Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

Amino acid composition of xylem (tracheal) sap and ethanolicextracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae)and their hosts were compared, using material collected in theirnative habitats. Data indicated that certain host xylem soluteswere transferred directly to the parasite xylem, while otherswere either not absorbed or were metabolized prior to transfer.Certain solutes were major constituents of parasite xylem, butundetected or only in trace amount in the host. Shoot aminoacid pools of parasites differed markedly from those of hosts.The mistletoe, Amyema preissii, exhibited differential storageand transport of arginine when parasitizing three differentspecies, but accumulated proline on only two of these hosts.Host- specific amino acids (djenkolic acid in Acacia saligna,and tyramine in Acacia acuminata) were transported and accumulatedin relatively large amounts by the parasite, but were not detectedin other associations. Proline was the major solute of Amyemalinophyllum parasitizing Casuarina obesa, but arginine predominatedin Lysiana casuarinae on the same host. However, when L. csuarinaeparasitized A. linophyllum, in turn parasitic on C. obesa, theLysiana accumulated equal amounts of proline and arginine andmore asparagine than when directly on the Casuarina. Xylem feedingof ¹⁵N-labelled aspartic acid or ¹³N-(amide labelled) asparagineto cut shoots or whole haustoria-bearing plants of the mistletoeA. preissii resulted in 68–73% of the ¹⁵N of aspartateand 24–30% of that of asparagine appearing in ethanol-solubleshoot amino compounds other than the fed solute. ¹⁵N labellingpatterns of detached shoots were not noticeably different fromthat of whole plants suggesting that the haustorium had relativelylittle effect on processing incoming solutes. Alanine, glutamine,and arginine were principal recipients of ¹⁵N from aspartate,alanine and glutamine in the case of fed asparagine. It is estimatedthat 24% of the carbon requirements for dry matter accumulationin Amyema linophyllm were met by intake of xylem sap solutesfrom its host Casuarina obesa. Key words: Amino acids, xylem transport, mistletoes, host: parasite relations, N metabolism