Foliar Biostimulants Are Associated with Improved Physiological and Antioxidant Status of “Keitt” Mango Under Deficit Irrigation

Mango (Mangifera indica L.) cultivation is expanding into arid regions where high evaporative demand and restricted irrigation frequently constrain tree performance, creating a need for management strategies that reconcile water saving with sustained productivity. The present two-season field study evaluated the responses of “Keitt” mango grown in a hot, sandy environment to three foliar treatments, Ascophyllum nodosum extract (2%), nano-silicon (100 mg L−1), and ascorbic acid (200 mg L−1), under three deficit irrigation regimes (100%, 75%, and 50% of crop evapotranspiration, ETc). Growth, yield, plant–water relations, photosynthetic pigments, oxidative-stress markers, and antioxidant defenses were assessed and integrated through multivariate analysis. Deficit irrigation progressively reduced shoot growth, yield, relative water content, stomatal conductance, and leaf chlorophyll, while elevating proline, malondialdehyde, and hydrogen peroxide. Both A. nodosum and nano-silicon were consistently associated with attenuated responses across all irrigation levels, with higher antioxidant enzyme activity and retention of a disproportionate share of yield under severe deficit, corresponding to an increase in water productivity of up to approximately 31% at 50% ETc. The closely correlated, opposing shifts in growth and oxidative traits indicate associations among water status, photosynthetic characteristics, and antioxidant responses. These findings describe patterns of co-variation and should not be interpreted as evidence of direct causal mechanisms. Overall, these foliar biostimulants, especially A. nodosum, offer a promising, practical means of complementing deficit irrigation in arid mango production.