Effect of citrulline seed priming on growth, photosynthesis, redox equilibrium, secondary metabolic enzymes, metal sequestration and nutrient acquisition in sunflower (Helianthus annuus L.) under chromium toxicity.

Citrulline (CITRN) is a potent radical scavenger and osmolyte that plays a crucial role in plant drought stress tolerance. However, its role in mitigating chromium (Cr) phytotoxicity has not been studied yet. This study was conducted to appraise the potential of CITRN seed priming (1, 2, and 3 mM) for alleviating Cr toxicity (20 mg kg^‒1 soil) in sunflower plants. Chromium toxicity resulted in higher oxidative stress and membrane injury in plants, as evident by higher levels of superoxide radicals (O₂^·‾), hydroxyl radicals (·OH), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and electrolyte leakage (EL). Plants under Cr toxicity displayed higher cytotoxic methylglyoxal (MG) levels and lipoxygenase (LOX) activity, which exacerbated cellular damage. Consequently, plants suffered a significant reduction in growth attributes, photosynthetic pigments, total soluble proteins, leaf relative water content, and nutrient uptake. Chromium toxicity compromised photosystem II (PSII) health as reflected by diminished maximum efficiency of PSII (Fv/Fm), quenching coefficient (qP), and quantum efficiency of PSII (ΦPSII). However, CITRN significantly enhanced plant growth, chlorophyll concentration, PSII health, and nutrient acquisition by regulating reactive oxygen species scavenging, secondary metabolic pathways, and ionic equilibrium under Cr toxicity. Citrulline upregulated antioxidant defense and methylglyoxal detoxification in Cr-stressed plants. Furthermore, CITRN conferred protection against Cr-induced toxicity by elevating hydrogen sulfide, nitric oxide, glutathione, phenolic, and flavonoid compounds that boosted antioxidant defense and mitigated oxidative damage. The present study elucidates CITRN-priming as a potential strategy to mitigate Cr toxicity in Helianthus annuus plants.