Effects of Solution Applied and Different Encapsulation Method of Plant Growth Promoting Bacteria (PGPB) on the Physiological Response of Sunflower to Drought Stress

Sunflower (Helianthus annuus L.) is an important food, oil and bioenergy crop frequently affected by drought stress. Plant growth-promoting bacteria (PGPB) can enhance plant growth and resilience to water deficit. However, their efficiency often diminishes over time under prolonged stress due to reduced bacterial survival. This study investigated whether encapsulated forms of Bacillus subtilis, with and without humic acid supplementation, improves bacterial viability and photosynthetic performance of a drought-sensitive sunflower variety under water deficiency. Seedlings were grown in a greenhouse, with water deficit imposed at 30% field capacity for 29 days. Treatments included solution (B) or encapsulated forms (alginate beads alone (EB) or supplemented with humic acid (HEB)) under well-watered (W) and drought (D) conditions. Encapsulation, particularly with humic acid, increased bacterial viability, mitigating damage to photosynthetic reaction centers as reflected by subsequent morphophysiological measurements. Under drought, all PGPB treatments enhanced plant height (BD: 15.2%, HEBD: 14.6%, EBD: 6.1% vs. D), above-ground biomass (HEBD and BD significantly higher), Gsw (EBD: 3-fold, HEBD: 10-fold vs. D), ΦPSII (HEBD: 10%, BD: 4% vs. D), ETR (HEBD: 31% vs. D), and ChlF indices like PIABS (BD: 95%, HEBD: 81%, EBD: 53% vs. D). Stable encapsulation without premature release, especially with humic acid, sustained bacterial viability and prolonged benefits by protecting the bacteria against stress, improving root interactions and photosynthetic resilience. These findings demonstrate that encapsulation, particularly with humic acid, optimizes PGPB delivery and sustains physiological benefits under drought, highlighting its strong potential as a strategy for sustainable agriculture in semi-arid regions.