Arbuscular mycorrhizal (AM) fungi form mutualistic relationships with the majority of land plants and are an important part of the soil microbial community in natural and agricultural ecosystems. Those fungi promote water and nutrient acquisition of their host plant and regulate the allocation of photosynthetic carbon to soil. Both crop variety and environment affect naturally occurring mycorrhizal abundance in roots, but the relative importance of those factors for mycorrhization is largely unknown. In a field study covering a large pedoclimatic gradient across four European sites, we (i) compared the abundance of AM fungi in the roots of ten modern winter wheat (Triticum aestivum L.) varieties, (ii) evaluated the relative importance of variety and site for the variability in root colonization by AM fungi, and (iii) tested the relationship between mycorrhizal abundance and grain yield. Root colonization by arbuscules and hyphae ranged from 10-59% and 20-91%, respectively, across all samples and varied by 8% and 18%, respectively, among varieties when averaged across sites. Variance decomposition analysis revealed a ten times higher importance of site than variety for AM fungal root colonization. Specifically, we found the highest mycorrhizal abundance on the site with the most arid conditions and the lowest on the sites with low soil pH and high nutrient availability. Despite the low variability in mycorrhizal abundance among varieties, they showed significant differences in both arbuscular and hyphal root colonization. However, this did not translate into an increase in yield as no significant relationships between mycorrhizal abundance at flowering and grain yield were detected. The consistent differences between varieties in root colonization by AM fungi across European field sites underline that genetic drivers of mycorrhization are to some extent independent of the site. This highlights the relevance of breeding practices to shape a wheat variety's capacity for mycorrhizal symbiosis across a range of environmental conditions.
