Citric acid and hydrogen sulfide cooperate to mitigate chromium stress in tomato plants by modulating the ascorbate-glutathione cycle, chromium sequestration, and subcellular allocation of chromium.

The study aimed to investigate the role of hydrogen sulfide (H 2 S) in regulating chromium stress (Cr–S) tolerance of tomato plants treated with citric acid (CA). Prior to the Cr treatment, tomato plants were foliar-fed with CA (100 μM) daily for 3 days. Subsequently, the plants were grown for another ten days in a hydroponic system in a 50 μM Cr (VI) solution. Chromium treatment reduced photosynthetic pigments and plant biomass, but boosted the levels of hydrogen peroxide (H 2 O 2) malondialdehyde (MDA), H 2 S, phytochelatins (PCs), and glutathione (GSH), electrolyte leakage (EL), and antioxidant enzyme activity in tomato plants. However, the foliar spray of CA mitigated the levels of H 2 O 2 , MDA, and EL, promoted plant growth and chlorophyll content, enhanced antioxidant enzymes' activities, and increased H 2 S production in Cr–S-tomato plants. CA also increased the levels of GSH and PCs, potentially reducing the toxicity of Cr through regulated sequestration. Additionally, the application of sodium hydrogen sulfide (NaHS), a donor of H 2 S, improved CA-induced Cr stress tolerance. The addition of CA promoted Cr accumulation in root cell wall and leaf vacuoles to suppress its toxicity. To assess the involvement of H 2 S in CA-mediated Cr–S tolerance, 0.1 mM hypotaurine (HT), an H 2 S scavenger, was provided to the control and Cr–S-plants along with CA and CA + NaHS. HT reduced the beneficial effects of CA by decreasing H 2 S production in tomato plants. However, the NaHS addition with CA + HT inverted the adverse impacts of HT, indicating that H 2 S is required for CA-induced Cr–S tolerance in tomato plants. [Display omitted] • Chromium stress (Cr–S) decreased plant growth and enhanced oxidative stress • Citric acid (CA) suppressed the oxidative stress and Cr-induced growth reduction • CA modulates H 2 S, antioxidants and subcellular distribution of chromium • Hypotaurine, scavenger of H 2 S, reversed H 2 S content and provoked oxidative stress • CA and H 2 S are jointly responsible for enhancing Cr–S tolerance of tomato plants [ABSTRACT FROM AUTHOR]