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 was
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.