Evaluation of commercial arbuscular mycorrhizal fungal inoculants in grapevine rootstocks through root colonization, community profiles and plant responses to drought

Background Drought increasingly constrains viticulture and arbuscular mycorrhizal fungal (AMF) inoculants are claimed to enhance grapevine resilience to this stress. However, whether inoculated AMF strains establish in roots, restructure resident communities, and consistently enhance plant performance across different rootstocks and water regimes remains poorly understood. In a factorial glasshouse pot experiment, three ungrafted grapevine rootstocks (SO4, Börner and Vinto) were inoculated with one of three commercial AMF products (A, B or C) or left uninoculated, and were evaluated during establishment, drought and recovery phases. We quantified mycorrhization frequency and intraradical structures by microscopy, characterized root AMF communities by Glomeromycota-targeted partial SSU rRNA gene amplicon sequencing at establishment, and measured root and shoot dry weight, midday leaf water potential, proline, mineral nutrition, and leaf optical traits. Results Inoculation was associated with higher mycorrhization frequency and intraradical structure frequencies than the Control in several rootstock-inoculant comparisons, mainly during establishment; rootstock dependence was supported only for hyphal frequency. Most root profiles were dominated by one virtual taxon (VTX). Exploratory tests detected treatment differences in VTX00114 occurrence (Holm-adjusted P = 0.018), but not VTX relative abundance. VTX00114, assigned to Rhizoglomus irregulare , was detected in 50.0% and 33.3% of roots inoculated with Inoculants A and B, respectively, but not in Control roots or those receiving Inoculant C. In Börner, all inoculants increased root dry weight relative to the Control under well-watered conditions, whereas Inoculant C decreased root dry weight under drought (supported by an inoculation × water interaction). In SO4, Inoculants A and B were associated with reduced shoot dry weight during establishment. Drought-affected plants exhibited more negative leaf water potentials, and drought-stressed Control plants accumulated higher proline concentrations than well-watered Controls. However, no consistent inoculation-related improvements in leaf water potential, proline, mineral nutrition or optical traits were observed. Conclusions Inoculation-associated mycorrhization and community patterns were not consistently accompanied by improved plant performance. Root colonization, community profiles, inoculant establishment, and functional benefit must be evaluated as distinct outcomes. Furthermore, responses under glasshouse pot conditions should be interpreted within specific product × rootstock × water combinations, as they may not directly translate to field vineyard conditions.