Growth and herbivore defense of clonal plants under single and combined modes of interconnection

Both clonal plant capabilities for physiological integration and common mycorrhizal networks (CMNs) formed by arbuscular mycorrhizal fungi (AMF) can influence the growth and insect resistance among interconnected individuals. Using a microcosm model system, we disentangled how CMNs interact with clonal integration to influence plant growth, development and herbivore defense. We grew Sphagneticola trilobata clones with isolated root systems in individual, adjacent containers while preventing, disrupting, or allowing clonal integration aboveground via spacers and belowground CMNs to form. We assessed multiple metrics of plant development, ¹⁵N transfer from donor (mother) to receiver (daughter) plants, variation in AMF communities, and changes in chemical defenses. We show that spacer formation between ramets and the capacity to form CMNs promoted and inhibited the growth of smaller, daughter plants, respectively. However, the effects on defense signals were inconsistent. When the two modes of interconnection co-occurred, CMNs significantly weakened promotion of daughter plants by clonal integration but enhanced the defense signal transmittance. AMF species richness was also negatively correlated with overall plant growth. Our results demonstrate that two common modes of plant interconnection interact in non-additive ways to affect clonal plant integration, growth and defense, questioning the underlying assumptions of the positive effects of both AMF CMNs and species richness in comparison to direct plant interconnections.