The Nitrogen-Fixing Abilities of Cycads

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Long before the first legumes came onto the scene, the early ancestors of Cycads were hard at work fixing atmospheric nitrogen. However, they don't do this on their own. Despite being plentiful in Earth's atmosphere, gaseous nitrogen is not readily available to most forms of life. Only a special subset of organisms are capable of turning gaseous nitrogen into forms usable for life. Some of the first organisms to do this were the cyanobacteria, which has led them down the path towards symbioses with various plants on many occasions. 

Cycads are but one branch of the gymnosperm tree. Their lineage arose at some point between the Carboniferous and Permian eras. Throughout their history it would seem that Cycads have done quite well in poor soils. They owe this success to a partnership they struck up with cyanobacteria. Although it is impossible to say when exactly this happened, all extant cycads we know of today maintain this symbiotic relationship with these tiny prokaryotic organisms. 

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

The relationship takes place in Cycad roots. Cycads don't germinate with cyanobacteria in tow. They must acquire them from their immediate environment. To do so, they begin forming specialized structures called precoralloid roots. Unlike other roots that generally grow downwards, these roots grow upwards. They must situate themselves in the upper layer of soil where enough light penetrates for cyanobacteria to photosynthesize.

The cyanobacteria enter into the precoralloid roots through tiny cracks and take up residence. This causes a change in root development. The Cycad then initiates their development into true coralloid roots, which will house the cyanobacteria from that point on. Cycads appear to be in full control of the relationship, dolling out carbohydrates in return for nitrogen depending on the demands of their environment. Coralloid roots can shed and reform throughout the lifetime of the plant. It is quite remarkable to think about how nitrogen-fixing symbiotic relationships between plants and microbes have evolved independently throughout the history of life on this planet.

Photo Credits: [1] [2]

Further Reading: [1] [2]

 

Meet the Sweetfern

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I remember the first time I laid my eyes on Comptonia peregrina. I was new to botany at that point in my life so I didn't have a well developed search image for these sorts of things. I was scrambling down a dry ridge with a scattered overstory of gnarly looking chestnut oaks when I saw a streak of green just below me on a sandy outcropping. They were odd looking plants, the likes of which I had never seen before.

I took out my binoculars to get a better look. What were these strange organisms? Were they ferns? No, they seemed to have woody stems. Were they gymnosperms? No, I could make out what appeared to be male catkins. Luckily I never leave home without a field guide or two. Using what little terminology I knew, I was able to narrow my focus to a plant commonly called a "sweetfern."

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This was one of the first instances in which I grasped just how troublesome common names can be. C. peregrina is mostly definitely not a fern. It is actually an angiosperm that hails from the bay family (Myricaceae). Comptonia is a monotypic genus, with C. peregrina being the only species. It is a denizen of dry, nutrient poor habitats. As such, it has some wonderful adaptations to deal with these conditions.

To start with, its a nitrogen fixer. Similar to legumes, it forms nodules on its roots that house specialized nitrogen-fixing bacteria called rhizobia. This partnership takes care of its nitrogen needs, but what about others? One study found that not only do the roots form nodules, they also form dense cluster roots. Oddly, closer observation found that these clusters were not associated with mycorrhizal fungi. What's more, they also found that these structures were most prevalent in highly disturbed soils. It is thought that this is one way that the plant can maximize its uptake of phosphorus under the harshest growing conditions. 

Flowering in this species is not a showy event. C. peregrina can be monoecious or dioecious, producing male and female catkins towards the ends of its shoots. After fertilization, seeds develop inside bristly fruits. Seed banking appears to be an important reproductive strategy for this species. One study found that germinated seeds had lain dormant in the soil for over 70 years until disturbance opened up the canopy above. It is expected that seeds of this species could exhibit dormancy periods of a century or more. 

In total, this is one spectacular species. Not only does it have a unique appearance, it is also extremely hardy and an excellent species to plant in drought-prone soils wherever it is native. I do see it in landscaping from time to time. If you encounter this species in the wild, take the time to observe it in detail. You will be happy you did!

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

Of Gunnera and Cyanobacteria

Nitrogen is a limiting resource for plants. It is essential for life functions and yet they do not produce it on their own. Instead, plants need to get it from their environment. They cannot uptake gaseous nitrogen, which is a shame because it makes up 78.09% of our atmosphere. As such, some plants have developed very interesting ways of obtaining nitrogen from their environment. Some, like the legumes, produce special nodules on their roots, which house bacteria that fix atmospheric nitrogen. Other plants utilize certain species of mycorrhizal fungi. One family of plants, however, has evolved a symbiotic relationship that is unlike any other in the angiosperm world.

A  Gunnera  inflorescence

A Gunnera inflorescence

Meet the Gunneras. This genus has a family all to itself - Gunneraceae. They can be found in many tropical regions from South America to Africa and New Zealand. Some species of Gunnera are small while others, like Gunnera manicata, have leaves that can be upwards of 6 feet in diameter. Their leaves are well armed with spikes and spines. All in all they are rather prehistoric looking. The real interesting thing about the Gunneras though, is in the symbiotic relationship they have formed with cyanobacteria in the genus Nostoc.

Traverse section of a  Gunnera  stem showing cyanobacteria colonies (C) and the cup-like structures (S) where they enter the stem.

Traverse section of a Gunnera stem showing cyanobacteria colonies (C) and the cup-like structures (S) where they enter the stem.

Gunnera produce cuo-like glands that house these cyanobacteria. The glands are filled with a special mucilage that not only attracts the cyanobacteria, but also stimulates it to grow. Once inside the glands, the cyanobacteria begins to grow into the plant, eventually fusing with the Gunnera cells. From there the cyanobacteria earn their keep by producing copious amounts of usable nitrogen and in return, the Gunnera supplies carbohydrates. This relationship is amazing and quite complex. It also offers researchers an insight into how such symbiotic relationships evolve.

Photo Credit: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5]