Several plant lineages evolved the ability to obtain carbon from the mycorrhizal fungi in their roots. These mycoheterotrophic plants provide outstanding opportunities for studying convergent evolution (representing at least 47 losses of photosynthesis in land plants, likely far exceeding the number of origins of parasitic plants), plant-microbe interactions (typically they indirectly parasitize green plants through shared mycorrhizal fungi), conservation biology (there are many rare and poorly known lineages), plant morphology and anatomy (unusual reductions in vegetative form; adaptations for plant-fungal interactions; floral modifications), evolution, systematics and phylogeny (e.g., modification, reduction and rate elevation in plastid and other plant genomes), and biogeography (disjunct distributions). Crucial developments in the fields of ecology and evolution have led to significant progress in our understanding of mycoheterotrophic plants. Here we pull together recent insights on these fascinating organisms in an intregrative symposium on their biology.
Wanek W, Huber W, Arndt SK, Popp M (2002) Mode of photosynthesis during different life stages of hemiepiphytic species. Functional Plant Biology 29, 725–732.
Beginning with flowering the leaves begin to wither. So the flower starts its additional CAM photosynthesis and secures a sufficient supply with nutrients even under conditions of increasing heat and drought.
The seedling is developing under conditions of a mycoheterotrophic supply with nutrients. Developing the first leaves, photosynthesis becomes possible. It can be assumed that the C3 mechanism of photosynthesis is still dominant, since temperatures are moderate in spring and there is enough water. And the plant needs to grow quickly, to build up biomass. Here the C3 photosynthesis has a clear advantage.
-Mesophyll cells perform only initial fixation in C4 plants.
CAM- Separation of initial CO2 fixation and calvin cycle is between night and day.
In this study, we investigate the occurrence of CAM photosynthesis in 200 native orchid species from Panama and 14 non-native species by carbon isotopic composition (δ13C) and compare these values with nocturnal acid accumulation measured by titration in 173 species.
An advantage of the floral photosynthesis is the ideal position of flowers to the sun light. The higher amount of light energy absorbed enables a higher capacity to store CO2. Though there is no detailed research yet, there are many signs that especially the Mediterranean Ophrys master a floral photosynthesis. The CAM metabolism enables them to survive in dry climates, on rock grounds, with early withering leaves. The evolution of the Ophrys labellum fulfilled two functions: the adaption to pollinators and the ability of floral photosynthesis.
Compare & Contrast
C3- No separation of initial CO2 fixation and calvin cycle
-Stomata opens in the day
-Best adapted to cool, wet environments
-mesophyll cells perform complete photosynthesis in C3 plants
C4- Separation between mesophyll and bundle-sheath cells (in space)
-Stomata opens in the day
-Best adapted to Hot, sunny environments.
Since CAM plants have to store CO2 by night in order to enable the daily photosynthesis, they have enlarged vacuoles. Thick leaves can store more organic acid which conserves CO2 in night time. It may well be that this is also the case with the floral photosynthesis of Ophrys. One clear sign is the thick Ophrys labellum – the form which also has its special function to imitate female pollinators to induce pseudo copulation. The often low height of Ophrys is an additional sign that those species are at least facultative CAM plants, since there is less biomass produced.
Most orchids with thin leaves use a C3 photosynthesis. Among them are the genera Orchis, Dactylorhiza, Anacamptis and Neotinea. Those with thick leaves can master CAM photosynthesis, for example the tropical genera Phalaenopsis, Cymbidium or Cattleya.
Although thick leaves were related to δ13C values in the CAM range, some thin-leaved orchids were capable of CAM photosynthesis, as demonstrated by acid titration.
There are also plants with a facultative CAM photosynthesis: the water saving mode will only be used in dry summer times. In spring, the budding plants still use the C3 photosynthesis. While CAM has the advantage of a lesser water consumption it also has a big disadvantage: The production of biomass is quite lower. CAM plants grow much slower than C3 plants.
The advantage of this CAM photosynthesis compared with the standard C3 photosynthesis: The plant cells receive carbon dioxide at night. Then, the stomata of the plant tissue are open. In the daily heat they can be closed, so the plant is protected from dehydration. The CO2 received by night is stored in the vacuoles of the cell in form of malic acid. By day, it will be transformed to carbon dioxide and oxygen by the help of light energy. The CAM photosynthesis only needs less than a fifth of the water amount which is normally necessary in the case of C3 photosynthesis. Therefore, the plant can survive in dry times and is protected against lack of water.