Interactive effects of citric acid and tryptophan on cobalt bioavailability and oxidative stress responses in Salvia officinalis under controlled contamination conditions

The accumulation of cobalt (Co) in agricultural systems poses a serious threat to plant productivity, particularly in high-value medicinal species. This study evaluated the potential of exogenous citric acid (CA) and tryptophan (Trp), applied individually or in combination, to mitigate Co-induced phytotoxicity in hydroponically grown Salvia officinalis L. Plants were exposed to four Co concentrations (0, 1, 10, and 30 ppm) and four biostimulant treatments (control, 2 mM CA, 2 mM Trp, and CA + Trp) for four weeks. Increasing Co levels, especially 10 and 30 ppm, markedly impaired plant growth, biomass accumulation, and photosynthetic pigment contents (chlorophyll a, chlorophyll b, carotenoids, β-carotene, and anthocyanins). These effects were accompanied by enhanced lipid peroxidation, increased activities of antioxidant enzymes (CAT, POD, SOD, APX, and GPX), and significant disruption of primary metabolism, including reductions in soluble sugars, proteins, amino acids, malate, and oxalate. Application of CA or Trp alone partially alleviated Co toxicity; however, their combined application exerted a pronounced synergistic effect, resulting in substantial recovery of growth performance, photosynthetic capacity, and metabolic balance. Moreover, the combined treatment significantly reduced oxidative damage and moderated antioxidant enzyme activities, reflecting a lower oxidative burden, and limited Co accumulation in plant tissues. These findings suggest that CA may reduce Co bioavailability through chelation, while Trp likely enhances internal metabolic and antioxidative capacity. Their synergistic use represents an effective and sustainable biostimulant strategy to improve cobalt tolerance and ensure the safe production of medicinal plants under heavy metal stress in hydroponic systems.