Cleopatra mandarin (Citrus reshni Hort. Ex Tan.) modulate physiological mechanisms to tolerate drought stress due to arbuscular mycorrhizal fungi and mycorrhizal helper bacteria

The influence of Glomus intraradices colonisation on growth and reactive oxygen metabolism of Cleopatra mandarin (Citrus reshni Hort. Ex Tan.) seedlings treated with mycorrhizal helper bacteria, that is, phosphate solubilising bacteria containing a mixture of Bacilus subtilis and B. megaterium, Azospirillum braisilence or Providencia sp.) in potted culture was studied under well-watered or drought stress conditions. Results indicate that mycorrhizal inoculation increased plant growth and nutrient acquisition independent of the water regime, particularly when associated with mycorrhizal helper bacteria. Exposure of plant to drought stress led to generation of superoxide radicals and hydrogen peroxide in leaf tissues, however, their concentrations were lower in seedlings inoculated with G. intraradices and phosphate solubilising bacteria, as compared to other treatments including control. This particular treatment also increased total glutathione content and enhanced antioxidant enzyme activity in plant and microbial activity in soil. Mycorrhizal colonization was positively correlated with antioxidant metabolite, plant antioxidant enzymes, rhizospheric microbial activity and microbial biomass carbon and negatively correlated with reactive oxygen species under drought stress, which indicated that its inoculation could enhance plant defence system and alleviates oxidative damages to membrane lipids and proteins.