Roots are specialized organs that plants use to collect water. Unless a plant has specialized structures for collecting water via other means, vitually 100% of water is absorbed through the roots. Nearly all orchids absorb 100% of their water through their roots, with some exceptions (e.g. black-haired (nigrohirsute) Dendrobium, Phalaenopsis violacea/bellina types…etc.) that have evolved the ability to absorb water via humidity entering the stomatal pores[1]. Since the majority of orchids in cultivation are epiphytes, we will only talk about epiphytic roots and their modifications from “normal” terrestrial roots.
Root Anatomy
Epiphytic orchid roots (hereon referred to as just roots) have evolved with one purpose in mind – to collect water as fast as possible before the next dry spell. You’re probably wondering “But they are tropical, and the tropics are always wet, right?” No, the sun is so strong, that even with frequent rains up to a few times a day, the plants still manage to get crispy dry when the sun hits them. Additionally, (and this is important to note) epiphytes do not have access to moisture from the soil. That means that when the rain stops, so does access to water. That’s why we at the AOS say that to be most successful at orchids, you must master making your plants’ wet-dry cycles to be very quick. This can be achieved in many ways depending on your conditions, habits, and potting media. But first, let’s take a look at what roots are, and how they function.
Figure 3 – A healthy orchid root tip with velamen. The longer the green at the tip, the faster the growth! Courtesy Dr. Lew Feldman, https://botanicalgarden.berkeley.edu/glad-you-asked/epiphytes
Orchid roots have two visible parts – the velamen and the root tip. The root tip is the nexus of growth, and tells you about the health of the overall orchid. Often, the root tips will be the “canary in the coalmine” giving warnings about the future of the orchid’s health in the time to come. The longer and more robust the root tips are, the faster and healthier the orchid is growing. The root tip is alive, but once the cells mature and the root elongates, they become the dead (but functional!) velamen layer.
The velamen is a layer of the root that serves three functions – to absorb water, hold water, and house beneficial fungi.
If you squeeze an orchid root, it feels hard, but on the cellular level, it’s rather porous and spongy. Interestingly, if you look closely at the cellular structure of the velamen, the sponginess pattern is like a fingerprint – each pattern corresponds to a species! Again, the spongy layer is formed by the dying and hollowing out of cells created by the root tip. While you may think that the velamen needs to be alive to absorb water, it does not. Because of the adhesive-cohesive nature of water, the plant can take advantage of this quirk of physics to absorb water without exerting any energy. Water adheres to other surfaces that have a slight static charge (like the spongy velamen cell walls) and water coheres to itself – sticking to itself to pull more of itself. This is why straws in beverages work. The water adheres to the straw’s inner edges, and coheres to itself, bringing itself up as you suck the air out of the straw.
Figure 4 - SEM images of (a) Dendrobium densiflorum, longitudinal section. (b) Dendrobium nobile, cross section. (c) Dendrobium nobile, longitudinal section. (d) Microcoelia exilis, cross section. Courtesy Hauber, F., Konrad, W. & Roth-Nebelsick, A. Aerial roots of orchids: the velamen radicum as a porous material for efficient imbibition of water. Appl. Phys. A 126, 885 (2020). https://doi.org/10.1007/s00339-020-04047-7
If you have ever broken an orchid root, you may be wondering what that stringy thing is inside the orchid. The “string” inside of the orchid root is the true root and is alive at maturity. The velamen is a spongy layer that is created to house fungi and absorb/hold water, but it all feeds into the wiry root at the center. Epiphytes generally don’t have media to draw water from when it stops raining, so they have grown a part of themselves to be that media to trap as much rainwater as possible before it drips off.
New roots are only formed with new growth, but older healthy roots may branch with new growth – a sign that you are doing very well! While roots may be many colors, the true way to distinguish between a healthy and a not healthy root is to squeeze it. If the root is hollow (which means that the velamen rotted away), then it is dead and may be removed. Root tip colors also vary with species, ranging from green to pink to orange to purple.
Orchid Root Mycorrhizal Associations
As humans, we are programmed to think that most fungi are bad because we associate them with rot. This is not the case for many fungi. Think of the fungi that you eat (mushrooms) and the fungi you drink (yeast for beer and wine)! There are many beneficial fungi, and orchids have made relationships with hundreds of species of different fungi[2]. The velamen is a layer where the orchid purposely cultivates mycorrhizal fungi to help absorb water and trace minerals for the orchid. This symbiosis benefits both the orchid and the fungi – the orchid receives minerals and water from the fungus, and the fungus receives sugars from the orchid. The term “mycorrhizal fungi” generally refers to all beneficial fungi that live in the root zone and inside the plant.
Orchids that have more diverse beneficial fungi in their root zones grow significantly more robustly than plants that do not have as much diversity. Think of mycorrhizal fungi as the “gut bacteria” of the plant. The mycorrhizal fungi will take on the burden of helping out with some metabolic processes within the plant, which will allow the plant to grow bigger, faster, and more floriferously.
Some examples of commonly associated genera of fungi for orchids are typically basidiomycetes: Ceratobasidium[3] (Rhizoctonia), Sebacina, Tulasnella[4] Thelephora, Tomentella, and Russula species[5][6]. Most orchids associate with saprotrophic fungi (dead-material-feeders) or endophytic/endomycorrhizal fungi (fungi that penetrate living cells), while a few associate with ectophytic/ectomycorrhizal fungal species (fungi that do not penetrate living cells, but live between the cells).
What’s interesting is that Rhizoctonia-type fungi are also opportunistic saprotrophs – they will consume decaying matter around the orchid root, freeing the nutrients to be absorbed, but also will consume the orchid root if the root dies. Rhizoctonia root rot is one of the more common rots of all commonly cultivated orchids. Keeping your media fresh and frequently drying out the media between waterings is the key to preventing this normally beneficial species from “turning on/betraying” the orchid.
If you are interested in supplementing beneficial fungi for your orchids, Professor Paul Stamets is the leader in fungal research in the US and has a company that sells endophytic supplements for plants. Although the research is based on plants that grow in soil and associated soil mycorrhizae, based on the research, these supplements might be worth a try[8]!
[1] Unravelling foliar water uptake pathways: The contribution of stomata and the cuticle
Paula Guzmán-Delgado,Emilio Laca,Maciej A. Zwieniecki
First published: 04 March 2021
[2] https://serc.si.edu/research/research-topics/biodiversity-conservation/orchids-fungi-symbioses
[3] Novotná, A., Benitez, A., Herrera, P., Cruz, D., Filipczykova, E., & Suárez, J. P. (2018). High diversity of root-associated fungi isolated from three epiphytic orchids in southern Ecuador. Mycoscience, 59(1), 24-32. https://www.sciencedirect.com/science/article/abs/pii/S1340354017300803
[4] Suárez, J. P., Weiß, M., Abele, A., Garnica, S., Oberwinkler, F., & Kottke, I. (2006). Diverse tulasnelloid fungi form mycorrhizas with epiphytic orchids in an Andean cloud forest. Mycological research, 110(11), 1257-1270.
[5] Cevallos, S., Sánchez-Rodríguez, A., Decock, C., Declerck, S., & Suárez, J. P. (2017). Are there keystone mycorrhizal fungi associated to tropical epiphytic orchids?. Mycorrhiza, 27(3), 225-232.
[6] https://serc.si.edu/research/research-topics/biodiversity-conservation/orchids-fungi-symbioses
[7] Quote, Christopher Satch, Professor at NYBG
[8] This is not an endorsement
