DYNAMICS OF PHOSPHORUS TRANSLOCATION IN INTACT ECTOMYCORRHIZAL SYSTEMS - NONDESTRUCTIVE MONITORING USING A BETA-SCANNER

Phosphorus uptake and translocation through intact mycelial systems of Paxillus involutus and Suillus variegatus infecting Pinus contorta se edlings was monitored non-destructively using a beta-scanner. Mycorrhi zal plants were grown in flat perspex chambers (20 x 6 cm(2)) and root growth was restricted to the upper portion of each chamber enabling m ycelial translocation to be studied over distances of up to 15 cm. P-3 2 was supplied, either directly to distal parts of the extending mycel ium, or to single, cut mycelial strands in feeding dishes. Two-dimensi onal patterns of activity were accumulated as scans with a lateral res olution of 5 mm and a longitudinal resolution of 3-4 mm. No distinct t ranslocation front could be detected but patterns of accumulation of l abel in the mycorrhizal roots were not consistent with movement by sim ple diffusion. Activity in translocating hyphae became visible only af ter the activity in mycorrhizal root lips had been visible for a few d ays. In all cases there was a lag period of 20-50 hours before P-32 co uld be detected in mycorrhizal root tips. Pre-feeding with unlabelled phosphate had no effect on this lag period. This implies continuous tr anslocation of phosphate at low concentrations and a lag period due to the time needed for detectable levels of phosphate to accumulate in m ycorrhizal roots. Thus the minimum velocity of phosphate movement in t he hyphae would be 7.5 mm/h, if the first molecules of P-32 arriving a t the roots could be detected and the transport distance is 15 cm. Acc umulation of phosphate to the roots was fairly constant, but not linea r. The phosphorus uptake rate by intact mycelial margins was nearly fo ur orders of magnitude higher than the uptake rate of cut mycorrhizal strands. The results indicate that the fine, foraging hyphae are bette r suited for nutrient uptake than mycelial strands and that phosphorus translocation in the hyphae occurs by active translocation of small a mounts rather than by mass flow.