Plants use a variety of cues from neighbors to obtain information about their surroundings (Karban 2021; Ninkovic et al. 2021). Selection can presumably favor responding to those cues that contain information that is reliable and that plants are able to perceive and interpret. Once plants obtain such cues, they adjust allocation decisions to more effectively compete with neighbors and to defend against biotic and abiotic threats such as herbivory. These plastic systems of perception, decision-making, and allocation allow them to increase fitness although their competitors and antagonists also manipulate plants by producing misleading cues that cause plants to make maladaptive choices. For example, cues from whiteflies cause tomato plants to downregulate defenses that would be effective against whiteflies and to emit volatiles that cause their neighbors to respond inappropriately as well (Zhang et al. 2019).
Plants have access to many different cues from different sources involving different modes of transmission (Karban 2021). These cues include airborne volatiles, volatiles in the soil, physical touch stimulation to leaves and roots, light inputs that vary in quality and quantity, soil inputs that include nutrients and secondary metabolites, and others that are less well known. We have only a cursory understanding of how plants prioritize these various cues which may provide conflicting information. Some plants responses increase in proportion to the strength of the stimulus, acting in essentially a dosage-dependent manner (Horiuchi et al. 2003; Mithofer et al. 2005). In some cases, cues involving multiple sensory modalities elicit stronger responses; for example, lima bean responses to mechanical damage were intensified when other cues such as insect oral secretions were also detected (Bricchi et al. 2010). In these cases, confirmatory or redundant cues may reduce the likelihood of responding to inaccurate or misleading information.
Plants often face tradeoffs in allocating resources to defending against antagonists and competing with neighbors (Herms and Mattson 1992). Plants that receive cues of the existence of neighboring conspecifics might be expected to delay reproduction since resources for growth may be less available (Stearns 1976; Young and Augspurger 1991). On the other hand, plants that receive cues from neighbors indicating a high risk of herbivory might be expected to flower quickly at a smaller size if risk of mortality is great (Stearns 1976; Reznick and Endler 1982; Duncan and Williams 2020).
In this experiment we manipulated the cues that focal tomato plants received from neighbors with clipped leaves. Some plants had access to airborne volatile cues emitted by clipped neighbors. Other plants had access to soilborne cues emitted by clipped neighbors. Some plants had access to both cues while some had access to neither type of cue. We asked whether airborne and soilborne cues would reduce levels of chewing damage by naturally occurring herbivores. Was one of these two communication modalities more effective at inducing resistance than the other? Did the two communication modalities act synergistically to reduce damage relative to either cue alone? Did plants that received cues from damaged neighbors initiate flowering more rapidly than controls that did not receive cues?