Irradiance governs exploitation of fungi: fine-tuning of carbon gain by two partially myco-heterotrophic orchids

Abstract

While all members of the Orchidaceae are fully dependent on mycorrhizal fungi during their achlorophyllous juvenile stages, mature plants may remain fully myco-heterotrophic, become fully autotrophic or develop a nutritional mode where the carbon gain through photosynthesis is complemented by organic carbon from fungal partners. This so-called partial myco-heterotrophy is intriguingly complex. Current knowledge indicates a large range in the proportion of fungus-derived carbon between and within partially myco-heterotrophic plant species. However, the driving factors for this variation are so far mostly unknown. Here we show for two green species of the orchid genus Cephalanthera that light availability is the major determinant of the degree of myco-heterotrophy. Using leaf stable isotope natural abundance analysis together with time-integrated microscale light climate monitoring we could demonstrate that there is a sensitive reaction to varying light availability within forests. Low light levels result in strong myco-heterotrophy while higher irradiances successively drive the orchids towards autotrophy. Our results demonstrate that partial myco-heterotrophy in these species is not a static nutritional mode but a flexible mechanism driven by light availability which allows a balanced usage of carbon resources available in nature.

Figure 1.

δ¹³C values of 12 autotrophic plant species (n = 177) including the orchid C. calceolus, two partially myco-heterotrophic Cephalanthera spp. (n = 18 each) and the fully myco-heterotrophic orchid N. nidus-avis (n = 11), plotted against mean light availability. Samples were collected from an open P. sylvestris stand (diamonds), a forest dominated by Fagus sylvatica (circles) and a mixed stand (squares) in northeast Bavaria. Regression curves (solid lines) are given with 95% confidence intervals (dashed lines).

Figure 2.

Correlation between relative enrichments in ¹³C (ε) calculated per plot (Preiss & Gebauer 2008) and mean light availability based on the data shown in figure 1. Regression lines (±95% confidence intervals) represent the range of isotope signatures of autotrophic (green), partially myco-heterotrophic (light and dark red) and fully myco-heterotrophic plants (blue). Arrows indicate the variable proportion of carbon derived from fungi (C_df) or photosynthesis (C_dp), respectively.