Are carbon and nitrogen exchange between fungi and the orchid Goodyera repens affected by irradiance?

Abstract

Background and aims: The green orchid Goodyera repens has been shown to transfer carbon to its mycorrhizal partner, and this flux may therefore be affected by light availability. This study aimed to test whether the C and N exchange between plant and fungus is dependent on light availability, and in addition addressed the question of whether flowering and/or fruiting individuals of G. repens compensate for changes in leaf chlorophyll concentration with changes in C and N flows from fungus to plant.

Methods: The natural abundances of stable isotopes of plant C and N were used to infer changes in fluxes between orchid and fungus across natural gradients of irradiance at five sites. Mycorrhizal fungi in the roots of G. repens were identified by molecular analyses. Chlorophyll concentrations in the leaves of the orchid and of reference plants were measured directly in the field.

Key results: Leaf δ(13)C values of G. repens responded to changes in light availability in a similar manner to autotrophic reference plants, and different mycorrhizal fungal associations also did not affect the isotope abundance patterns of the orchid. Flowering/fruiting individuals had lower leaf total N and chlorophyll concentrations, which is most probably explained by N investments to form flowers, seeds and shoot.

Conclusions: The results indicate that mycorrhizal physiology is relatively fixed in G. repens, and changes in the amount and direction of C flow between plant and fungus were not observed to depend on light availability. The orchid may instead react to low-light sites through increased clonal growth. The orchid does not compensate for low leaf total N and chlorophyll concentrations by using a (13)C- and (15)N-enriched fungal source.

Keywords: 13C; 15N; Ceratobasidium; Goodyera repens; Tulasnella; chlorophyll concentration; light availability; mycoheterotrophy; mycorrhiza; nutrition; orchid; stable isotopes.

Fig. 1.

(A) Roots of Goodyera repens. Notice the abundant root hairs. (B) Low chlorophyll concentration of a flowering shoot of G. repens surrounded by dark green non-flowering rosettes.

Fig. 2.

Overview over all leaf δ¹³C and δ¹⁵N values of Goodyera repens and of reference plants investigated in this study. Note the unique δ¹⁵N values found for Rubus chamaemorus.

Fig. 3.

Comparison of the enrichment factors ε for ¹³C and ¹⁵N in Goodyera repens leaves collected at the different sampling sites. For the reference plants, mean ε is zero by definition.

Fig. 4.

Leaf δ¹³C (A) and δ¹⁵N values (B) dependent on light availability for each investigated Goodyera repens individual and for the respective autotrophic reference plants.

Fig. 5.

Comparison of the leaf enrichment factors ε for ¹³C and ¹⁵N in Goodyera repens individuals associated with different mycorrhizal fungi (numbers in parentheses indicate the frequency of fungi detected in orchid roots): Tulasnella (17), Ceratobasidium (11), Russula (4), Lactarius (1), Sebacina (1). In some cases, Tulasnella was also found in roots together with Ceratobasidium, Lactarius, Russula and Sebacina. For the reference plants, mean ε is zero by definition.

Fig. 6.

Comparison of N, C and chlorophyll concentrations and of δ¹⁵N and δ¹³C values in the leaves of flowering/fruiting and non-flowering individuals of Goodyera repens: (A) total N concentration; (B) chlorophyll concentration; (C) δ¹⁵N; (D) total C concentration; (E) δ¹³C.