Can arbuscular mycorrhizal fungi mitigate drought stress in annual pasture legumes?

Aims Root colonization by arbuscular mycorrhizal fungi (AMF) enhances plant growth and the symbiosis can play a major role in enhancing drought tolerance of host plants. Our aim was to determine whether AMF can mitigate negative effects of drought stress on productivity in annual legume species used in seed mixtures for permanent pastures in agroecosystems in the Iberian Peninsula. Methods Five species - Trifolium subterraneum, Trifolium incarnatum, Trifolium resupinatum, Trifolium vesiculosum and Ornithopus sativus - were grown in pots, non-inoculated or inoculated with AMF, at different watering regimes (no-stress, moderate drought stress, severe drought stress). We measured colonization levels, mycorrhizal dependency, biomass, and plant and substrate phosphorus (P) status. Results In all species, AMF led to a significantly higher biomass and tissue P, and water deficit significantly decreased biomass, with the responses to mycorrhization and watering being species-specific. In all Trifolium spp. watering did not affect mycorrhizal growth dependency (MGD), but mycorrhizal P-dependency (MPD) was significantly higher with drought. However, in Ornithopus, MGD was lower with severe drought, while MPD in this species was not affected by watering. Conclusions Biomass accumulation in the studied species benefitted from AMF symbiosis, although mycorrhizae did not mitigate drought effects on growth. Drought stress severity determined the carbon allocation to the different fungal structures. Under moderate drought stress carbon allocation to fungal structures increased, outweighing plant growth benefits by P or water gain of the symbiosis, while severe drought stress led to carbon limitation for both plant and fungi.