School of Agriculture, Food and Ecosystem Sciences - Theses

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Start date: 2000 × End date: 2019 × Subject: Eucalyptus × Subject: Heartwood × Type: PhD thesis ×

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    The Cell biology of eucalypt heartwood formation
    Wilson, Lawrence Francis ( 2001)
    Trees are among the most successful of organisms. Despite voracious enemies, diverse and treacherous habitats, individuals may thrive for hundreds of years, or even millennia. Their success is due to adaptability, and the development of powerful multi-faceted defensive systems. The least understood of these is the heartwood/tylosis system. Once thought to be merely a metabolic waste dump, or an irrelevant consequence of homeostatic processes, heartwood is now, with the benefit of new perspectives, emerging as a crucial and dynamic defensive element. This thesis proposes that ray parenchyma cells actively produce heartwood to provide a micro-organism resistant base for the vulnerable sapwood transpiration system and carbohydrate reserve. Statistical analyses of a 20-year-old Eucalyptus nitens tree showed that heartwood existed as a central pillar within the tree, and tyloses within heartwood were optimally distributed to seal the largest and most vulnerable vessels against invasion. Ultrastructural analysis showed that ray cells in the middle and inner sapwood were essentially dormant, but were reactivated in the heartwood transition zone. Ray cells forming tyloses showed evidence of a high level of activity with the nucleus directing expansion from within tyloses. The coordinated activity and subsequent deaths of these cells suggest the possibility of programmed death as their ultimate fate. Ray cell deaths result in release of phytotoxins into the surrounding tissue. This is in contrast to animal cell apoptosis where release of toxins into the surrounding tissue is actively prevented. Research into the microstructural and molecular basis of heartwood formation in vivo is limited by the reactivity of extractives within the transition zone, the inaccessibility of cells, and the transitory and asynchronous nature of heartwood transformation. In vitro systems, in which initiation and inhibition of heartwood-like transformation can be easily manipulated, may ultimately provide solutions to these problems. The development of two of these systems is discussed.