Foliar Magnesium Application Modulates Growth and Physiological Responses of Soursop and Custard Apple Under Salt Stress

Salt stress significantly limits the growth of Annonaceae species in tropical regions. This research aimed to test foliar Mg applications to alleviate the effects of salt stress on the growth and physiology of Annonaceae species. Two simultaneous experiments were conducted in a randomized block design in a 2 × 3 factorial scheme (n = 4). The factors were: two levels of control water salinity (0.5 and 2.5 dS m−1), three doses of foliar Mg (0, 1, and 2 mL L−1), and four replicates. A salinity of 2.5 dS m−1 reduced growth in both species. However, Mg supplementation effectively mitigated these deleterious effects. In Soursop (Annona muricata L.), 1 mL L−1 of Mg increased salt tolerance, enhancing carboxylation efficiency and total dry mass by approximately 33% and 44%, respectively, compared with 0 Mg under saline conditions. For custard apple (Annona squamosa L.), a dose of 2 mL L−1 yielded the best results, maintaining higher water use efficiency (17.67%), intrinsic water use efficiency (18.51%), and total chlorophyll content (32.27%) compared with plants receiving 0 Mg under saline conditions. Foliar supplementation with Mg is a viable strategy to alleviate saline stress in Annonaceae, with A. muricata requiring 1 mL L−1 and A. squamosa 2 mL L−1 to optimize physiological performance and biomass production at 2.5 dS m−1.