Root and shoot phenotyping elucidate plant growth‐promoting microorganisms effects on wheat under water deficit

Water scarcity represents a major constraint to crop productivity, increasing interest in sustainable strategies such as plant growth-promoting microorganisms (PGPM). This study investigated the effects of a commercial PGPM consortium, composed of four Bacillus spp. strains and two arbuscular mycorrhizal fungi species, on wheat (Triticum aestivum L. ‘Julie’) grown under well-watered and non-watered regimes. The product was applied once as seed inoculation (MIX1) or repeatedly during plant growth (MIXR). Two controlled-environment experiments were performed using high-throughput phenotyping platforms: a 15-day rhizobox experiment to monitor root and shoot growth (Experiment 1) and a 25-day pot experiment to assess shoot growth, biomass, and photosynthetic activity (Experiment 2). Water regime was the main factor influencing plant performance. In Experiment 1, MIX1 promoted greater root expansion under water deficit, whereas MIXR was more associated with root area efficiency, suggesting modulation of root functional traits rather than improvements in root elongation. In Experiment 2, non-watered MIXR-treated plants maintained a high performance in shoot growth, root and shoot fresh biomass, and photosystem II efficiency compared to non-watered MIX1- and non-inoculated- treated plants. PGPM effects on wheat at early developmental stages were strongly influenced by the application strategy. High-throughput phenotyping enabled the identification of microbial inoculation strategies that sustain plant functions under water deficit by resolving temporal dynamics of growth allocation and photosys- tem II activity. Therefore, the results provide preliminary evidence for optimizing PGPM application strategies under water-limited conditions, while field-scale trials are needed to assess agronomic feasibility.