Hormonal modulation by omeprazole improves tomato growth under single and combined water and nitrogen deficits

Omeprazole (OMP) has emerged as a promising protective compound against abiotic stress and a potential plant growth promoter, yet its role under combined stress and underlying mechanisms remains unclear. This study assessed the hormonal response of tomato plants to single and combined nitrogen (N) and water (W) deficits (50%N and/or 50%W) and the impact of OMP application to the root zone (0 and 1 μM) under these conditions. Combined deficit induced distinct hormonal shifts, including decreased shoots' abscisic acid (ABA) and indole-acetic acid (IAA), alongside increased salicylic acid (SA), jasmonic acid (JA) and jasmonic acid-isoleucine (JA-Ile). OMP consistently improved plant performance irrespective of the growth conditions, increasing total dry weight by 23% and enhancing nitrogen use efficiency and water use efficiency by 23% and 17%, respectively. Root growth stimulation in OMP-treated plants correlated with higher root IAA under control (+9%), N deficit (+27%), and W deficit (+20%) conditions, while combined deficit plants exhibited 35% lower root ABA without significant root biomass gain. OMP also improved photosynthetic rates and decreased shoots' lipid peroxidation, suggesting a protective effect. Hormonal modulation by OMP included lowering stress-related hormones - restoring JA and JA-Ile to control levels and reducing phaseic acid and SA under W deficit - while promoting IAA, a growth-related hormone. Collectively, these findings indicate that the combined deficit triggers a distinct hormonal profile from single deficits, and that OMP acts as both a stress alleviator and a growth promoter under N and W limitations by modulating hormonal, physiological and biochemical responses in a condition-specific manner.