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Open Access

ARTICLE

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

Dilek Killi1,2,#,*, Hüsna Dolu1,2, Masud Omar Barre3, Gamze Kaya4,5, Deniz Sezlev Bilecen4
1 National Research Council of Italy, Institute of Sustainable Plant Protection (CNR-IPSP), Florence, Italy
2 Plant Production and Technologies Department, Konya Food and Agriculture University, Konya, Türkiye
3 Department of Biotechnology, Graduate School, Konya Food and Agriculture University, Konya, Türkiye
4 Department of Molecular Biology and Genetics, Faculty of Agriculture and Natural Sciences, Konya Food and Agriculture University, Konya, Türkiye
5 Department of Molecular Biology and Genetics, Faculty of Science, Bilkent University, Ankara, Türkiye
* Corresponding Author: Dilek Killi. Email: email, email
# Present Address: Department of Soil Science and Plant Nutrition, Faculty of Agricultural Sciences and Technologies, Yasar University, Bornova, Izmir, Türkiye
(This article belongs to the Special Issue: Plant Growth Regulators (PGRs) and Plant Stress)

Phyton-International Journal of Experimental Botany https://doi.org/10.32604/phyton.2026.082888

Received 25 March 2026; Accepted 05 June 2026; Published online 20 July 2026

Abstract

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.

Keywords

Bacillus subtilis; bacterial encapsulation; drought stress; humic acid; photosynthetic efficiency

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