Microautoradiographic localization of phosphate and carbohydrates in mycorrhizal roots of Populus tremula x Populus alba and the implications for transfer processes in ectomycorrhizal associations

Microautoradiographic studies were carried out to examine the distribution and exchange of phosphate and labeled carbohydrates in mycorrhizal roots of Populus tremula x Populus alba L. following application of P-33-orthophosphate (P-i) and (CO2)-C-14. Labeled P-i was not homogeneously distributed along the mycorrhizal longitudinal axis. The fungal sheath and the Hartig net contained more P-33(i) in the median parts of the root than in the apical or basal root zones, indicating that uptake and transfer of P-i to the host plant was localized mainly in this area. The P-i was translocated by the Hartig net and the interfacial apoplast to the host plant. It was distributed by way of the stele within the plant. Young leaves and meristematic tissue in the shoot tip were the main sinks for P-i. In plants that were left in the dark for 5 days before P-33(i) application, the reduced carbohydrate supply caused a decrease in P-i absorption by mycorrhizal roots. Microautoradiography of mycorrhizal roots after assimilation of (CO2)-C-14 revealed that: (1) the fungal partner had a high capacity to attract photosynthates; (2) the main transfer of carbohydrates was localized in the median zone of a mycorrhizal root; (3) carbohydrates that were absorbed by the mycorrhizal fungus were translocated to the fungal sheath and were homogeneously distributed; and (4) in the main exchange zone, cortical cell nuclei showed a high sink capacity, indicating increased metabolic activity in these cells. We postulate that (1) the phosphate demand of the host plant regulates absorption of P-i by the fungus, and (2) a bidirectional transfer of carbohydrates and P-i occurs across the same interface structure in ectomycorrhizal roots of Populus.