PS 40-117 - Legume species with different life histories differ in their strategies of regulation of symbiotic nitrogen fixation and nitrogen use

Abstract: Symbiotic dinitrogen fixation is a trait commonly shared by plant species in the legume family (Fabaceae) and, in many ecosystems, is the primary natural source of new nitrogen. Legume species evolved different strategies to regulate symbiotic nitrogen fixation as a function of nitrogen availability in the soil and plant demand. The strategies were defined as a scale between downregulation of the fixation (facultative), and no regulation at all (obligatory). Recent work suggested a link between the strategy of regulation of symbiotic nitrogen fixation and within-plant nitrogen allocation. Where strong downregulation of fixation in response to nitrogen availability in the soil was associated with low and plastic nitrogen concentrations in the leaves. In this study, we examined the relationship between regulation of symbiotic nitrogen fixation and nitrogen allocation traits in a large selection of legume species. To achieve this, we collected and analysed data on thirteen legume species that were grown in three similar controlled pot experiments. In each experiment, the plants grew under two nitrogen availability treatments and inoculated with a small amount of soil to supply them with the necessary bacteria for symbiosis. The plants were harvested in spring and sorted into different organs (root, stem, and leaves) to measure biomass and nitrogen concentration.

We found a negative correlation between the level of regulation of nitrogen fixation and leaf nitrogen concentrations and a positive correlation between the regulation of fixation and intraspecific variation in leaf nitrogen (change in nitrogen concentrations in response to nitrogen availability). Stronger regulation of symbiotic nitrogen fixation was associated with low leaf nitrogen and high intraspecific variation in leaf nitrogen. Our results suggest that the relationship between the regulation of symbiotic nitrogen fixation and nitrogen allocation traits characterizes legume species with different life histories. Ephemeral annual herbaceous species had weak regulation of symbiotic fixation coupled with high and constant leaf nitrogen concentrations, a strategy that might allow intense growth during their short lifespan. While perennials had strong downregulation of fixation coupled with relatively high intraspecific variation and low values of leaf nitrogen concentrations, a strategy that might contribute to their ability to compete and survive over many growing seasons. In conclusion, we suggest that lifespan and habitat characteristics explain the evolution of the symbiotic relationship between legumes and their Rhizobium symbionts and sheds a light on the selection of different nutrient-use strategies in plants.