Biostimulants (AMF and PGPRs) have the potential to mitigate the negative impacts of water-deficit stress on dwarf chicory (Cichorium pumilum Jacq.)

Water deficit represents a critical abiotic stress factor that adversely affects multiple physiological processes, reproductive development, and metabolic pathways in plants, ultimately compromising crop productivity. To investigate the effects of biostimulants on physiological traits, fruit yield components, and inulin content in dwarf chicory roots under varying irrigation regimes, a split-plot experiment arranged in a randomized complete block design with three replications was conducted during the 2017–2018 growing season at the experimental station of Azarbaijan Shahid Madani University's Faculty of Agriculture, Tabriz, Iran. Plants were irrigated at three different soil moisture levels: 55 %, 35 %, and 20 % of the total available water capacity. The biostimulant treatments consisted of an untreated control, plant growth-promoting rhizobacteria (PGPRs), and arbuscular mycorrhizal fungi (AMF). Water-deficit stress resulted in a reduction in the leaf area index (LAI), relative water content (RWC), chlorophyll a, b, and total chlorophyll content, as well as carotenoid content, in leaves, while increasing proline and total soluble sugar levels. The highest LAI, RWC, and soluble sugar content in leaves were obtained from the application of PGPRs. In contrast, the highest proline content was obtained when AMF was applied. All physiological traits significantly decreased in the control compared to the biostimulant treatments, except for proline and soluble sugar contents, which increased higher than the control with rising water-deficit stress using biostimulants. Water-deficit stress significantly reduced fruit yield and its components in dwarf chicory plants. This reduction was less pronounced in plants treated with biostimulants, particularly PGPRs, compared to those inoculated with non-biostimulants. The occurrence of moisture stress and the plant's defensive response to the induced stress resulted in a significant reduction in inulin content in dwarf chicory roots, particularly in the non-biostimulant treatment, which is considered a key factor in determining the economic yield of this crop. However, the application of biostimulants- particularly PGPRs -enabled dwarf chicory plants to tolerate better and significantly enhanced inulin accumulation in their roots compared to the control under severe water-deficit stress conditions. • PGPRs/AMF could diminish water-deficit stress adverse effects on some physiological characteristics. • PGPRs or AMF improved fruit yield and its components under water-deficit stress conditions. • The highest inulin content in roots obtained from biostimulants especially PGPRs at all irrigation regimes.