Nacl-Altered Oxygen Flux Profiles And H+-Atpase Activity In Roots Of Two Contrasting Poplar Species

Maintaining mitochondrial respiration is crucial for proving ATP for H+ pumps to continuously exclude Na+ under salt stress. NaCl-altered O-2 uptake, mitochondrial respiration and the relevance to H+-ATPase activity were investigated in two contrasting poplar species, Populus euphratica (salt-tolerant) and Populus popularis 35-44 (salt-sensitive). Compared with P. popularis, P. euphratica roots exhibited a greater capacity to extrude Na+ under NaCl stress (150 mM). The cytochemical analysis with Pb(NO3)(2) staining revealed that P. euphratica root cells retained higher H+ hydrolysis activity than the salt-sensitive poplar during a long term (LT) of increasing salt stress (50-200 mM NaCl, 4 weeks). Long-sustained activation of proton pumps requires long-lasting supply of energy (adenosine triphosphate, ATP), which is delivered by aerobic respiration. Taking advantage of the vibrating-electrodes technology combined with the use of membrane-tipped, polarographic oxygen microelectrodes, the species, spatial and temporal differences in root O-2 uptake were characterized under conditions of salt stress. Oxygen uptake upon NaCl shock (150 mM) was less declined in P. euphratica than in P. popularis, although the salt-induced transient kinetics were distinct from the drastic drop of O-2 caused by hyperosmotic shock (255 mM mannitol). Short-term (ST) treatment (150 mM NaCl, 24 h) stimulated O-2 influx in P. euphratica roots, and LT-treated P. euphratica displayed an increased O-2 influx along the root axis, whereas O-2 influx declined with increasing salinity in P. popularis roots. The spatial localization of O-2 influxes revealed that the apical zone was more susceptible than the elongation region upon high NaCl (150, 200 mM) during ST and LT stress. Pharmacological experiments showed that the Na+ extrusion and H+-ATPase activity in salinized roots were correspondingly suppressed when O-2 uptake was inhibited by a mitochondrial respiration inhibitor, NaN3. Therefore, we conclude that the stable mitochondrial respiration energized H+-ATPase of P. euphratica root cells for maintaining Na+ homeostasis under salt environments.