Complementary power of Bacillus strains: mitigating drought–dust stress and boosting yield in safflower (Carthamus tinctorius L.)

In arid regions, dust often accompanies drought, acting as a complementary abiotic stressor that exacerbates its detrimental effects on plants. This study investigated the potential of two plant growth-promoting bacteria (PGPB), Bacillus amyloliquefaciens and B. halotolerans, to mitigate the physiological, biochemical, and growth impacts of combined drought-dust stress on safflower (Carthamus tinctorius L.). A factorial experiment, arranged in a randomized complete block design, tested individual and combined bacterial inoculations under three irrigation intervals (well-watered to severe drought: every 4, 6, and 8 days) and two dust levels (0 or 0.57-1.13 g m⁻² day⁻¹, simulating natural deposition). Results demonstrated that combined stress inflicted greater damage than either stressor alone, with drought being the more severe factor. Drought significantly reduced mineral element uptake, soluble sugars (by up to 40%), chlorophyll b, seed weight per plant (by 45%), and plant dry weight, while increasing phenols, proline, and carotenoids. Dust primarily reduced phosphorus and soluble sugars but increased ascorbic acid and phenols. Bacterial inoculation generally enhanced key physiological and yield traits. The combined inoculation of both PGPB generally provided the most consistent and broad-spectrum improvement across traits, although single-strain inoculations were superior for specific parameters, indicating functional complementarity. Specifically, the dual inoculation enhanced nitrogen content by 35-64% and chlorophyll b levels across all conditions. While B. halotolerans and the combined treatment were superior for increasing seed weight (by up to 384% under dust stress), B. amyloliquefaciens inoculation resulted in the greatest increase in plant dry weight. Therefore, the combined inoculation of B. amyloliquefaciens and B. halotolerans is identified as a highly effective biological strategy to bolster safflower resilience and yield under concurrent drought and dust stress, demonstrating immediate relevance for sustainable agriculture in arid regions.