Hydrogen peroxide and nitric oxide mediate K+/Na+ homeostasis and antioxidant defense in NaCl-stressed callus cells of two contrasting poplars

Using callus cells of a salt-tolerant Populus euphratica Oliver and a salt-sensitive P. popularis 35–44 (P. popularis), the effects of NaCl stress on hydrogen peroxide (H₂O₂) and nitric oxide (NO) production and the relevance to ionic homeostasis and antioxidant defense were investigated. Results show that P. euphratica exhibited a greater capacity to tolerate NaCl stress in terms of cell viability, membrane permeability and K⁺/Na⁺ relations. NaCl salinity (150 mM) caused a rapid increase of H₂O₂ and NO in P. euphratica cells, but not in P. popularis. Moreover, salinised P. euphratica cells retained a high and stable level of H₂O₂ and NO during the period of 24-h salt stress. Noteworthy, P. eupratica cells increased activities of superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase under salinity stress, but these antioxidant enzymes were significantly inhibited by the salt treatment in P. popularis cells. Pharmacological experiments proved that the NaCl-induced H₂O₂ and NO was interdependent and contributed to the mediation of K⁺/Na⁺ homeostasis and antioxidant defense in P. euphratica cells. Given these results, we conclude that the increased H₂O₂ and NO enable P. euphratica cells to regulate ionic and ROS (reactive oxygen species) homeostasis under salinity stress in the longer term.