Citrulline Regulates Photosynthesis, Water Relations, Redox Equilibrium, Ion Homeostasis, and Osmotic Adjustment to Lessen Alkaline Stress in Sunflower (Helianthus annuus L.)

Citrulline (CIT) mediates plant stress responses through osmoregulation and ROS detoxification, improving membrane integrity and nutrient acquisition. The specific mechanisms by which CIT promotes tolerance to alkaline stress remain unclear. Therefore, a study was conducted to evaluate the efficacy of CIT seed priming (1, 1.5, and 2.5 mM) in protecting sunflower plants from phytotoxicity induced by alkaline stress. In order to simulate alkaline stress the soil was watered at field capacity with Na2CO3 and NaHCO3 in a 9 : 1 ratio to achieve 15 dS/m electrical conductivity. Alkaline stress reduced PSII efficiency, chlorophyll, leaf relative water content, and nutrient uptake due to increased oxidative damage. Alkaline stress increased levels of oxidized glutathione (GSSG), methylglyoxal (MG), leaf relative membrane permeability (LRMP), and production of reactive oxygen species (ROS), including hydrogen peroxide (H2O2), superoxide radicals ( $${\text{O}}_{2}^{{\bullet - }}$$ ), and hydroxyl radicals (•OH), leading to oxidative damage. Elevated lipoxygenase (LOX) activity promoted lipid peroxidation in cellular membranes, as measured by malondialdehyde (MDA) levels, exacerbating oxidative damage. Seed priming with 2.5 mM CIT increased phenylalanine ammonia-lyase and polyphenol oxidase activities, augmented antioxidant defense, enhanced nitric oxide and hydrogen sulfide levels, and diminished MG and ROS accumulation, thereby lessening oxidative stress and ion excess toxicity to promote membrane integrity, nutrient uptake, and photosynthetic efficiency.