1. Introduction: The excessive and prolonged use of chemical fertilizers has adversely affected soil ecosystems and environmental sustainability. A considerable proportion of these fertilizers is lost through leaching caused by rainfall or irrigation, resulting in groundwater contamination. Continued dependence on chemical fertilizers therefore poses serious environmental challenges. Replacing chemical fertilizers with biological and organic alternatives is considered one of the most effective strategies for reducing these negative impacts. In this context, biofertilizers have emerged as sustainable substitutes that improve soil fertility while supporting environmentally friendly agricultural production. Among these, plant growth-promoting rhizobacteria (PGPR) have attracted considerable attention because of their ability to enhance plant growth through a wide range of direct and indirect mechanisms, ultimately improving crop productivity and quality.
2. Materials and methods: A factorial experiment was conducted in a completely randomized design (CRD) with three replications under greenhouse conditions at the Islamic Azad University, Kermanshah Branch, Iran, to evaluate the effects of plant growth-promoting rhizobacteria (PGPR) and putrescine on the quantitative and qualitative characteristics of tomato (Lycopersicum esculentum L.) cv. Sinda.The first experimental factor consisted of three bacterial inoculation treatments: no inoculation (control), Pseudomonas fluorescens, and Azospirillum lipoferum. The second factor included three concentrations of putrescine (0, 1.5, and 3 mM). Growth parameters, including growth rate, days to flowering, plant height, number of leaves, stem diameter, and total fruit yield, were recorded throughout the growing period. After harvest, fruit quality attributes, including soluble solids content (TSS), titratable acidity, lycopene concentration, and vitamin C content, were determined using standard analytical procedures.
3. Results and discussion: The combined application of PGPR and putrescine significantly improved both the quantitative and qualitative characteristics of tomato plants. Fruit soluble solids, titratable acidity, lycopene, and vitamin C contents increased following bacterial inoculation and putrescine application. These improvements can be attributed to the ability of PGPR to produce phytohormones, siderophores, and organic acids, thereby enhancing nutrient availability and uptake, particularly nitrogen and iron. Improved nutrient acquisition subsequently promotes photosynthetic activity, carbohydrate accumulation, and overall plant performance, leading to enhanced plant growth and improved fruit quality. Analysis of variance revealed that PGPR significantly affected all evaluated traits, including plant height, fruit diameter, number of flowers, number of fruits, root dry weight, shoot dry weight, total fruit yield, soluble solids, titratable acidity, lycopene, potassium, phosphorus, and calcium contents (P < 0.01). However, the effect on vitamin C content was not statistically significant. Likewise, putrescine significantly influenced all measured characteristics at the 1% probability level (P < 0.01). Moreover, the interaction between PGPR and putrescine significantly affected plant height, flower number, fruit number, fruit yield, titratable acidity, and phosphorus content (P < 0.01), indicating a synergistic effect between bacterial inoculation and putrescine application. The highest fruit yield (16 kg plant⁻¹) was obtained from plants inoculated with Pseudomonas fluorescens and treated with 1.5 mM putrescine. Plants inoculated with Azospirillum lipoferum and sprayed with 1.5 mM putrescine also exhibited high fruit yield, although lower than that achieved with P. fluorescens. In contrast, the lowest yield (7.75 kg plant⁻¹) was recorded in the untreated control, where neither bacterial inoculation nor putrescine was applied. The beneficial effects of putrescine under non-stress conditions may be associated with its antioxidant properties, its contribution to maintaining cellular cation–anion balance, and its potential role as a nitrogen source. Furthermore, PGPR have been reported to enhance phosphorus and potassium uptake, thereby improving plant nutritional status and stimulating vegetative and reproductive growth. The increased number of flowers and fruits observed in the present study is likely the result of enhanced nutrient uptake, improved photosynthetic efficiency, and increased metabolic activity induced by both PGPR and putrescine. The stimulation of the plasma membrane H⁺-ATPase by PGPR may further facilitate the uptake of essential mineral nutrients, particularly nitrogen, phosphorus, and potassium, ultimately contributing to higher fruit yield and improved overall crop productivity.
4. Conclusions: The combined application of plant growth-promoting rhizobacteria and foliar application of putrescine effectively enhanced tomato growth, fruit quality, and yield under greenhouse conditions. Among all treatments, inoculation with Pseudomonas fluorescens in combination with 1.5 mM putrescine produced the highest fruit yield. Therefore, this treatment can be recommended as a sustainable and environmentally friendly strategy for improving greenhouse tomato production while reducing reliance on chemical fertilizers.