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Monash University (2003)

Comparative ecology of four Pittosporum species from contrasting rainfall regimes in south-eastern Australia

Rayner, Gerard Michael

Titre : Comparative ecology of four Pittosporum species from contrasting rainfall regimes in south-eastern Australia

Auteur : Rayner, Gerard Michael

Université de soutenance : Monash University

Grade : Doctor of Philosophy 2003

The evolutionary pathways and adaptational responses of the Australian flora to Late Tertiary-Quaternary climate change are yet to be fully elucidated. Congeneric species distributed across a precipitation gradient should possess different responses to water deficit, which may provide an opportunity to investigate the adaptations of plants to such climate changes. In southeastern Australia, four Pittosporum species (P. bicolor, P. undulatum, P. revolution and P. angustifolium) occupy habitats with contrasting rainfall regimes. Climate analyses showed important differences in the climates of these species to be the higher temperatures and predominantly summer rainfall of sites supporting tropical and arid species, grading to winter-spring rainfall in the southernmost part of their distributions, compared with the temperate species. Patterns of seasonal water relations indicated the greater ability of P. angustifolium, in comparison to the more mesic P. undulatum and P. revolutum, to decrease leaf water potential whilst maintaining low stomatal conductance during periods of maximum water deficit. Nevertheless, the latter two species exhibited considerable differences in the seasonal and diurnal water relations. Under common conditions of drought, species responses were broadly consistent with their seasonal field water relations. Maintenance of transpiration, stomatal conductance and high osmotic potential by P. bicolor indicated its low resistance to water deficit. P. undulatum exhibited high drought resistance, incorporating stomatal closure, osmotic adjustment and increased water use efficiency. P. revolutum and P. angustifolium, which decreased leaf water potential and displayed intrinsically low rates of transpiration and osmotic adjustment to water deficit, appear to possess very high resistance to water deficit. The seed and seedling biology of these species revealed considerable intra- and interspecific variation in growth and biomass allocation variables, with seedlings of P. undulatum exhibiting the highest RGR. Species differed significantly in seed mass, with a broad pattern of decreasing seed mass with increasing latitude. Seedlings of species inhabiting permanently or seasonally dry environments exhibited higher specific leaf area and root:shoot ratio but lower specific root length, with the opposite pattern for those of species from wetter environments. Species’ responses to environmental variables were compared in terms of comparative germination rate and quotient. A lack of germination by P. bicolor seed indicated primary dormancy due to after-ripening or alternating temperature requirements. For other species, germination responses to temperature were broadly consistent with temperature variables across their distribution ! ;. P. angustifolium and P. undulatum seed germinated most rapidly but across a narrower range of temperatures than the more slowly-germinating P. revolutum. The slower germination and lower quotients of P. angustifolium and P. revolutum seed from populations subject to low rainfall and/or high seasonality illustrated their high degree of intraspecific variation. While P. angustifolium seed germinated readily under suitable field conditions of temperature and soil moisture, there was no seedling survival. However, this species demonstrated a high capacity for vegetative reproduction and a capacity to adjust reproductive allocation in response to climatic or biotic factors. The close relationship between P. bicolor and P. angustifolium suggests the recent evolution and spread of the latter species into arid environments. However, differences between these species in selected life-history traits, and similarities in these traits amongst other Pittosporum species, contrast with their purported phylogenetic positions. This may be of evolutionary importance, as it suggests a capacity among these species for broad adaptive shifts, possibly in response to climate changes during the Late Tertiary-Quaternary, in life history traits such as seed size and germination biology, biomass allocation and reproductive strategy.

Subject words : Pittosporum Seeds Victoria Physiology • Pittosporum Climatic factors Victoria • Pittosporum Drought tolerance Victoria • Seeds Victoria Growth • Seeds Victoria Anatomy • Seeds Victoria Physiology


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