Silicon enrichment alters functional traits in legumes depending on plant genotype and symbiosis with nitrogen‐fixing bacteria

1. Silicon (Si) uptake and deposition (silicification) in tissues is known to alleviate stresses and generally improve plant health. This is mostly studied in Si-high accumulators, such as grasses, with comparatively less known about its effects on other plant functional groups, such as legumes. There is speculation that Si may positively impact the symbiosis between legumes and the nitrogen-fixing bacteria (rhizobia) they associate with, but this is poorly understood. This study examined the effects of Si enrichment on legume species associated with rhizobia and the potential underlying mechanism of Si impacts.

2. We conducted a glasshouse experiment with lucerne Medicago sativa and barrel medic M. truncatula associated with a model rhizobial strain. Six genotypes (three per species) were either supplemented with Si (+Si) or untreated (−Si). We quantified 16 functional traits which could be classified as plant growth, physiology, elemental chemistry, nodule activity and nitrogen fixation.

3. The two legume species responded to Si distinctively. For example, Si supplementation increased shoot biomass by more than 10% in lucerne but growth was unaffected in barrel medic. Conversely, nitrogen-fixing enzyme (nitrogenase) activity was promoted by more than 85% in +Si barrel medic plants but not in lucerne. Moreover, Si supplementation of lucerne increased the concentrations of Si in leaves by more than 36% but not in root nodules. Increased foliar concentrations of Si in lucerne were positively associated with increased shoot and root biomass in Sequel and Trifecta genotypes, respectively. Conversely, Si supplementation of barrel medic increased the concentration of Si in root nodules by 29% but not that in foliar tissues. Nitrogenase activity and where silicification occurred, differed between genotypes in barrel medic; nitrogenase activity was correlated with concentrations of Si in root nodules rather than that in foliar tissues in one genotype (Sephi) but the reverse was true in another (Hannaford).

4. This study demonstrates that two closely related legume species can respond to Si in distinct ways, depending on plant genotype and symbiosis. These results present the overlooked function of Si in legume–rhizobia interactions, which could potentially enhance productivity of this important group of plants.