Mycorrhizal Inoculation Enhances Drought Resilience in Citrus Seedlings of Two Cultivars by Modulating Gas Exchange...

Water scarcity and climate variability threaten citrus production in semi-arid regions, requiring strategies to improve drought resilience. This study evaluated the physiological and hormonal responses of two citrus cultivars, alemow (Citrus macrophylla Wester) and 'Cleopatra' mandarin (Citrus reshni Hort. Ex Tanaka), inoculated with arbuscular mycorrhizal (AM) fungi (Rhizophagus irregularis + Funneliformis mosseae) and subjected to drought stress imposed by progressive soil drying (water withholding) and quantified by volumetric soil water content (θ_v) classes: >0.20 cm³ cm^-3 (well-watered), 0.05-0.20 cm³ cm^-3 (moderate drought), and <0.05 cm³ cm^-3 (severe drought). Gas exchange, plant water status, and abscisic acid (ABA) dynamics were monitored to assess cultivar-specific effects of AM symbiosis. Under well-watered conditions, +AM plants exhibited higher photosynthetic rates than non-inoculated plants, with a stronger response in Macrophylla. During drought, contrasting patterns emerged: +AM Macrophylla maintained higher stomatal conductance and photosynthesis, with foliar ABA increasing only under severe stress, suggesting that non-hormonal mechanisms support gas exchange. In Cleopatra, AM inoculation was associated with higher root-derived ABA and earlier stomatal closure, suggesting a more conservative water-use strategy under soil drying conditions; however, the benefits were limited to moderate stress and decreased beyond a stomatal conductance threshold. These findings reveal that AM symbiosis enhances drought resilience through contrasting mechanisms: hydraulic stabilization predominates in Macrophylla, whereas hormonal (ABA-mediated) regulation drives the response in Cleopatra. This cultivar-dependent modulation highlights the importance of developing AM-based strategies adapted to each cultivar for effective citrus drought management. Combining AM inoculation with irrigation-saving practices could improve water productivity and support climate-smart citrus production.