The control of bacterial plant diseases is very challenging and often relies on the application of copper compounds, although the frequent emergence and spread of resistant bacterial strains compromise their efficacy. Additionally, copper-based compounds raise environmental and human health concerns, leading to their inclusion in the European Commission’s list of candidates for substitution. As a promising and sustainable alternative, we investigated the efficacy of biogenic zinc oxide nanoparticles (ZnO-NPs) in protecting tomato plants against Pseudomonas syringae pv. tomato (Pst), the causal agent of bacterial speck disease. ZnO-NPs exhibited significant in vitro antibacterial activity (EC₉₅ = 17.0 ± 1.1 ppm) against the pathogen. Furthermore, when applied to the foliage of tomato plants at 100 ppm before or following Pst inoculation, they induced significant reductions in symptom severity and bacterial growth in planta, which were comparable to those shown by plants treated with acibenzolar-S-methyl, a plant defense inducer. Gene expression assessed by qPCR revealed the involvement of the systemic acquired resistance (SAR) pathway in tomato plants treated with ZnO-NPs before inoculation, suggesting that the observed protection could be due to a priming effect. Finally, infected plants showed oxidative stress, with higher H₂O₂ and malondialdehyde (MDA) contents. ZnO-NPs reverted this effect, containing the content of the above molecules, and stimulated the production of metabolites involved in dealing with oxidative perturbations (carotenoids and phenols), while unaffecting flavonoids and anthocyanins.

Excessive nitrogen (N) fertilizer usage in agriculture has prompted the exploration of sustainable strategies to enhance nitrogen use efficiency (NUE) while maintaining crop yield and quality. Processed tomatoes (Solanum lycopersicum L.) were grown for two years (2023 and 2024) following a two-way factorial randomized complete block (RCBD) design, considering three biostimulants and three N regimes as two factors, to assess their morphophysiological, biochemical, anatomical and yield performances. Nitrogen application significantly influenced biomass accumulation, the leaf area index (LAI), nitrogen uptake and yield with notable comparable values between reduced and optimal nitrogen dose, indicating improved nitrogen use efficiency. Biostimulants showed limited effects alone but enhanced plant performance under reduced nitrogen conditions, particularly improving chlorophyll content, crop growth, N uptake, yield and anatomical adaptations. Moreover, compared to 2024, biostimulant application enhanced tomato growth more evidently in 2023 due to environmental variations, likely due to the occurrence of stress conditions. Importantly, biostimulants, together with N regimes, i.e., optimal and reduced doses, showed improved anatomical traits, especially regarding leaf thickness and thickness between the two epidermises, indicating adaptive responses that may support sustained productivity under N-limited conditions. Among the biostimulants used, the processed tomatoes responded better to protein hydrolysate and endophytic N-fixing bacteria than to seaweed extract. These findings suggest that although biostimulants alone were not affected, integrating them with reduced N fertilization provides a viable strategy for optimizing tomato production, conserving resources and minimizing the environmental impact without compromising yield or quality.

The excessive nitrogen (N) fertilizers usage in agriculture has prompted the exploration of sustainable strategies to enhance nitrogen use efficiency (NUE) while maintaining crop yield and quality. This two-year (2023 and 2024) open-field experiments assessed the effects of three biostimulants, i.e., seaweed extract (BIOS1), protein hydrolysates (BIOS2), and endophytic N-fixing bacteria (BIOS3), under three N fertiliza-tion regimes, i.e., no nitrogen (N0), optimal (STD) and reduced (RED: 30% reduced than optimal) on processing tomatoes (Solanum lycopersicum L.) morphological, physiological, biochemical, anatomical and yield performances. Nitrogen application significantly influenced biomass accumulation, leaf area index (LAI), nitrogen uptake and yield with notable comparable results with reduced nitrogen rates to the optimal dose, indicating improved nitrogen use efficiency. Biostimulants alone had limited effects but showed synergistic benefits under reduced N conditions by enhancing chlorophyll content, crop growth, N-uptake and yield. Moreover, biostimulants application enhanced tomato growth more evidently in 2023 than 2024 due to environmental variations, likely due to the occurrence of stress conditions, with a prominent increment in dry matter accumulation and N uptake. Importantly, biostimulants together with N regimes i.e., STD and RED showed improved anatomical traits specially for Leaf thickness and thickness between the two epidermises, indicating adaptive responses that may support sustained productivity under N-limited conditions. Among biostimulants BIOS2 and BIOS3 responded better than BIOS1 by processing tomatoes. These findings suggest that although biostimulants alone were not affected but integrating them with reduced N fertilization provides a viable strategy for optimizing tomato production, conserving resources and minimizing environmental impact without compromising yield or quality