Arbuscular mycorrhizal fungi mitigate drought stress in citrus by modulating root microenvironment

This study aimed to evaluate effects of Funneliformis mosseae on plant growth and root exudate compositions and contents, soil and root phosphatase activity, soil glomalin concentrations, and thus soil aggregate stability and distribution in trifoliate orange under well-watered (75% of maximum water holding capacity) and drought stress (55% of maximum water holding capacity) conditions. After eight weeks of drought treatment, mycorrhizal fungal inoculation improved plant growth and exhibited altered composition of root exudates than non-inoculated treatment. Mycorrhizal fungal inoculation dramatically increased the relative abundance of phenolics (e.g., 2 H,8 H-Benzo[1,2-b:3,4-b’] dipyran-2-one,8,8-dimethyl), terpenoids (e.g., geijerene), and acids (n-hexadecanoic acid), while notably reduced the relative abundance of alkanes (e.g., tridecane, 2-methyl-), esters (e.g., hexanedioic acid and dimethyl ester), and amides (e.g.,13-docosenamide) in root exudates. Mycorrhizal fungal colonization profoundly increased easily extractable and total glomalin-related soil protein levels under two soil water regimes, which cemented soil macroaggregate (2–4 mm size) formation, thereby, improving soil aggregate stability. Mycorrhizal fungal-inoculated plants represented higher soil acid, alkaline and total phosphatase activities, irrespective of well-watered and drought treatment. The results suggest that mycorrhizal plants had improved root microenvironment to mitigate drought damage through changes in root exudate components along with glomalin, phosphatase, and soil aggregate stability in the mycorrhizosphere.