• A new method is described for monitoring hyphal 32P transport in compartmented, monoxenic mycorrhizal root cultures. Nondestructive time-course measurements of P transport in hyphae were obtained by capturing digital autoradiograms on P-imaging screens, and comparing with growth observed by optical scanning. 32P distribution measured by densitometry on the day of harvest closely agreed with values obtained by liquid scintillation counting after destructive harvest.
• Virtually all labeled PO4 was absorbed by arbuscular mycorrhizal (AM) hyphae, but transfer to the roots appeared to be incomplete. P transport was not unidirectional towards the roots, as 32P was also transported from the root compartment to the hyphal compartment. Net P flux rates were calculated for hyphae crossing between compartments, taking bidirectional flow into account.
• Amounts of transported P were poorly correlated with extra-radical hyphal length and root d. wt, but highly correlated with the number of hyphae crossing the barrier separating the two compartments. Such correlations were highest when only hyphae with detectable protoplasmic streaming were considered.
• The method was tested using radiolabeled P sources, H2PO4− and cytidine triphosphate (CTP), and the AM fungi, Glomus intraradices and G. proliferum. Fungal transport of 32P from CTP was much slower than from PO4 for both fungi.