The roles of root-nodulating bacterial associations and cyanogenesis in the freezing sensitivities of herbaceous legumes.

Premise: Reduced snow cover and increasing temperature variability can increase freezing stress for herbaceous plants in northern temperate regions. Legumes have emerged as a plant functional group that is highly sensitive to these changes relative to other herbaceous species in these regions. We explored root-nodulating bacterial associations and cyanogenesis as potential mechanisms explaining this relatively low freezing tolerance of legumes.

Methods: To examine the influence of bacterial associations, we grew four legume species with or without crushed-nodule inoculum at three severities of freezing, and three concentrations of nitrogen to disambiguate the direct benefits of increased nitrogen from the total bacterial effect. We quantified cyanogenesis via hydrogen cyanide production in both true leaves and cotyledons for nine legume species.

Results: Root nodulation generally only affected legume survival under low nitrogen, when freezing severity was moderate or low. However, for the frost-surviving plants, the growth advantage provided by nodulation decreased (it was often no longer significant with increasing freezing severity), and greater freezing severity reduced total nodule mass. In contrast, cyanogenesis was only detected in two of the nine species.

Conclusions: The diminished performance of nodulated plants in response to freezing could place legumes at a competitive disadvantage and potentially explain their high sensitivity to freezing relative to other herbaceous species in northern temperate regions. Overall, this result has important implications for changes in soil fertility, community composition, and plant productivity in these ecosystems in the context of a changing winter climate.