Xylem Transfer of Organic Solutes inSantalum albumL. (Indian Sandalwood) in Association with Legume and Non-legume Hosts

Abstract

Indian sandalwood (Santalum album), a commercially important root hemi-parasitic angiosperm, was partnered singly in pot culture with one of three nitrogen-fixing legumes or a eucalypt host. Xylem (tracheal) sap of stems of host and parasite and ethanolic extracts of endophytic tissue of haustoria of the parasite were analysed for amino acids, organic acids and sugars to determine which sets of solutes were available to and obtained by the parasite from different hosts. There were high concentrations of asparagine, followed by glutamate, aspartate and γ-amino butyrate in the xylem sap solutes of the three legume hosts (Sesbania formosa, Acacia trachycarpa and A. ampliceps) and much higher levels of glutamate, aspartate, glutamine and arginine in the non-legume, Eucalyptus camaldulensis. Close resemblances between Santalum and legume hosts in concentration and composition of xylem sap amino acids, and in the amino acid spectra of the corresponding parasite endophytic tissue, indicated substantial direct intake of xylem N by Santalum from these hosts. By contrast, low N levels in xylem sap of E. camaldulensis and dissimilarities between its amino acid composition and that of partnered Santalum indicated that the parasite obtained little N from the xylem sap of this host. Xylem sap of hosts contained variable amounts of sucrose. glucose and fructose, whereas that of matching parasites was dominated by fructose. Dissimilarities were also evident in the proportional amounts of xylem-borne organic acids between hosts and parasite particularly for the eucalypt: Santalum partnership. Leaf extracts of the host: parasite pairings generally showed substantial differences in sugar and organic acid balance between partner species. Similarly, where amino acid spectra of host and parasite xylem sap and corresponding haustorial endophytes were closely similar, respective leaf compositions were markedly dissimilar. This implied that substantial metabolic patterns of incoming xylem solutes were highly idiosyncratic of the species in question. Data are related to previous information showing superior growth performance and higher photosynthetic rates and foliar N concentrations in Santalum partnered with the three legumes than with the eucalypt.