Microbial Inoculation Differentially Affected the Performance of Field-Grown Young Monastrell Grapevines Under Semiarid Conditions, Depending on the Rootstock

A trial was conducted from 2017 to 2023 in a 0.2 ha irrigated vineyard located in a semiarid area of southeastern Spain, using field-grown young vines (0–6 years old) of Vitis vinifera L. cv. Monastrell grafted onto three rootstocks: 140Ru, 161-49C, and 110R. The main objective was to evaluate the effect of early co-inoculation in the field using commercial microbial inoculants containing arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), and a mycorrhizal helper bacterium (MHB) on young vine performance. We assessed the impact of microbial inoculation and its interaction with the rootstock on soil environment, plant water relations, leaf gas exchange, plant nutrition, growth, yield, and berry quality. Mycorrhizal colonization rates in root samples showed similar values in inoculated and non-inoculated vines across all of the rootstocks; however, inoculated vines grafted onto 140Ru showed significantly higher concentrations of total glomalin in the soil compared to their non-inoculated counterparts. Microbial inoculation altered the soil environment, leading to increased oxygen diffusion rate (161-49C), organic matter decomposition rate (140Ru), soil CO2 flux (110R, 140Ru), and soil H2O flux (110R) values in the rhizosphere of inoculated vines. Additionally, inoculated vines grafted onto 140Ru and 161-49C exhibited improved vegetative and reproductive development, enhancing productive water use efficiency (WUEyield), whereas inoculated vines on 110R showed poorer soil–plant water relations, growth, yield, and WUEyield compared to non-inoculated vines. Microbial inoculation also led to a significant decrease in must phenolic content, particularly in 140Ru, unlike 110R and 161-49C. These findings indicate that early microbial inoculation had a rootstock-dependent impact on the performance of young grapevines.