NaCl-altered oxygen flux profiles and H+-ATPase activity in roots of two contrasting poplar species.

Abstract

Maintaining mitochondrial respiration is crucial for proving ATP for H+ pumps to continuously exclude Na+ under salt stress. NaCl-altered O2 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 (150mM). 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 (50-200mM NaCl, 4weeks). 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 O2 uptake were characterized under conditions of salt stress. Oxygen uptake upon NaCl shock (150mM) was less declined in P. euphratica than in P. popularis, although the salt-induced transient kinetics were distinct from the drastic drop of O2 caused by hyperosmotic shock (255mM mannitol). Short-term (ST) treatment (150mM NaCl, 24h) stimulated O2 influx in P. euphratica roots, and LT-treated P. euphratica displayed an increased O2 influx along the root axis, whereas O2 influx declined with increasing salinity in P. popularis roots. The spatial localization of O2 influxes revealed that the apical zone was more susceptible than the elongation region upon high NaCl (150, 200mM) during ST and LT stress. Pharmacological experiments showed that the Na+ extrusion and H+-ATPase activity in salinized roots were correspondingly suppressed when O2 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.