Direct measurement of arbuscular mycorrhizal phosphorus uptake into field-grown winter wheat

The aim of the study was to measure hyphal P uptake by native arbuscular my corrhizal fungi using a test system designed for field conditions. Phosphor us uptake by external hyphae of a native arbuscular mycorrhizal (AM) fungal community was studied in the winter wheat (Triticum aestivum L, ev. Stava) crop of an organic cropping rotation. Mesh containers containing a soil of volume accessible only to external hyphae were buried in the ground. The s oil inside these hyphal compartments (HCs) was labeled with P-32 to disting uish P taken up from inside the HCs from overall P uptake. As an additional control treatment, half of the HCs received the fungicide carbendazim [met hyl benzimidazol 2-y carbamate] at a rate known to disrupt AM hyphal P upta ke. Wheat tillers were harvested throughout the experimental period to moni tor the time course of hyphal P uptake. final harvest was at plant maturity , at which time the HCs a ere excavated and the soil inside was analyzed fo r hyphal length, NaHCO3-extractable P and specific activity of the extracte d P. External hyphae of the native AM fungi grew into the HCs without added carbendazim and took up substantial amounts of P. Addition of carbendazim at a rate known to inhibit hyphal P uptake resulted in no P uptake from ins ide those HCs. This is due to the design of the HCs used in this study, whi ch successfully prevented P uptake by plant roots and root hairs from insid e the HCs. The HCs are therefore well suited for field measurements of plan t P uptake,ia external hyphae of the native MI fungi. Estimates for length- specific hyphal P uptake per unit time (37.8 +/- 5.2 fmol P m(-1) s(-1)) we re similar to published estimates obtained in growth chamber studies with c ultured isolates of AM fungi (1-430 fmol P m(-1) s(-1)). The results of thi s study demonstrate the considerable contribution of native Mi fungi to ove rall P uptake of field-grown winter wheat, even at typical field soil ferti lity levels (28 mu g NaHCO3-extractable P g(-1) soil).