor selection and oviposition preference of leaf herbivores through the changes in leaf volatile organic compounds (VOCs) is still unknown. In this study, we used a local woody plant species, Populus yunnanensis, to investigate whether soil cadmium (Cd) stress could affect plant VOC production and whether Cdmediated changes in leaf VOC emissions will further influence the host plant odor and oviposition preferences of female adults of a specialist and a generalist herbivore species. The results clearly showed that the soil Cd stress could prominently induce leaf total VOC emissions of P. yunnanensis and such induction was positively correlated with leaf Cd accumulation. Herbivore olfactometer bioassays further demonstrated that the VOCs released by P. yunnanensis under Cd exposure are far less attractive to both of the specialist and generalist female adults compared to control plants, leading to significant reduction in oviposition on Cd-treated plants. Moreover, the host plant odor selection and oviposition preference of the two herbivore species were all negatively correlated with leaf total VOC emissions, which confirmed the defensive role of Cd-induced VOCs for deterring the female insects. The result will extend the existing knowledge of the Elemental defense hypothesis and provide newinsight into predicting the herbivore damage level of poplar species that naturally occurred in metal-polluted habitats.

Automated summary

This study demonstrated that soil cadmium (Cd) stress can significantly induce leaf volatile organic compound (VOC) emissions in Populus yunnanensis, and the released VOCs are less attractive to both specialist and generalist herbivores. Furthermore, the herbivores' odor selection and oviposition preference were negatively correlated with leaf total VOC emissions, confirming the defensive role of Cd-induced VOCs. This finding expands our understanding of the Elemental defense hypothesis and provides new insights into predicting herbivore damage in metal-polluted habitats.