The tiny-leaved orchid Cephalanthera subaphylla obtains most of its carbon via mycoheterotrophy

The evolution of mycoheterotrophy has been accompanied by extreme reductions in plant leaf size and photosynthetic capacity. Partially mycoheterotrophic plants, which obtain carbon from both photosynthesis and their mycorrhizal fungi, include species with leaves of normal size and others that are tiny-leaved. Thus, plant species may lose their leaves in a gradual process of size reduction rather than through a single step mutation. Little is known about how the degree of mycoheterotrophy changes during reductions in leaf size. We compared the degree of mycoheterotrophy among five Japanese Cephalanthera species, four with leaves of normal size (Cephalanthera falcata, Cephalanthera erecta, Cephalanthera longibracteata and Cephalanthera longifolia), one with tiny leaves (Cephalanthera subaphylla), and one albino form of C. falcata (as reference specimens for fully mycoheterotrophic plants). The levels of mycoheterotrophy were determined by stable isotope natural abundance analysis. All Cephalanthera species were relatively enriched in ¹³C and ¹⁵N in comparison with surrounding autotrophic plants. Cephalanthera subaphylla was strongly enriched in ¹³C and ¹⁵N to levels similar to the albinos. Species with leaves of normal size were significantly less enriched in ¹³C than C. subaphylla and the albinos. Thus, C. subaphylla was strongly mycoheterotrophic, obtaining most of its carbon from mycorrhizal fungi even though it has tiny leaves; species with leaves of normal size were partially mycoheterotrophic. Hence, during the evolutionary pathway to full mycoheterotrophy, some plant species appear to have gained strong mycoheterotrophic abilities before completely losing foliage leaves.