The Leafless Orchid Cymbidium macrorhizon Performs Photosynthesis in the Pericarp during the Fruiting Season

Photosynthesis with highly photoreactive chlorophyll (Chl) provides energy for plant growth but with simultaneous risk of photooxidative damage and photoprotection costs. Although the leafless orchid Cymbidium macrorhizon mostly depends on mycorrhizal fungi for carbon, it accumulates Chl particularly during fruiting and may not be fully mycoheterotrophic. In fact, stable isotopic analysis suggested that the fruiting C. macrorhizon specimens obtain a significant proportion of its carbon demands through photosynthesis. However, actual photosynthetic characteristics of this leafless orchid are unknown. To reveal the functionality of photosynthetic electron transport in C. macrorhizon, we compared its photosynthetic properties with those of its relative mixotrophic orchid Cymbidium goeringii and the model plant Arabidopsis thaliana. Compared with C. goeringii and A. thaliana, maximum photochemical efficiency of PSII was substantially low in C. macrorhizon. Chl fluorescence induction kinetics revealed that the electron transport capacity of PSII was limited in C. macrorhizon. Chl fluorescence analysis at 77K suggested partial energetic disconnection of the light-harvesting antenna from the PSII reaction center in C. macrorhizon. Despite its low PSII photochemical efficiency, C. macrorhizon showed photosynthetic electron transport activity both in the field and under laboratory conditions. Cymbidium macrorhizon developed strong nonphotochemical quenching in response to increased light intensity as did C. goeringii, suggesting the functionality of photoprotective systems in this orchid. Moreover, C. macrorhizon fruit developed stomata on the pericarp and showed net O-2-evolving activity. Our data demonstrate that C. macrorhizon can perform photosynthetic electron transport in the pericarp, although its contribution to net carbon acquisition may be limited.

Automated summary

Photosynthesis with chlorophyll in Cymbidium macrorhizon, a leafless orchid, provides energy for growth but comes with risks. Despite mostly relying on mycorrhizal fungi for carbon, this orchid shows some carbon acquisition through photosynthesis during fruiting. However, its photosynthetic characteristics are not fully understood. Compared to other orchids and a model plant, C. macrorhizon has lower maximum photochemical efficiency and limited electron transport capacity in its PSII. Despite this, it still shows some photosynthetic activity and has photoprotective systems in place.