Arbuscular mycorrhizal fungi (AMF) are expected to benefit their host plants, contributing to better growth, especially in stressful conditions. The plant-AMF relationship is highly context dependant; growth responses of the host plant vary among and within species and often depend on environmental conditions. While general AMF responsiveness of a species may be known, it is unclear how inter-varietal differences affect mycorrhizal growth responses. Plant breeders typically target traits like high yield, fast regrowth, or hardiness, all traits that can affect AMF symbioses (e.g. by diverting carbon resources above ground or changing root morphology), Such breeding effects are often ignored. To test how mycorrhizal symbioses could change between varieties We selected nine alfalfa varieties expressing diverse traits that we inoculated with the AMF Rhizophagus intraradices in a factorial greenhouse trial. Plants were exposed to one of four treatment conditions: high salinity, low nutrients, drought, or a low stress control. We hypothesized that mycorrhizal growth responses would differ across varieties, and that those differences would depend on stress treatment. We measured the growth response in dry biomass, as well as seed production. Multiple fine root traits associated with AMF symbioses were assessed along with colonization levels as a measure of AMF compatibility.
Results/Conclusions
Rhizophagus intraradices inoculation reduced growth in some plants but the response changed depending on the stress treatment. On average, AMF reduced growth in the high salinity and drought treatments but did not affect growth in the control or low nutrient treatments. We did see differences between varieties as well, some varieties that were not affected by high salinity or drought when uninoculated had less biomass in those stress treatments when they were grown with R. intraradices. AMF inoculation and stress treatment also modified root trait expression across varieties. Inoculated alfalfa had higher root diameter and lower specific root length in saline conditions than the control plants, but the same pattern was not seen in uninoculated root systems. Though low specific root length and higher diameter indicate a more collaborative phenotype in the root economic space, those traits were associated with a negative mycorrhizal response in the salinity treatment. Our findings indicate that R. intraradices did not collaborate well with alfalfa. Hosting AMF penalized some alfalfa varieties when they were stressed.
