Environmental influences of salinity resistance in forest plants

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Oil sands mining involves a complete removal and reconstruction of landforms in sensitive boreal ecosystems in northeastern Alberta. Land reclamation and revegetation efforts following oil sands mining are challenging due to the severity of land disturbance and the properties of oil sands tailings. It is predicted that soils in many reclamation areas will be affected by high pH, limited nutrient supply, hypoxia, and elevated salt levels. Boreal plants are known to be sensitive to salt; however, it is unclear how they respond to salinity in the presence of other unfavorable soil factors which often accompany salinity in reclamation sites. This information is essential to ensure successful revegetation of oil sands mining areas. It is also important to develop understanding of the processes contributing to plant recovery from salt stress and learn how to recognize the critical stages beyond which severe injury is irreversible and vegetation survival is threatened. In the proposed project, we will examine the effects of root zone pH, soil oxygen availability and mineral nutrition on salt resistance and sodium uptake in important oil sands reclamation tree species. We will also study the processes of recovery from salt exposure and determine whether tissue sodium concentration could be used as a more reliable parameter than soil salinity level for monitoring and predicting salinity effects on vegetation. Since salt avoidance and salt sequestration are the most effective salt tolerance mechanisms in plants, the expected consequences of increased sodium uptake include significant increases in mortality and decreases in growth rates of reclamation plants. It has been well documented that salt injury is triggered once sodium levels in shoots reach a certain threshold level. Therefore, if elevated tissue sodium levels are found to correlate with plant injury and mortality, they could be used as a more effective monitoring tool for mitigation and management of salinity-affected sites than the soil salinity data alone. Since plant responses to tissue sodium accumulation may be influenced by site conditions, it will be necessary to examine how environmental factors such as soil pH, hypoxia, and mineral nutrition affect this relationship. The principal objective of the present study is to generate fundamental knowledge which is required to develop protocols for successful reclamation of oil sand areas affected by salinity that is accompanied by other unfavorable soil factors. The knowledge generated by this project will also help improve basic understanding of the processes which plants use to resist these detrimental soil factors. We will examine the following hypotheses: 1) Sodium tissue levels correlate with plant injury and mortality, 2) High and neutral pH aggravate the effects of salinity by increasing shoot sodium uptake and the magnitude of this effect varies between plant species, 3) Elevated calcium levels reduce sodium uptake and alleviate salt effects in plants due to the stabilizing effect on root aquaporins, 4) Mineral deficiencies aggravate the effects of salt, 5) Effective long-term recovery mechanisms in trees from salt stress include both Na+ sequestration in vacuoles and leaf abscission by deciduous trees, 6) Hypoxia increases the apoplastic transport of water and ions in roots resulting in a greater uptake and accumulation of Na+ in shoots and acceleration of salt injury.