Mixotrophy in orchids: facts, questions, and perspectives

Area of Science:

• Plant biology
• Mycology
• Ecology

Background:

• Orchids rely on mycorrhizal fungi for carbon, especially during germination.
• Adult orchids exhibit varied carbon exchange with fungi, ranging from autotrophy to full mycoheterotrophy.
• Mixotrophic (MX) orchids are photosynthetic but also import carbon from fungi.

Purpose of the Study:

• To classify mixotrophic (MX) orchid nutrition into distinct types.
• To differentiate nutritional strategies based on fungal associations and isotopic signatures.
• To explore the characteristics and research potential of Type III mixotrophy.

Main Methods:

• Classification of MX orchid nutrition based on fungal symbiont taxonomy and ecology.
• Isotopic analysis (²H, ¹³C, ¹⁵N) to trace carbon and nutrient sources.
• Case study of Mediterranean *Ophrys* variants and review of published data.

Main Results:

• Three types of MX orchid nutrition were identified based on fungal associations.
• Type I and Type II mixotrophy involve diverse fungi (Asco- and Basidiomycota) and enrich orchids in ²H, ¹³C, and ¹⁵N.
• Type III mixotrophy, associated with specific rhizoctonias, enriches orchids primarily in ²H and ¹⁵N.

Conclusions:

• Mixotrophic orchid nutrition is diverse, with distinct classifications based on fungal partners.
• Type III mixotrophy presents unique nutritional dynamics and warrants further investigation.
• Understanding these nutritional strategies is crucial for orchid conservation and ecology.