Date of Award

1-1-2008

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

Abstract

Isohydric and anisohydric regulation of plant water status has been observed over several decades of field, glasshouse and laboratory studies, yet the functional significance and mechanism of both remain obscure. W e studied the seasonal trends in plant water status and hydraulic properties in a natural stand of Eucalyptus gomphocephala through cycles of varying environmental moisture (rainfall, groundwater depth, evaporative demand ) in order to test for isohydry and to provide physiological information for the mechanistic interpretation of seasonal trends in plant water status. Over a 16-month period of monitoring, spanning two summers, midday leaf water potential correlated with pre-dawn leaf water potential, which was correlated with water table depth below f:,>rnund level, which in tum was correlated with total monthly rainfall. Eucalyptus gomphocephala was therefore not isohydric. Despite strong stomatal downregulation of transpiration rate in response to increasing evaporative demand, this was insufficient to prevent midday leaf water potential from falling to levels below -2. 0 MPa in the driest month, well into the region likely to induce significant xylem air embolisms. However, even though midday leaf water potential varied by over 1 .2 MPa across seasons, the hydrodynamic plant water potential gradient (delta psi plant), inferred as the difference between pre-dawn and midday leaf water potential, was relatively constant across seasons, averaging about 0.6 MPa. This unusual pattern of hydraulic regulation, referred to here as isohydrodynamic, is predicted by a hydromechanical stomatal control model, but only when plant hydraulic conductance is dependent on transpiration rate. We observed a correlation between midday transpiration rate and whole-plant hydraulic conductance that was consistent with this requirement, although conditions did not allow dependence of one on the other to be established . The accuracy of the model is improved slightly with the addition of a root-shoot signal allowing guard cell osmotic pressure to decline in response to soil water potential. The implications of the observed pattern of hydraulic regulation are discussed in the context of mechanistic requirements in the stomatal control system, and its possible function in related physiological processes .

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