Nitrogen and phosphorus nutrition in mycorrhizal Epacridaceae of south-west Australia

Xylem transport of nitrogen and phosphorus was examined in mature mycorrhizal plants of 41 species in 15 genera of Epacridaceae in native habitat in south-west Australia. Glutamine was the principal nitrogenous solute of xylem of all but four species. In the latter species, arginine or asparagine predominated. Nitrate and ammonium comprised minor fractions of xylem (tracheal) sap N, except in two species in which nitrate contributed over half of the N. Ratios of total-N:phosphate-P in xylem sap varied widely (mean 67 +/- 18, range 0.2-495) between species and habitats. Plants of Croninia kingiana (syn. Leucopogon kingeanus) from the one habitat showed higher levels of N and P in xylem early than late in the mycorrhizal season, but there was no consistent evidence of higher N and P levels from upper than deeper parts of their root systems. Study of juvenile populations of four species of epacrids indicated that substantial fractions of the yearly increment of N, P and dry matter was accumulated during the three summer months when infected mycorrhizal hair roots were absent. Glasshouse culture of mycorrhizal plants of Epacridaceae in habitat soil enriched with decomposed and leached double (C-13, N-15)-labelled dry matter of wheat showed substantial labelling of shoots with N-15 but not with C-13. Plants fed similarly treated N-15-labelled root residues of maize acquired N-15 but failed to generate partial derivative(13)C values different from those of control plants. Possible avenues of mycorrhizal and nonmycorrhizal nutrition of Epacridaceae are discussed. (C) 1996 Annals of Botany Company