Root responses to neighbors depend on neighbor identity and resource distribution

In a complex soil environment, competitive and environmental factors will interact with individual traits to influence a plant’s root growth patterns and ability to compete for resources. Here, we examine how root growth of a focal plant, Plantago lanceolata L., responds to resource heterogeneity and to presence of two neighbor species, Centaurea jacea L.and Poa pratensis L. A full factorial experiment tested the effects of nutrient heterogeneity, neighbors, and their interaction on root responses of Plantago. Roots in shared quadrants of a pot were harvested and quantified by qPCR for plants grown alone or with a neighbor, in patchy or even soil. The effects of experimental treatments on Plantago root mass distribution were tested with two-way ANOVA. When soil resources were evenly distributed, Plantago individuals increased root allocation to soil shared with a Centaurea neighbor but not a Poa neighbor. When soil resources were patchy, Plantago responded more strongly to Poa than to Centuarea, and placed more roots in the high-resource patch. These results demonstrate that plants can respond differently to neighbors depending on species and that integrating multiple cues results in non-additive effects on root behavior.

Methods
The experimental design is described in the publication. Shoot mass was harvested, dried, and weighed in mg. Root mass was harvested from two shared quadrants of each pot and wet weight in mg recorded. For pots with neighbors, qPCR was used to estimate the percentage of root mass belonging to each plant.