Drought accentuates the role of mycorrhiza in phosphorus uptake

Arbuscular mycorrhizal fungi (AMF) greatly facilitate uptake of phosphorus (P) by most plant species. Whether or how this changes if water becomes limiting remained little explored. Medicago truncatula plants were grown in previously sterilized cultivation substrate and inoculated or not with AMF isolate Rhizophagus irregularis 'PH5' (M or NM, respectively) in two-compartment rhizoboxes. The compartments were separated or not with root-excluding meshes. A 15-step soil moisture gradient (maintained over 5 weeks) spanned generous water supply through strong water deficiency. Two radioisotopes (P-33 and P-32) were injected into the plant and distant compartments, respectively, to distinguish cumulative and immediate-term plant P uptake from either of the compartments at each moisture level. M plants were larger than NM plants at high and medium moisture levels and accumulated markedly more P throughout the whole gradient. The immediate-term P uptake did not differ between M and NM plants at high moisture, but at medium and low moistures, M plants acquired significantly more P-33 than NM plants, even after accounting for differences in root biomass. Increased immediate-term P uptake by M plants grown under medium and low substrate moistures was probably due to complementary effect of direct hyphal uptake and indirect alterations of substrate hydraulic properties by AMF. This research illustrates that AMF remain functional part of plants even under severe drought stress and thus they should be considered when plant functioning under water deficiency is to be fully understood.

Automated summary

The research demonstrates that arbuscular mycorrhizal fungi (AMF) play a significant role in enhancing phosphorus uptake by most plant species, even under limited water resources. The increased phosphorus uptake in plants grown under medium and low substrate moistures is likely due to the complementary effects of direct hyphal uptake and indirect alterations of substrate hydraulic properties by AMF.