Herbivory and Attenuated UV Radiation Affect Volatile Emissions of the Invasive Weed Calluna vulgaris.

Evans Effah,Murray A Potter,D Paul Barrett,Jason J Wargent,Andrea Clavijo Mccormick,Paul G Peterson,Jarmo K Holopainen

doi:10.3390/molecules25143200

Abstract

Calluna vulgaris (heather) is an aggressive invasive weed on the Central Plateau, North Is., New Zealand (NZ), where it encounters different environmental factors compared to its native range in Europe, such as high ultraviolet radiation (UV) and a lack of specialist herbivores. The specialist herbivore Lochmaea suturalis (heather beetle) was introduced from the United Kingdom (UK) in 1996 as a biocontrol agent to manage this invasive weed. Like other plant invaders, a novel environment may be challenging for heather as it adjusts to its new conditions. This process of “adjustment” involves morphological and physiological changes often linked to phenotypic plasticity. The biochemical responses of exotic plants to environmental variables in their invaded range is poorly understood. The production and release of volatile organic compounds (VOCs) is essential to plant communication and highly susceptible to environmental change. This study therefore aimed to explore the VOC emissions of heather in response to different levels of UV exposure, and to feeding damage by L. suturalis. Using tunnel houses clad with UV-selective filters, we measured VOCs produced by heather under NZ ambient, 20% attenuated, and 95% attenuated solar UV treatments. We also compared VOC emissions in the field at adjacent sites where L. suturalis was present or absent. Volatiles produced by the same target heather plants were measured at four different times in the spring and summer of 2018–2019, reflecting variations in beetle’s abundance, feeding stage and plant phenology. Heather plants under 95% attenuated UV produced significantly higher amounts of (E)-β-farnesene, decanal, benzaldehyde, and benzeneacetaldehyde compared to 25% attenuated and ambient UV radiation. We also found significant differences in volatiles produced by heather plants in beetle-present versus beetle-absent sites on most sampling occasions. We also recorded a lower number of generalist herbivores on heather at sites where L. suturalis was present. Interactions between invasive plants, a novel environment, and the native communities they invade, are discussed.

Highlights

Climate change and the spread of species beyond their natural geographic boundaries are major ways in which humans have altered the environment, with consequences to species development, Molecules 2020, 25, 3200; doi:10.3390/molecules25143200 www.mdpi.com/journal/moleculesMolecules 2020, 25, 3200 fitness and competitiveness [1]
We investigated the volatile production of heather in response to these abiotic and biotic factors; i.e., exposure to ultraviolet radiation (UV) radiation in tunnel house conditions and feeding damage by L. suturalis under natural field conditions
Days of UV exposure, foliar volatile compounds producedusing by heather plants exposedheadspace to ambient, 20% attenuated and 95%

Summary

Introduction

Climate change and the spread of species beyond their natural geographic boundaries are major ways in which humans have altered the environment, with consequences to species development, Molecules 2020, 25, 3200; doi:10.3390/molecules25143200 www.mdpi.com/journal/moleculesMolecules 2020, 25, 3200 fitness and competitiveness [1]. Climate change and the spread of species beyond their natural geographic boundaries are major ways in which humans have altered the environment, with consequences to species development, Molecules 2020, 25, 3200; doi:10.3390/molecules25143200 www.mdpi.com/journal/molecules. Invasive plants have high phenotypic plasticity that enhances their competitiveness [2,3]. New environments are unlikely to contain specialist herbivores creating enemy-poor or enemy-free spaces [4]. Invasive plants can allocate more resources towards competition instead of defence. Some invasive plants are known to release root exudates that are phytotoxic to natives [5,6]. Our understanding of the chemical mechanisms behind the success of invasive plants in novel environments is still limited considering that phytochemicals mediate interactions with microbes, pollinators, herbivores and their natural enemies and plant responses to stress [7]

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