Nickel-induced biphasic regulation of morpho-physiological, biochemical, and essential oil traits in drought-stressed Fumaria parviflora Lam.

Nickel (Ni) acts as an essential micronutrient at low concentrations but induces toxicity when accumulated excessively. Drought stress likewise restricts plant growth and secondary metabolism in medicinal plants. This study investigated the single and interactive effects of foliar-applied Ni and drought stress on growth, physiological responses, antioxidant metabolism, and essential oil production of Fumaria parviflora under greenhouse conditions using a factorial completely randomized design. Drought stress markedly reduced biomass accumulation, root and shoot growth, chlorophyll, carotenoids, and essential oil yield, while significantly increasing malondialdehyde, proline, soluble carbohydrates, phenolics, flavonoids, and antioxidant activity, reflecting oxidative stress and adaptive metabolic adjustments. Ni application induced a concentration-dependent response. Moderate Ni levels (0.08-0.24 mg L-1) were associated with reduced drought-induced damage through improved photosynthetic pigments, antioxidant activity, and secondary metabolite accumulation, whereas higher concentrations (0.33-0.50 mg L-1) intensified oxidative injury, particularly under severe drought. The highest essential oil yield (0.42% v/w) was obtained at 0.24 mg L-1 Ni under moderate drought, accompanied by enhanced accumulation of major constituents including carvacrol, octadecane, and hexadecanoic acid. The results suggest a hormesis-like biphasic response of Fumaria parviflora to Ni, where moderate Ni supplementation may enhance drought tolerance by supporting antioxidant activity and secondary metabolite accumulation, while excessive Ni becomes phytotoxic. These findings highlight the importance of optimizing Ni concentration to maximize beneficial effects while avoiding toxicity under water-limited conditions.