Water relations and gas exchange of the root hemiparasite Santalum acuminatum (quandong)

The water relations and leaf gas-exchange characteristics of the root hemiparasite quandong (Santalum acuminatum (R.Br.) A. DC) and its neighbouring plants were examined at three field sites in central Southern Australia. This paper examines the role of water potential and osmotic gradients in facilitating the movement of water from host plants to quandong. Quandong exhibited a significantly more negative water potential than the neighbouring plant species at both field sites during summer and winter. A significant proportion of the osmotic potential was accounted for by mannitol, Na+, K+ and Cl-. A water potential difference of 1.7 MPa was maintained between quandong and its putative host over a measurement period of 24 h. Xylem sap and leaves of quandong contained considerable concentration (0.1-0.4 mol (kg tissue water)(-1)) of mannitol. Stomatal conductance and assimilation of quandong were lower than those of the neighbouring plants at both Middleback and Aldinga during both summer and winter measurements. Measurements of transpiration for quandong differed between the two sites. The lower transpirational water loss resulted in quandong at Middleback having an instantaneous water-use efficiency higher (0.13-2.2 mu mol (CO2) mmol(-1) (H2O)) than the neighbouring plants. Daily sap flow and calculated hydraulic conductivity were not significantly different between quandong and putative host plant.