HETEROTROPHIC GAIN OF CARBON FROM HOSTS BY THE XYLEM-TAPPING ROOT HEMIPARASITE OLAX-PHYLLANTHI (OLACACEAE)

Heterotrophic gains of carbon from various host species by the root he miparasitic shrub Olax phyllanthi (Labill) R.Br. were assessed using t echniques based on carbon isotope discrimination (delta(13)C) On C3 an d C4 hosts and C:N ratios of xylem sap and dry matter of host and para site. Heterotrophic benefits (H) to Olax based on delta(13)C values we re 30% and 19% from two non-native C4 hosts (Portulaca oleracea and Am aranthus caudatus respectively) compared with 13% and 15% from these h osts when computed on the basis of C:N ratios of host xylem sap and C and N increments of Olax dry matter. Nitrate was the source of N avail able to pot cultures of the above species and estimates based on C:N r atios assumed that all N accumulated by Olax: had come from nitrate ab sorbed by the host. Equivalent estimates of H for Olax, grown in nitro gen-free pot culture with the native N-2-fixing host Acacia littorea a s its sole source of N, indicated 63% and 51% dependence on host carbo n when assessed in terms of xylem sap composition of host and parasite respectively. Comparisons of xylem sap solutes of Olax and a range of partner hosts indicated marked selectivity in haustorial uptake and t ransfer of nitrate, amino compounds, organic acids and sugars. Possibl e implications of variations between hosts in absolute levels of C and inorganic and organic forms of N in xylem are discussed in relation t o evidence of much better growth performance of Olax on Acacia littore a and other N-2-fixing legumes than on non-fixers.