Photosynthesis - How Orchids Feed Themselves

At the end of the day, orchids are plants.  All plants photosynthesize, and “light is food for plants."  We may refer to fertilizer as “plant food,” but that is a terrible misnomer – fertilizer is merely the vitamins and minerals that plants need.  There are no calories in fertilizer; therefore it is not food.  Ultimately, success with orchids or any plant is dependent on how much light they get.  Light should always be the first consideration when getting an orchid.

Whereby the energy from light is combined with carbon dioxide and water to yield glucose and oxygen.  The reverse equation of photosynthesis is respiration.  All organisms respire, including plants.  While plants both produce and consume oxygen, the net rate of photosynthesis is greater than the rate of respiration, which is why plants yield a net amount of oxygen. 

The orchid family is an ancient lineage of plants that evolved roughly 110-120MYA[1], though Epidendroideae (mainly the orchids we know such as Cattleya, Epidendrum, etc.) and much of the orchid family underwent their greatest evolutionary radiation after the K/T Extinction event (the one that killed the dinosaurs) about 65MYA.  To put that into perspective, the first true flowering plants in the world evolved at the beginning of the Cretaceous Period, about ~135MYA[2]; making the orchid family one of the most ancient flowering plant groups on earth.

Having a lot of time to evolve, and many environments to evolve into, orchids have unsurprisingly evolved alternative forms of photosynthesis to cope with the diversity of environments that they are in.  Orchids exhibit all three forms of photosynthesis that exist – C3, C4, and CAM photosynthesis.  Each form has a different mechanism to harvest carbon dioxide and light to make sugars for the plant to feed on, as well as other metabolites that the plant uses.  These metabolites that are not used in photosynthesis are called “secondary metabolites” (more on them later) and are used for other purposes for the plant, such as compounds for insect defense or fragrances to attract pollinators.  A rundown of each form of photosynthesis is below:

  • C3 – The most ancient form of photosynthesis.  Evolved to be optimal in cooler, wetter conditions.  Stomatal pores are open during the day and closed at night.  Oxygen and carbon dioxide freely flow throughout the plant.
  • C4 – Separates carbon-fixation in space.  Evolved to be optimal in high light and high heat environments.  Stomatal pores are selectively open during the day and closed at night.  Specialized cells selectively filter in carbon dioxide.  This increases the concentration of carbon dioxide around chloroplasts, increasing photosynthetic efficiency.
  • CAM – Crassulacean Acid Metabolism separates carbon fixation in time.  Stomatal pores are open at night, and closed during the day.  CAM plants have evolved to save water by closing pores during the day but also trapping carbon dioxide inside themselves at night.  This limits the amount of sugars that can be produced in a day because the supply of carbon dioxide is finite in the plant.

For C4 and CAM photosynthesis, carbon dioxide is trapped in a liquid form by converting it to malic acid – the same acid that’s responsible for apple flavor. 

In general, photosynthesis occurs in two phases –the light-independent (dark reactions), and the light-dependent reactions{3].  The light-dependent reactions occur in two photosystems, each of which harvests the power of the sun’s photons and stores their energy in various molecules inside the chloroplasts.  The light-independent reactions, AKA the Calvin-Benson Cycle, occur at any time, and the Calvin-Benson cycle is where the actual fixation of carbon occurs via the magic of an enzyme called RuBisCO.  RuBisCO is one of the only six known ways to fix carbon[4] and turn it into useful things to grow.  Without RuBisCO, most life on Earth would not exist.

A diagram[5] of what’s happening inside the plant can be seen below for each photosynthesis form, with a few examples:

Different orchid groups have evolved different forms of photosynthesis.  While it’s not critical to know which form of photosynthesis your orchid is using, it is interesting to understand how they use these types of photosynthesis to survive in nature.

Interestingly, orchids have unique anatomies that they have evolved to do alternative forms of photosynthesis in the pseudobulbs and leaves.  It’s been found in Oncidium, Cattleya, and other genera that orchids can switch which photosynthesis system that they are using[6], in response to environmental changes – very uncommon in the plant kingdom.  Typically, orchids can facultatively use their pseudobulbs as storage mechanisms for carbon dioxide, taking advantage of air pockets in the pseudobulb cellular structure called aerenchyma.  This allows them to act as if they were a CAM plant – storing carbon dioxide at night.  However, the pseudobulb photosynthesizes poorly compared to the leaves.  To maximize efficiency, the orchid also stores carbon dioxide (in the form of malic acid) in the leaves where it is used in the Calvin Cycle.  This allows the orchid to selectively close its pores in the daytime if the environment becomes too dry.

Figure 7 - A diagram of photosynthesis in a facultative CAM orchid.  Pseudobulbs help store carbon dioxide at night for photosynthesis during the day.  This allows the orchid to close its stomatal pores when it gets too dry but still allows it to photosynthesize. Spatial patterns of photosynthesis in thin-and thick-leaved epiphytic orchids: Unravelling C3-CAM plasticity in an organ-compartmented way Article  in  Annals of Botany · April 2013 DOI: 10.1093/aob/mct090 · Source: PubMed

Photosynthesis provides the energy that a plant needs to grow and flower.  Oftentimes, a grower wonders why a plant isn’t flowering or doing what it is supposed to be doing.  A good grower will adjust the light before other factors for non-growth or non-blooming.  Plants also take cues from the environment as well to optimize photosynthesis – they can sense far-red light (>700nm) to know if they are being shaded, as well as UV light to know if they are being exposed[7].  Orchids under too much light stress will sense light stress by picking up UV light signals and moving their chloroplasts away from the light[8].

Light has two properties – quality and quantity.  More on this will be discussed in a future article about artificial lighting.  Just know for now that light quality is the appropriate color spectrum, measured in nanometers/CRI, and light quantity is the brightness/intensity of light, measured in lumens.  Each lightbulb that you select to grow orchids under should produce more than 1,600 lumens per square foot at a distance of 1 foot above the plants.  The CRI should be >92.

For those of you interested, orchids use all colors in the visible spectrum, as well as UV and far-red light in their biochemical reactions.  Therefore, beware of those purple plant lights – they are not spectrally complete, and your plants may not bloom as well, or sometimes at all.  There is a myth that plants are green because they reflect the green light that’s not used, but that is not true.  Plants are green because chlorophyll happens to be the dominant pigment, and it absorbs green the least, but it still absorbs some green.

Figure 8 - McCree, 1972, Agric. Meteorology 9:191-216.  You can see UV light being absorbed by plants from 380-400nm, and far-red light absorbed above 700nm.  All wavelengths in between are used for different purposes.

[1] Givnish, T. J., Spalink, D., Ames, M., Lyon, S. P., Hunter, S. J., Zuluaga, A., ... & Cameron, K. M. (2015). Orchid phylogenomics and multiple drivers of their extraordinary diversification. Proceedings of the Royal Society B: Biological Sciences, 282(1814), 20151553.

[2] Friedman, William E. (January 2009). "The meaning of Darwin's "abominable mystery"". American Journal of Botany. 96 (1): 5–21. doi:10.3732/ajb.0800150. ISSN 0002-9122. PMID 21628174.



[5] Yamori W, Hikosaka K, Way DA. Temperature response of photosynthesis in C3, C4, and CAM plants: temperature acclimation and temperature adaptation. Photosynth Res. 2014 Feb;119(1-2):101-17. doi: 10.1007/s11120-013-9874-6. Epub 2013 Jun 26. PMID: 23801171.

[6] Spatial patterns of photosynthesis in thin-and thick-leaved epiphytic orchids: Unravelling C3-CAM plasticity in an organ-compartmented way Article  in  Annals of Botany · April 2013 DOI: 10.1093/aob/mct090 · Source: PubMed

[7] Teixeira, R. T. (2020). Distinct responses to light in plants. Plants, 9(7), 894.

[8] Lin, Y. J., Chen, Y. C., Tseng, K. C., Chang, W. C., & Ko, S. S. (2019). Phototropins mediate chloroplast movement in Phalaenopsis aphrodite (moth orchid). Plant and Cell Physiology, 60(10), 2243-2254.