Abstract Salinity has considerable adverse effects on agriculture through reduction in plant growth and water use. Sodium chloride salinity has both an osmotic effect on plant water relations, and a toxic effect on cellular processes. The relative contribution of these two effects to plant growth depends on a range of factors including plant specific tolerance mechanisms, such as Na and Cl exclusion, compartmentation of ions at a whole plant and cellular level, and synthesis of organic osmotic compounds for plant osmotic adjustment. Plants growing in saline soil would also experience reduced plant available water, due to the additional osmotic effect on soil water potential. The effect of salinity on plant growth is further complicated by the interactions of environmental conditions with plant water and ion uptake. This thesis examines the osmotic and toxic effects of salinity on wheat (Triticum aestivum L.) and chickpea (Cicer arietinum L.), with particular focus on plant water availability, effects of Na and Cl toxicity, and temperature and humidity effects. While considerable research has been undertaken into the physiological response of plants to NaCl, our understanding of the capacity of plants to extract water from saline soils has remained largely theoretical. Total plant available water is largely determined by the matric potential of the soil. Presence of sodium chloride would have an additional osmotic effect, and previous theory stated that the salt tolerance of the plant determined the extent to which this osmotic potential reduced plant available water. The capacity of wheat and chickpea to extract water from saline soils was examined in a soil experiment where water stress was imposed on established plants, which were then grown until permanent wilting point (PWP) was reached. Wheat extracted to lower soil water potentials (-1.5 MPa), than chickpea (-900 kPa) in 0 NaCl treatments. Where salinity was low to moderate, plants were able to extract water to a PWP determined by the combined total of matric and osmotic potentials. Wheat extracted water to PWP in salinity treatments up to soil ECse of 5.3 dS/m, and chickpea to 2.9 dS/m. As salinity increased, toxic effects of salinity dominated, and water extraction by plants was significantly lower than that determined by total soil water potential. Solution culture experiments investigated the comparative toxic effects of Na, Cl and salt mixtures. Growth of wheat was reduced by Na toxicity, but not Cl toxicity, with Na causing a small, but significant additional reduction in growth, compared to high Cl or a salt mixture. Reductions in growth of 50% from control treatments occurred at -500 kPa for the Na treatment, and -630 kPa for the Cl and mixed salt treatments. In contrast, growth of chickpea was significantly reduced by both Na and Cl toxicity, with a large difference in growth compared to the salt mixture. Growth reductions of 50% occurred at -330 kPa for the Cl treatments, and -450 kPa for the Na treatment. A 50% growth reduction was not observed in the mixed salt treatment. Tolerance of saline conditions is reduced under stressful environmental conditions, such as high temperature and low relative humidity. Hot and dry conditions were shown to reduce the tolerance of saline conditions by both wheat and chickpea, compared to cool or humid conditions. Tissue concentrations of Na in wheat were disproportionately high in treatments with high evaporative demand, while tissue Cl was not related to evaporative demand. Tissue concentrations of Na in chickpea increased with temperature, but not relative humidity, while tissue Cl concentrations were highly correlated with evaporative demand. The relationships between NaCl salinity, plant water use, and environmental conditions were examined, allowing further development of the two phase salinity model. In particular, the transition point between the osmotic and toxic salinity effects. While the concentration of NaCl in the soil remains the primary factor, soil water status, environmental stresses and presence of other salts may dictate whether salinity be tolerated by the plant or not. The ability of the plant to extract water to PWP, as determined by total matric and osmotic potential has been identified as a useful indicator of salinity tolerance. The point at which toxicity of Na and/or Cl is observed is associated with a rapid increase in Na and Cl uptake by the shoot tissue, and a decrease in the amount of water the plant is able to extract from the soil. Within the osmotic region of salinity stress, the plant is able to extract water to PWP, but as NaCl becomes toxic the plant is unable to utilize this water.
| Identifer | oai:union.ndltd.org:ADTP/279325 |
| Creators | Anna Sheldon |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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