The effect of microbial fertilizers on the growth, some physiological and biochemical characteristics, and yield of cucumber plants grown under water deficit

Cucumber (Cucumis sativus L.) is among the most extensively cultivated vegetable crops worldwide. Its yield potential is highly sensitive to the amount of irrigation water supplied throughout the plant's developmental stages. Drought stress induces a range of physiological and biochemical alterations in plants, notably disrupting the homeostasis of reactive oxygen species (ROS) within chloroplasts of the leaves. These ROS cause oxidative damage to cellular components such as proteins, lipids, and DNA, thereby impairing normal cellular functions. Nonetheless, the plant's response to water stress can be modulated by beneficial soil microorganisms, particularly arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR). Despite growing evidence, the role of these microorganisms in enhancing plant tolerance to deficit irrigation conditions remains incompletely understood. In the present study, the effects of three irrigation regimes (100%, 75%, and 50% of available soil water capacity, ESTK) and five fertilization strategies (control, chemical fertilizer, PGPR, AMF, and a combined PGPR+AMF treatment) were evaluated on the morphological, physiological, biochemical, fruit quality attributes, and soil biological activities of cucumber plants under controlled conditions. Regarding morphological traits, microbial fertilizer applications significantly improved plant height, stem diameter, leaf number, leaf area, and both fresh and dry biomass. While water stress led to a general decline in growth parameters, AMF and PGPR applications effectively mitigated these adverse effects. For physiological responses, leaf relative water content (YOSI), chlorophyll a, chlorophyll b, and total chlorophyll content, declined under increasing water deficit. However, microbial applications alleviated these reductions. AMF treatments were particularly effective in maintaining chlorophyll content and leaf water status, whereas PGPR improved membrane stability; the combined application enhanced these responses more markedly. In terms of biochemical indicators, drought stress resulted in elevated levels of malondialdehyde (MDA), superoxide dismutase (SOD), peroxidase (POD), proline, and total phenolics, reflecting heightened oxidative and osmotic stress responses. Microbial treatments reinforced these biochemical defenses, as AMF and PGPR notably reduced MDA accumulation while increasing antioxidant enzyme activities and proline content. Microbial fertilizer applications also significantly enhanced fruit yield and quality traits, including fruit number, total yield, external appearance, taste, firmness, and color. Moreover, the concentrations of essential macro- and micronutrients (N, P, K, Ca, Mg, Fe, Mn, Zn, and Cu) in fruits increased, with AMF-based treatments particularly improving phosphorus and zinc content. Soil microbial activity indicators—such as root colonization rates, AMF spore counts, and bacterial population densities—were substantially elevated under microbial inoculation treatments. Combination treatments yielded the highest values, though single microbial applications also outperformed both the chemical fertilizer and control treatments. Additionally, microbial applications led to significant increases in soil enzyme activities, including glomalin-related soil protein (GRSP), dehydrogenase, alkaline phosphatase (ALP), and β-glucosidase. GRSP accumulation contributed to improved soil structure, while the other enzymes indicated enhanced microbial activity and nutrient transformation potential. In conclusion, microbial fertilizer applications, especially the combined use of AMF and PGPR, conferred multiple agronomic and ecological benefits by enhancing cucumber plant growth, physiological and biochemical resilience under water-limited conditions, fruit quality, and soil biological function. While combined treatments offered the most pronounced effects, single microbial inoculants also demonstrated excellent performance across several parameters. Nevertheless, further investigations focusing on the comparative efficacy of diverse microbial strains, dynamics of the rhizosphere microbiome, and molecular level soil–microbe–plant interactions are warranted to advance the understanding of microbial fertilization.