Assessing functional benefits of specific apple rootstock/AMF associations within context-dependent frameworks

Arbuscular mycorrhizal fungi (AMF) are root-colonizing endophytes present in most terrestrial ecosystems, including agricultural soils. AMF have been shown to provide a spectrum of benefits to their plant hosts. Their functional benefits can be both environmental-context dependent and fungus-plant-species dependent. How contextual controls govern AMF-mediated benefits is not well-understood, yet this knowledge is key to successful "real-world" application. The objective of this study was to assess contextual benefits of specific apple rootstock/AMF associations, including improved tolerance to water stress, nutrient access, and disease resistance. In conjunction, we sought to better understand how certain context-dependent factors, such as host genotype and soil microbial context influenced anticipated functional benefits. Specifically, in the first experiment, we tested the ability of Rhizophagus irregularis to improve water and inorganic nitrogen (¹⁵NH₄NO₃) uptake in G.11 rootstock with (and without) water stress in "live" (replant disease-conducive) vs. pasteurized (pathogen-free) orchard soil. While AMF inoculation did positively influence plant water and nitrogen uptake, these benefits were modulated by watering regime and dependent on soil microbial context. Under replant conditions, pre-colonization of G.11 rootstocks by R.irregularis led to significant increases in stomatal conductance in both well-watered and water-limited conditions. By notable comparison, soil pasteurization appeared to negate this benefit. At the same time (in live soil), AMF inoculation was associated with significantly greater total ¹⁵N isotope tracer recovery under well-watered (but not water-stressed) conditions. The second experiment focused on the role host genotype/AMF species identity plays in reducing pathogen infection in apple roots. The ability of the AMF Claroideoglomus etunicatum and Claroideoglomus claroideum to reduce infection by the fungal replant pathogen Rhizoctonia solani AG-5 in G.41 and G.890 was tested. Findings showed AMF directly limited pathogen establishment. The degree of protective effect varied, however, according to the particular host genotype/AMF species combination. Both experiments establish clear evidence of AMF species directly functioning in beneficial roles with commercially available apple rootstock genotypes and provide insight into specific AMF-rootstock relationships with the potential to promote more resilient and sustainable apple production. The context-dependency of these outcomes highlights the need to move beyond generalized strategies for enhancing mycorrhizal benefits in orchard systems.