Arbuscular mycorrhizal fungi and mycorrhizal helper bacteria co-inoculation alters drought-associated physiological traits and leaf abscisic acid concentration in citrus rootstocks

Plants rely heavily on root-associated microflora to mitigate various environmental stresses such as drought and the use of arbuscular mycorrhizal fungi (AMF) as a biological instrument to enhance plant drought tolerance, through the formation of mutualistic associations, seems like a novel strategy for sustainable agriculture. Thus, a glasshouse study was undertaken with two citrus rootstocks (Troyer citrange and Cleopatra mandarin) under 20 days of differential moisture regimes (well-watered and water-stress condition), after 270 days of inoculation with two AMF strains (Glomus intraradices and mixed strains of Gigaspora decipiens + Glomus mosseae) alone or in combination with mycorrhizal helper bacteria (MHB), i.e., phosphate solubilising bacteria (PSB), Azospirillum or Providencia, with an objective of testing the hypothesis that AMF and MHB might influence ABA accumulation in citrus leaves, thereby altering physio-biochemical response to declining soil water status. The seedlings dually inoculated with AMF and MHB exhibited significantly higher relative water content of leaf as well as rhizospheric soil. Glomus intraradices in association with PSB or Azospirillum accumulated higher concentrations of osmolytes (phenols, proline and total soluble sugars) and antioxidant metabolites (glutathione and carotenoids) in leaves under WS condition. Significantly altered leaf tissue ABA accumulation was noticed in dual inoculated treatment of Glomus intraradices and Azospirillum for Troyer citrange (3.517 µg g-1 F.W.) and G. intraradices and PSB for Cleopatra mandarin seedlings (3.079 µg g-1 F.W.), under water-stress condition, as compared to other microbial inoculants including control. Thus, water deficit stress induced physiological and biochemical response pathways could be modified by AMF and MHB. Under these experimental conditions, Troyer citrange exhibited higher relative water content, photosynthetic activity, and stomatal conductance under drought conditions than Cleopatra mandarin seedlings, offering a potential microbial intervention strategy for mitigating osmotic stress in semi-arid environments.