School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemPhysiological and phylogenetic patterns in eucalyptus spp. responses to salinity and drought( 2004)Increasing aridity is regarded as a major driving force behind the evolution of the genus Eucalyptus. Previous investigations have identified both morphological and physiological characteristics whose properties, both in distribution and function, correlate strongly with adaptation to saline and arid environments. One such response is the regulation of osmotic potential via both acclimating and constitutive mechanisms. The identity of solutes contributing to these responses has hitherto remained unknown. Substantial evidence for the role(s) of cyclitols (cyclic polyols) during stressful conditions has been gleaned from previous investigations in both tree and herbaceous species. Here I uncover contrasting biochemical mechanisms for the regulation of osmotic potential among eucalypt species, with broad implications for taxonomy and evolution. For the first time, targeted biochemical profiling has uncovered a quantitative yet discrete link between eucalypt taxonomy and adaptation to arid environments. The distribution of cyclitols among eucalypt species correlates strongly with other demonstrated adaptations to arid conditions. Analysis of Eucalyptus leptophylla leaf tissues from trees growing along a topographic gradient encompassing hyper-saline lakes, suggests that cyclitols contribute significantly to osmotic adjustment induced by drought. The presence of substantial quantities of cyclitols in xylem sap of E. leptophylla suggests additional roles in the signalling of plant stress reponses. Determination of the contribution of low molecular weight carbohydrates, polyols and inorganic ions towards the regulation of osmotic potential in glasshouse grown E. spathulata (Hook.) show that salt and drought stress elicit contrasting mechanisms of solute concentration. Under salt stress, seedlings reduce osmotic potential by accumulating large quantities of inorganic ions in leaf tissues, presumably sequestrating them into the vacuole. Under drought stress, seedlings reduce osmotic potential through the concentration of constitutively present solutes - particularly that of the cyclitol quercitol. Targeted biochemical profiling of 13 species of Eucalyptus originating from contrasting rainfall environments revealed equally contrasting biochemical responses to drought stress. In species originating from low rainfall environments, the constitutive concentration of quercitol is the major mechanism of osmotic regulation whilst species originating from high rainfall environments osmotically adjust through the active accumulation of sucrose. These results suggest that the solutes governing both acclimating and constitutive regulation of osmotic potential in Eucalyptus species have finally been identified. The implications of these findings are discussed in the context of plant adaptation to saline and arid environments.