Exhausted plant cell culture media as potential biostimulants to enhance plant growth and nitrogen use efficiency in tomatoes under optimal and reduced nitrogen supply

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Background Inefficient nitrogen (N) use leads to economic losses and environmental harm. Improving N use efficiency (NUE) is crucial. The use of plant biostimulants (PBs), integrated with good agronomic practices, offers a sustainable solution. Aims The study explores the potential use of exhausted cell culture media from plant cell cultures ( Nicotiana sp. and Gardenia sp.) as PBs, assessing their role in enhancing plant physiological status and NUE. It also compares their effectiveness against commercial PBs. Methods Two experiments (Experiments 1 and 2) were conducted using cherry tomato plants. In the first one, the two media were applied at different concentrations as a foliar spray to define the optimal dose. This outcome was used to set up Experiment 2, in which both media were compared to commercial PBs under optimal and reduced N supply. Measurements included plant growth, chlorophyll (Chl) content, and NUE indices. Moreover, Experiment 2 examined gene expression related to N assimilation, transport, and response to oxidative stress. Results Both experiments demonstrated a significant increase in leaf area (≈11%) and aboveground biomass (≈13%) using exhausted media. Furthermore, Experiment 2 demonstrated enhancements in Chl content (≈10%) and plant N accumulation (≈20%). These findings indicate that exhausted media has a comparable efficacy to commercial PBs. Conclusions The study underscores the practical viability of exhausted cell culture media as effective PBs for tomato growth in both N conditions. This approach aligns with sustainability objectives by repurposing by‐products to enhance plant resilience against abiotic stress and potentially improve the efficiency of N fertilizers.