Mitigating negative impacts of drought on oak seedlings performances through plant growth-promoting rhizobacteria

Drought stress during the plant's growing season is a serious constraint to plant establishment in arid and semiarid Mediterranean ecosystems. Plant growth promoting rhizobacteria (PGPR) as environmentally friendly and innovative management approach can be used to produce seedlings better adapted to these environments. We tested native PGPR strains isolated from drought-tolerant tree and shrub species originating from two climatically contrasting regions: hot-dry (Dehloran) and milder Mediterranean climate (Ilam). These strains were used as a biological input to improve the morphological and physiological traits of Brant's oak (Quercus brantii Lindl.) seedlings in a greenhouse experiment. Seedlings were inoculated with various drought-tolerant strains or a combination of all bacteria (Bmix) and their morpho-physiological traits were measured under three levels of water stress (80% field capacity, FC80: no stress; 60% FC, FC60: moderate water stress and 40% FC, FC40: severe water stress). Strains from Dehloran were the most performant (Bacillus anthracis, B. licheniformis, B. cereus) except Stenotrophomonas maltophilia which originated from Ilam. They demonstrated the ability to solubilize insoluble phosphates (except for the B. licheniformis). They were also all able to produce auxin and the ACC-deaminase enzyme (except S. maltophilia). PGPR application, water stress treatments, and their interaction significantly influenced all the morphological and physiological seedling traits. Most traits significantly (p < 0.01) decreased under FC40, in particular average leaf area, shoot fresh weight, and shoot dry weight. This decrease was particularly marked for the control (no inoculation), as the leaf area was reduced by 60% in FC40. In FC40, all bacterial treatments, especially B. cereus and Bmix efficiently improved seedling morphological traits and seedling quality index. The highest amount of chlorophyll a in FC40 was recorded for Bmix and B. licheniformis isolates. Total phenolic content and malondialdehyde levels, as well as proline, showed an increase under water stress conditions, particularly at FC40, irrespective of inoculation conditions. The amounts of malondialdehyde without inoculation increased significantly under water stress and the content were 1.3 and 2 times higher in FC60 and FC40, respectively, compared to FC80 with no inoculation. Under water stress, seedling performances and quality were impacted and the production of substances associated with physiological defense mechanisms, increased. However, the application of PGPR was able to mitigate these effects (in particular for B. cereus and Bmix). We concluded that application of drought tolerant PGPRs can be a cost effective and ecologically sustainable method for producing oak seedlings better adapted to water-scarce environments.