Effect of NaCl on leaf H⁺-ATPase and the relevance to salt tolerance in two contrasting poplar species

During a 30-day period of increasing salinity, we examined the effects of NaCl on leaf H⁺-ATPase and salinity tolerance in 1-year-old plants of Populus euphratica Oliv. (salt resistant) and P. popularis 35-44 (P. popularis) (salt sensitive). Electron probe X-ray microanalysis of leaf mesophyll revealed that P. euphratica had a higher ability to retain lower NaCl concentrations in the cytoplasm, as compared to P. popularis. The sustained activity of H⁺ pumps (by cytochemical staining) in salinised P. euphratica suggests a role in energising salt transport through the plasma membrane (PM) and tonoplast. Salt-induced alterations of leaf respiration, ATP content and expression of PM H⁺-ATPase were compared between the two species. Results show that P. euphratica retained a constant respiratory rate, ATP production and protein abundance of PM H⁺-ATPase (by Western blotting) in salt-stressed plants. P. euphratica was able to maintain a comparatively high capacity of ATP hydrolysis and H⁺ pumping during prolonged salt exposure. By contrast, the activity and expression of PM H⁺-ATPase were markedly decreased in P. popularis leaves in response to salt stress. Furthermore, NaCl-stressed P. popularis plants showed a marked decline of respiration (70%) and ATP production (66%) on day 30. We conclude that the inability of P. popularis to transport salt to the apoplast and vacuole was partly due to the decreased activity of H⁺ pumps. As a consequence, cytosolic ion concentrations were observed to be comparatively high for an extended period of time, so that cell metabolism, in particular respiration, was disrupted in P. popularis leaves.