Soil origin and light heterogeneity interact to affect trait expression in perennial plants

Background/Question/Methods

The outcome of competition in plants is dependent on individuals’ responses to their environment, including phenotypic trait expression. The expression of different leaf or root traits amongst individuals of the same genotype across environments could influence the outcome of competition. For example, phenotypic plasticity may result from plant responses to soil or light conditions. However, the extent to which soil and light heterogeneity interact to influence traits remains an open question. In a greenhouse experiment conducted in Northeast Ohio, we planted two congeneric pairs of perennial species (Plantago major/P. lanceolata and Rumex crispus/R. obtusifolius in a factorial design with soil heterogeneity (heterogeneous/homogeneous) and light heterogeneity (heterogeneous/homogeneous) treatments. The soil treatment included analysis of soils; taken from the rhizosphere of the conspecific or congener in the field, which were either mixed (homogeneous) or kept in separate patches (heterogeneous). The light treatment used spectral filters to change the red: far red ratio experienced by the plant, thereby simulating patchy (heterogeneous) or uniform (homogeneous) shade. We tested for interactive effects between heterogeneity in soil and light of the growing environment using root:shoot ratio (RS) and specific leaf area (SLA) as measures of resource allocation. 

Results/Conclusions

RS ratio was significantly influenced by light treatment and the interaction between the light and soil heterogeneity treatment for the Plantago congeners; these individuals also exhibited significantly different responses of SLA to soil heterogeneity and a marginally significant response to the interaction between the light and soil heterogeneity treatments. There were no significant differences between Rumex congeners, but we observed trends for differences in RS ratio due to soil origin with higher values in R. obtusifolius soil and for SLA with light treatment. Additionally, the homogeneous soil treatment had a non-additive effect on the RS ratio of R. crispus—the RS ratio in this species was greater in homogeneous soil than either conspecific or congeneric soil. The different types of environmental heterogeneity interacted to produce significantly different resource allocation trait values, highlighting the importance of factorial design in experiments manipulating environmental heterogeneity. We found that soil origin and heterogeneity drive plastic trait expression in the studied congeneric pairs. Given the responses to soil environment of the individuals in this experiment, further research with soil biota, nutrient availability and quality of plant litter as possible mechanisms of action is warranted.