Carbon nutrition of mature green orchid Serapias strictiflora and its mycorrhizal fungus Epulorhiza sp.

We studied the nutritional modes of the orchid Serapias strictiflora and its mycorrhizal fungus Epulorhiza sp. using the differences in carbon isotopic composition (delta C-13) of C-3 orchid and C-4 maize tissues. We found that if cultivated in substrate lacking any organic compounds, the mycorrhizal extraradical mycelia (delta C-13 = -26.3 +/- 0.2 %) developed well, despite being fully dependent on nutrition from orchid roots (delta C-13 = -28.6 +/- 0.1 %). If the mycorrhizal fungus had additional access to and colonized decaying maize roots (delta C-13 = -14.6 +/- 0.1 %), its isotopic composition (delta C-13 = -21.6 +/- 0.4 %) reflected a mixture of biotrophy and saprotrophy. No statistically significant differences in delta C-13 of new storage tubers were found between Epulorhiza-associated orchids with (delta C-13 = -28.2 +/- 0.1 %) and without access to maize roots (delta C-13 = -28.6 +/- 0.2 %). We conclude that autotrophy is the predominant nutritional mode of mature S. strictiflora plants and that they supply their mycorrhizal fungus with substantial amount of carbon (69 +/- 3 % of the fungus demand), even if the fungus feeds saprotrophically.