Potential roles of volatile organic compounds in plant competition

Volatile organic compounds (VOCs) are a major currency in plant communication where they mediate above-and below-ground interactions between plants and the surrounding organisms (i.e., other plants, microorganisms, pollinators, seed dispersers, herbivores, and their natural enemies). Considering the multiple interactions mediated by VOCs and their impact on a plant's reproductive success and survival, they can be a crucial weapon in plant-plant competition. However, this particular role of VOCs is underrepresented in the literature. Mechanisms by which volatiles can mediate plant competition can be direct or indirect. Direct mechanisms include establishing a neighbour's identity and status to select adequate responses and affecting competitor's seed germination or growth through VOC-mediated allelopathy. Indirect mechanisms can affect the plants competitive ability by modifying interactions with other trophic levels, for instance, through associational resistance or chemical camouflage. These mechanisms are not mutually exclusive and can be seen as part of a continuum. In this review, we present evidence from the literature to illustrate these roles and indicate how they could influence competition in plant communities. We propose new research avenues to test if and how these mechanisms affect competitive outcomes and suggest that, in addition to morphological traits, future competition studies should also incorporate data on plant-volatile emissions and measure their effects on the surrounding plants and other trophic levels. This information would allow us to understand competition from a broader perspective by acknowledging the existence of multiple (possibly coexisting) competition strategies and the role of other trophic levels in shaping competitive outcomes.