Comparative Effects of Chemical and PGPR-Based Biostimulants on Growth, Mineral Uptake, and Biochemical Responses of Rosmarinus officinalis under Drought Stress

Background Rosmarinus officinalis is a medicinal and aromatic plant valued for its high levels of phenolics, flavonoids, and essential oils. However, drought stress can negatively affect plant growth, nutrient uptake, and the accumulation of bioactive compounds. Biostimulants are increasingly used to mitigate the adverse effects of water deficiency and improve plant performance. This study aimed to evaluate the effects of different drought levels and selected biostimulants on growth, mineral nutrition, physiological characteristics, and phytochemical composition of rosemary. Results Restricted irrigation significantly reduced growth parameters, mineral uptake (except Fe and Zn), and most biochemical traits, while chlorophyll content remained largely unchanged. Drought stress also decreased the accumulation of several phenolic compounds. Biostimulant applications alleviated many of these negative effects. Gibberellic acid was the most effective treatment, significantly increasing seedling and root length, dry weight, potassium uptake, total flavonoids, antioxidant activity, and phenolic compounds such as vanillin, rutin, rosmarinic acid, and chrysin. Ascorbic acid improved root fresh weight, total phenolics, and resveratrol content. Frateuria aurantia enhanced total carotenoids, quercetin, and Ca, Mg, and Mn uptake, and produced the greatest reduction in MDA levels. In contrast, Bacillus megaterium showed relatively limited effects, mainly increasing cinnamic acid content. Conclusions The findings indicate that biostimulants can mitigate drought-induced stress in rosemary. In particular, gibberellic acid and ascorbic acid were highly effective in improving plant growth, physiological performance, and phytochemical quality under water-limited conditions. Overall, biostimulant applications represent a promising strategy to enhance rosemary productivity and bioactive compound accumulation under drought stress.