Light and C-13: Are orchids different from other vascular plants in their response to shade?

Orchids have symbiotic relationships with ectomycorrhizal or saprotrophic ("rhizoctonia") fungi, which provide plants with nutrients. The extent of plant dependency on fungi for carbon supply is reflected by plant tissue delta C-13 abundance values. Little is known about how wide-spread partial mycoheterotrophy (PMH) is among orchids in nature and it is debated whether rhizoctonia-associated orchids possess mycoheterotrophy. Using orchids with ectomycorrhizal fungi and rhizoctonia-associated orchid species, we tested one hypothesis: terrestrial green orchids decrease their delta C-13 values in response to shade less than neighboring non-orchid vascular plants (evidence for PMH). We examined eight pairs of plant species (orchid paired with reference autotrophic plant growing close-by) in well-lit versus shady field conditions in replicated sites across the European part of Russia. In each site illumination level was recorded and plant material was examined for delta C-13. We found that all tested orchids showed similar weaker response of tissue delta C-13 signature to relative light exposure in contrast to the stronger response in control plants, suggesting wide-spread PMH among orchids. There was no significant difference in orchids with ectomycorrhizal fungi and rhizoctonia-associated orchids in response of delta C-13 to shade, supporting mycoheterotrophic mechanisms of carbon transfer to plants in rhizoctonia-associated orchids. Proportions of fungi-derived carbon for individual orchid species ranged between 3 and 50%. Orchids reduced stomatal density in response to shade less or similarly to autotrophic plants, suggesting that mycoheterotrophic plants might be somewhat less water limited than autotrophic species due to fungal water supply. Our data provide support for broad prevalence of PMH among orchids and of the existence of PMH in rhizoctonia-associated orchids. These findings extend the ecological importance of mycorrhizal associations in belowground food webs.