The Plant kingdom is amazingly diverse and still quite unknown in many ways. As I have become fascinated with plants from the Proteaceae family, I have found out just how recent our understanding is about these plants and some of the things that make them special. I wrote about one of these some time ago, and that concerned their requirement for smoke as a way to break seed dormancy. In the past people thought they needed fire, but in the 1990s researchers found that it is chemicals in the smoke that was the important element. Since then, propagation techniques for many plants that appear after fire has swept through an area, have been revolutionized. Plants that used to have germination rates of a few percent, now see 90+% germination rates.
But what I want to talk about in this blog are roots. Again, this is a fairly recent discovery, this one going back a little further to 1960, but still not fully understood to this day. A hundred years ago, it was noticed that some Protea had roots that were different from others. Nobody knew why. The reason why I am writing this blog is because I saw these roots for the first time when I was repotting a Banksia. The term Proteoid roots was established in 1960 by H.M. Purnell who described them as a root with “dense clusters of rootlets of limited growth.” After extensive examination they were found in a number of species and genera within the Proteaceae family and today it is known that every genus except one grows these roots when required.
Purnell suggested that these roots were associated with the uptake of nutrients from the impoverished soils found in South Africa and Australia where these plants naturally grow – similar to the soils I have on the Oregon Coast. Most plants form a symbiotic relationship with mycorrhizal fungi.
Mycorrhizal fungi extract the minerals necessary for plant growth from the soil and give them to the plants in a liquid form that can be absorbed by the plant roots. In return, the plants support the fungi, giving them sugars and starches. But mycorrhizal fungi appear to not exist in the soils that these plants evolved in. This could be for one of two reasons: either the mycorrhizal fungi did not or could not live there, or that the soils are so lean that the plants cannot afford to give up any of their product of photosynthesis. These plants basically never shed their leaves.
Proteas evolved in the ancient supercontinent called Gondwana, but proteoid roots could be older than that because they have been shown to exist in plants from the Northern Hemisphere and thus arguably evolved when Pangea existed (about 250 million years ago). Other species have been shown to use this technique, especially those from the Mediterranean region. Some researchers disagree and say that the Proteoid root clusters from the Northern Hemisphere are different and may have been an independent development.
One paper suggests that proteoid roots in Lupins only developed 2.5 to 3 million years ago and that the only plants outside of the Proteaceae family that have these roots are nitrogen fixing plants. While that may be true for Northern Hemisphere plants, other Gondwanan plants do have similar root forms, although they appear to not be called Proteoid roots. The White Lupin (Lupinus albus) is an interesting Northern Hemisphere example because it is a food crop that exhibits this root formation, and so this plant is receiving a lot of attention from agricultural companies. This may finish up solving some of the many unknowns about these roots, but they may also be different from the true proteoid roots.
Without the mycorrhizal fungi, the plants have to extract nutrients themselves and that is the job of the proteoid roots. Normally the phosphorous is so tightly bound to other materials that it is not available to the plant. Proteoid roots exude acid, carboxylates, and other substances into the soil that break down minerals and allows phosphorous, iron, copper and other nutrients to be absorbed.
This is not a slow process – it is done in a burst over a period of just 2 or 3 days. It is believed that this is done to stop other microorganisms from being able to sniff them out and taking some for themselves. They have become so efficient at doing this that if you were to feed these plants with a balanced fertilizer, you would probably kill them. They would literally overdose on phosphorous. Some studies have found that Proteoid roots are up to 13 times more efficient at absorbing phosphorous that typical roots.
More Research Required
A lot remains unknown. The plant doesn’t always have these roots. They are grown at times and then reabsorbs them at others. It is expected that they are created when certain nutrients are deficient, but research so far has been inconclusive. It is also unclear why they then remove them rather than just keeping them until needed. If you want to learn more about this you might be interested in reading this research paper from 1999.