TRANSFER OF SYMBIOTICALLY FIXED NITROGEN FROM BERSEEM (TRIFOLIUM-ALEXANDRINUM L) TO MAIZE VIA VESICULAR ARBUSCULAR MYCORRHIZAL HYPHAE

In order to examine whether hyphae of VA mycorrhizal fungi may aid the transfer of symbiotically fixed nitrogen from a legume to a non-legume, the roots of berseem plants inoculated with the VAM fungus Glomus intraradices and Rhizobium leguminosarum biovar. trifolii (strain RCR 5) were separated by a 2 cm root-free zone from the roots of maize plants. A 40 mum pore size nylon net allowed VAM hyphae to pass through the root-free zone but prevented root penetration. Replacement of this net by a 0.45 mum pore size membrane prevented penetration of VAM hyphae to the root-free compartment. When the berseem plants were 63 d old, the plant chambers were sealed in plastic bags to allow the roots of the plants to be exposed to a N-15(2)-enriched atmosphere for 5 d. Berseem plants that were exposed to a N-15(2)-enriched atmosphere fixed more N-15 than did berseem plants maintained under air at natural N-15 abundance. The total N-15 excess content of berseem was similar whether or not the mycorrhiza had access to the maize roots. Plant mass (dry weight) of maize was not affected by the mycorrhizal fungus, but total nitrogen content tended to be higher in VAM-colonized maize than in non-colonized maize. Furthermore, both atom % N-15 excess and N-15 excess content of mycorrhizal maize were significantly increased as compared with non-mycorrhizal maize or with maize controls without berseem. The amounts of N-15 transferred were small, accounting for less than 4 % of the N-15 in the N2-fixing plant. In spite of the 2 cm root-free separating zone, the presence of the mycorrhizal fungus in the non-legume increased the transfer of symbiotically fixed nitrogen from berseem to maize plants that were physiologically younger. The implications of these results for nitrogen transfer from a legume to an accompanying non-legume via VAM hyphae are discussed.