Annals of Botany 80: 257-264, 1997
© 1997 Annals of Botany Company
Haustorium-related Uptake and Metabolism of Host Xylem Solutes by the Root Hemiparasitic ShrubSantalum acuminatum(R. Br.) A. DC. (Santalaceae)
Department of Botany, The University of Peradeniya, Peradeniya, Sri-Lanka Department of Botany, The University of Western Australia, Nedlands, WA, 6907, Australia Department of Botany, The University of Queensland, Brisbane, Queensland, 4072, Australia
January 30, 1997 ; March 20, 1997 .
Solute composition of root xylem sap of common native hosts of quandong (Santalum acuminatum) was compared with that of corresponding xylem sap and ethanolic extracts of endophytic tissues of haustoria of the hemiparasite. Each host transported a characteristic set of organic nitrogenous solutes, but little or no nitrate, and the data indicated only limited direct flow of amino compounds between xylem streams of hosts and parasite. Proline predominated in the haustorium and xylem ofSantalum, but was at negligible levels in the xylem of most hosts. Sucrose, fructose, glucose, malate and citrate were at high levels in all saps, and fructose especially prominent inSantalum. Chloride, sulphate and phosphate were the principal inorganic anions of the xylem. Based on C:N ratios of xylem and dry matter ofSantalumand assuming a 70% or more dependence on the host for N, it was estimated thatSantalumwould gain approximately one third of its C requirement for dry matter production heterotrophically from the xylem of its hosts. Infiltration of xylem of haustoria-bearing root segments of a major host (Acacia rostellifera) with a range of15N labelled substrates resulted in 40–80% of the15N of endophytes of the attached haustoria being received as proline. Nitrate reductase activity was induced in haustoria following host xylem feeding of nitrate. The study concludes that haustoria ofSantalumact as a major site of synthesis and export of proline and might therefore play an important role in osmotic adjustment of the parasite and its related acquisition of water from hosts.
Root hemiparasite; Santalum acuminatum; 15N labelled substrates; xylem transport; proline; osmoregulation
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