Ecophysiological aspects of the woody root hemiparasite Santalum acuminatum (R. Br.) A. DC and its common hosts in South Western Australia

Abstract

The water and nitrogen relationships of the xylem-tapping root hemiparasite, quandong (Santatum acuminatum) and its principal hosts were examined at a series of sites in native coastal heathlands of south west Australia. Assessments based on densities of above-ground biomass, ground cover and frequencies of haustoria on host roots indicated that woody N2 fixers (legumes and Allocasuarina) were principal hosts of Santalum. δ15N values for shoot dry matter of component species suggested these N2 fixers were strongly (70% or more) dependent on atmospheric N and that Santalum derived N principally from these species. Structural studies of haustoria showed the interface with host xylem to be almost entirely comprised of parenchymatous tissue. No luminal continuities were observed between xylem conducting elements of the partners. Formation and functional life of haustoria were closely coordinated with seasonal growth of hosts, with some haustoria surviving summer and overlapping functionally with new ones establishing in the next autumn. Transpiration and photosynthetic rates of the parasite were consistently less and water use efficiencies very similar to those of the principal host Acacia rostellifera. δ13C values of foliage of Santalum and this host were similar, but large variations in δ13C values for above-ground dry matter of parasite and hosts between study sites prevented evaluations of water stress or water-use-efficiency based on carbon isotope discrimination. Specific hydraulic conductivities of roots of Santalum were consistently lower than those of Acacia, a finding consistent with more conservative water use by the parasite than the host.