Antioxidant regulation of iron as a repressor for salt-induced leaf senescence in perennial grass species

Mechanisms of how iron (Fe) may affect salt-induced leaf senescence are still unclear despite the known effects of iron on overall plant tolerance to salt stress. The objectives of this study were to examine how iron may regulate leaf senescence induced by salt stress and determine whether Fe effects on salt-induced leaf senescence could be associated with its regulatory functions in antioxidant metabolism in bentgrass species. Two grass species, salt-tolerant Agrostis scabra (‘NTAS’) and salt-sensitive A. stolonifera (‘Penncross’) were supplied with deficient Fe (0 μM Fe(II)-EDTA), normal Fe (20 μM Fe(II)-EDTA) or supplemental Fe (40 μM Fe(II)-EDTA) in the hydroponic culture with 150 mM NaCl or without salt (control). Plants were subjected to salt stress for 35 days in growth chambers. Iron supplementation suppressed leaf senescence induced by salt stress in both grass species, as shown by increased chlorophyll content, photochemical efficiency and membrane stability, and reduced ROS accumulation (O₂^·− and H₂O₂) while Fe deficiency had opposite effects, compared to plants with normal Fe treatment. Additional iron resulted in increased activities of antioxidant enzymes in two grass species exposed to salinity stress, including catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), and superoxide dismutase (SOD). The antioxidant effects of Fe were most effective for SOD, as the enzymatic activity, isozyme abundance levels, and gene expression levels of Fe-SOD, Chl Cu/ZnSOD, Mn-SOD, and Cyt Cu/ZnSOD were increased due to Fe supplementation. These results indicated iron acted as a repressor for salt-induced leaf senescence in both Agrostis grass species, which was associated with its antioxidant regulation, particularly the superoxide dismutase pathway to scavenge highly reactive superoxide.