Dual Inoculations of Dark Septate Endophytic and Ericoid Mycorrhizal Fungi Improved the Drought Resistance of Blueberry (Vaccinium corymbosum L.) Seedlings

Dark septate endophytes (DSE) and ericoid mycorrhizal fungi (ERMF) are employed to augment the abiotic stress resistance of fruits. However, their potential functions in enhancing the drought resistance of blueberry, an economically important fruit, remain unclear. Thus, this study aims to identify optimal inoculation combinations to enhance the drought resistance of blueberry seedlings. Specifically, the effects of single and dual inoculations with DSE (Cladosporium cladosporioides, D79) and ERMF (Oidiodendron citrinum, N12) on seedling physiology and metabolism were explored under varying drought conditions. The results showed that dual inoculation significantly improved leaf physiological characteristics. Under severe drought stress, the 1:2 DSE:ERMF ratio (D1N2) notably increased leaf relative water content (RWC) and reduced electrolyte leakage by up to 42.1% compared with the non-inoculated control. Dual inoculation also significantly decreased malondialdehyde (MDA) content, with the smallest increase observed in D1N2. Regarding antioxidant enzymes, dual inoculation sustained higher superoxide dismutase (SOD) activity under moderate drought and minimized the decline in SOD activity under severe drought (the lowest decrease was 36.4% in D1N2 versus 56.7% in CK). Moreover, the antioxidant losses under drought stress were reduced by upregulating various metabolic processes, especially the biosynthesis of phenylalanine, tyrosine, and tryptophan. A comprehensive evaluation suggested that inoculation with a 1:2 mixture of DSE and ERMF most effectively improved blueberry drought resistance, primarily by enhancing water and metabolite supply and stimulating the antioxidant defenses.