Myora Springs is one of many groundwater discharge sites on North Stradbroke Island (Queensland, Australia). Here spring waters emerge from wetland forests to join Moreton Bay, mixing with seawater over seagrass meadows dominated by eelgrass, Zostera muelleri. We sought to determine how low pH / high CO2 conditions near the spring affect these plants and their interactions with the black rabbitfish (Siganus fuscescens), a co-occurring grazer. In paired-choice feeding trials S. fuscescens preferentially consumed Z. muelleri shoots collected nearest to Myora Springs. Proximity to the spring did not significantly alter the carbon and nitrogen contents of seagrass tissues but did result in the extraordinary loss of soluble phenolics, including Folin-reactive phenolics, condensed tannins, and phenolic acids by ≥87%. Conversely, seagrass lignin contents were, in this and related experiments, unaffected or increased, suggesting a shift in secondary metabolism away from the production of soluble, but not insoluble, (poly)phenolics. We suggest that groundwater discharge sites such as Myora Springs, and other sites characterized by low pH, are likely to be popular feeding grounds for seagrass grazers seeking to reduce their exposure to soluble phenolics.

Herbivore-induced plant volatile (HIPV)-mediated eavesdropping by plants is a well-documented, inducible phenomenon that has practical agronomic applications for enhancing plant defense and pest management. However, as with any inducible phenomenon, responding to volatile cues may incur physiological and ecological costs that limit plant productivity. In a common garden experiment, we tested the hypothesis that exposure to a single HIPV would decrease herbivore damage at the cost of reduced plant growth and reproduction. Lima bean (Phaseolus lunatus) and pepper (Capsicum annuum) plants were exposed to a persistent, low dose (~10 ng/h) of the green leaf volatile cis-3-hexenyl acetate (z3HAC), which is a HIPV and damage-associated volatile. z3HAC-treated pepper plants were shorter, had less aboveground and belowground biomass, and produced fewer flowers and fruits relative to controls, while z3HAC-treated lima bean plants were taller and produced more leaves and flowers than did controls. Natural herbivory was reduced in z3HAC-exposed lima bean plants, but not in pepper. Cyanogenic potential, a putative direct defense mechanism in lima bean, was lower in young z3HAC-exposed leaves, suggesting a growth–defense tradeoff from z3HAC exposure alone. Plant species-specific responses to an identical volatile cue have important implications for agronomic costs and benefits of volatile-mediated interplant communication under field conditions.

Highlight: Long-term exposure to a ubiquitous herbivore-associated volatile cue results in 30 species-specific fitness effects. Differential cue-mediated growth/defense tradeoffs have 31 important implications for plant-herbivore interactions. 32 33 34 35 36 2 Abstract 37 Herbivore-induced plant volatile (HIPV)-mediated eavesdropping is a well-documented, 38 inducible phenomenon that should-like any inducible phenomenon-incur fitness costs. Yet, 39 ecological costs associated with volatile exposure alone are unclear. In a common garden 40 experiment, we tested the hypothesis that exposure to a single HIPV would decrease herbivore 41 damage at the cost of reduced plant growth and reproduction. Lima bean (Phaseolus lunatus) and 42 pepper (Capsicum annuum) plants were exposed to a persistent, low-dose (~10ng/hour) of the 43 green leaf volatile cis-3-hexenyl acetate (z3HAC), a ubiquitous HIPV. z3HAC-treated pepper 44 plants were shorter, had less aboveground and belowground biomass, and produced fewer 45 flowers and fruits relative to controls while z3HAC-treated lima bean plants were taller and 46produced more leaves and flowers than did controls. Natural herbivory was reduced in z3HAC-47 exposed lima bean plants, but not in pepper. Cyanogenic potential, a putative defense 48 mechanism in lima bean, was lower in z3HAC-exposed leaves, suggesting a growth-defense 49 tradeoffs from z3HAC exposure alone. Plant species-specific responses to an identical priming 50 cue have important implications for ecological costs and benefits of volatile-mediated inter-plant 51 communication under field conditions. 52 53 Key Words -cis-3-hexenyl acetate, eavesdropping, lima bean, pepper, plant fitness, plant 54 volatiles 55 56 Abbreviations used-cis-3-hexenyl acetate (z3HAC), Herbivore-induced plant volatile (HIPV), 57 Green-leaf volatile (GLV) 58 59 60 Production and utilization of airborne chemical cues is prevalent within the plant 61 kingdom. Plants depend on airborne chemical signaling for pollination (Muhlemann et al., 62 2014), indirect defense (Mumm and Dicke, 2010), protection from pathogens (Farag et al., 632013), and herbivore resistance (Frost et al., 2008c). Volatile communication is also pivotal for 64 plant-plant signaling, and selection for such signaling depends on honest cues that reliably confer 65 ecologically relevant information. For example, herbivory is a fundamental ecological 66 interaction that impacts plant fitness, and many plants increase the production and emission of 67 volatile compounds in response to herbivore damage (Turlings et al., 1995). Such herbivore-68 induced plant volatiles (HIPVs) are potentially reliable cues around which plant-plant 69 eavesdropping could be evolutionarily adaptive(Rodriguez-Saona and Frost, 2010). Undamaged70

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