Soil water potential shifts: developmental responses and dependence on phosphorus translocation by the saprotrophic, cord-forming basidiomycete Phanerochaete velutina

Mycelial cord systems, of the basidiomycete Phanerochaete velutina, a commo n woodland saprotroph, were grown on unsterile soil in model laboratory mic rocosms from 4 cm(3) wood inocula. Systems were supplied after 37 d with a fresh 4 cm(3) beech wood 'bait', placed behind the foraging colony margin. Systems were subject to dry shift (-0.056 MPa) or wet shift (-0.009 MPa) ov er an 11 d period either immediately after, or 20 d after baits were suppli ed. Controls were maintained at constant soil matric potential (-0.019 MPa) . 57-d-old systems were supplied with NH4K2PO4 including P-32 tracer within soil compartments local to inoculum or bait. Image analysis was used to qu antify morphological responses to water regime and resource supply, and tra cer movement monitored non-destructively with a scintillation probe for 57 d. P-32 uptake was greatest when tracer was supplied local to the inoculum. Dry shift concurrent with bait supply caused system wide cord-thickening, prevented polarised growth towards the newly supplied bait, had a significa nt carbon (energy) cost compared to controls, significantly reduced P-32 ac quisition, and significantly increased P-32 relocation to the bait. Wet shi ft concurrent with bait supply caused considerable loss of extra-resource m ycelium in the unbaited region, resulting in highly polarised development a long the bait-inoculum line, but did not affect P-32 uptake and partitionin g. Delayed wet shift caused swifter polarisation towards the bait, quantifi ed in terms of fractal dimension, did not result in system wide regression of extra-resource mycelium, and resulted in correspondingly increased rates of P-32 acquisition. Delayed dry shift prevented polarised growth towards the bait and had only transient effects on P-32 uptake and partitioning. Re sults suggest that resource capture took priority over coordination of C re serves and reallocation of mycelial effort, and that mycelium colonising th e new resource was more dependent on P translocate during desiccation stres s.