Mycorrhizal Inoculation Enhances Drought Resilience in Citrus Seedlings of Two Cultivars by Modulating Gas Exchange and Hormonal Signaling

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 (<i>Citrus macrophylla</i> Wester) and '<i>Cleopatra</i>' <i>mandarin</i> (<i>Citrus reshni</i> Hort. Ex Tanaka), inoculated with arbuscular mycorrhizal (AM) fungi (<i>Rhizophagus irregularis</i> + <i>Funneliformis mosseae</i>) and subjected to drought stress imposed by progressive soil drying (water withholding) and quantified by volumetric soil water content (<i>θ_v</i>) 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.