Translocation of carbon by Rhizoctonia solani in nutritionally-heterogeneous microcosms

Responses of Rhizoctonia solani to spatial heterogeneity in sources of carbon, and associated translocation of carbon (C), were studied in a simple microcosm system comprising two discrete domains of agar gels separated on a glass slide and overlain with a porous membrane. Two arrangements of the gel pairs were used, one containing two equally large resources (representing 'homogeneous' conditions) and one containing a large and a negligible resource (representing 'heterogeneous' conditions). The nutrient sources were a standard mineral salt medium with or without glucose as sole C source. The fungus was inoculated onto one domain and growth responses determined by direct measurement of biomass. Translocation of C was quantified by use of 13C-enriched glucose. This substrate was either added to the agar at the outset, when studying newly developing colonies, or as a pulse into already established colonies. When growing in heterogeneous conditions, the fungus actively translocated C from a glucose-containing domain to sustain growth in the adjacent region lacking such a resource. In homogeneous conditions there was evidence of passive translocation (diffusion), but the fungus preferentially used local resource to maintain growth. Active translocation was only observed in newly growing colonies, whereas passive translocation occurred in both growing and established colonies. When the fungus was pulsed with a 13C-enriched glucose solution after 10 d growth, 2.5 times more 13C was taken up by the fungus grown in heterogeneous than homogeneous conditions, suggesting uptake exceeded local demands. In heterogeneous conditions, the total amount of 13C enriched glucose taken up by the fungus was independent of the location of the enriched glucose in the underlying medium. When the nylon membrane was replaced by Cellophane (an additional C source), degradation of the membrane and an increase in biomass occurred only in the heterogeneous system. The possible implications for these results in soil systems are discussed.