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Troubles in the Rhizosphere Part 3

A Closer Look at Roots

Mycorrhizae covered by hyphae. Water and elements often are absorbed into the hyphae and then the tree. The hyphae extending from the mycorrhizae greatly increase the area for absorption.

 

Woody tree roots are organs that support the tree mechanically, store energy reserves, transport water and the substances dissolved in it and synthesis substances such as growth regulators, amino acids and vitamins that are essential for growth.

Trees have different types of root systems. For example, mangroves along coastlines have stilt roots. Many trees growing in tropical areas have aerial roots that become prop roots when they grow into the soil. Other trees have strangling roots that eventually kill the host tree that first supported their growth. Trees in sandy soils can have roots that grow downward over 90 feet. Palms have roots that are adventitious and grow from meristematic regions in their base. Many tree species have deep roots when they are young and more shallow roots later. It would be nearly impossible for the strongest person to pull out young saplings of beech, oak or hickory from forest soil.

Woody roots have cells with walls of cellulose, hemicellulose and lignin. Lignin is that natural "cementing" substance that gives wood its unique characteristic for strength. Woody roots also have an outer bark or periderm made up of three layers: the phellogen, phelloderm and phellem. The phellogen is the bark cambium. The phelloderm is a thin layer of cells on the inner side of the phellogen. The phellem is the outer corky layer. Phellem cells are impregnated with a substance called suberin, which is a fatty substance that prevents water absorption.

Some characteristics of woody roots are:

* They do not absorb water.

* They have no pith.

* Their conducting elements are usually wider than those in the trunk.

* They have a greater proportion of parenchyma cells than is usual for trunks. The living parenchyma store energy reserves, usually as starch.

A soft cortex without chlorophyll may be in the bark. In some tree species that thrive in wet soils or have deep roots, the cortex may have many open spaces that act as channels for air to reach the living cells in the roots. It is important to remember that the parenchyma in the woody roots store energy reserves, and root defense is dependent on energy reserves. When reserves are low, defense is low. When defense is low, weak or opportunistic pathogens attack. It is nature's way.

Non-Woody Roots

Non-woody tree roots are organs that absorb water and elements dissolved in it. The two basic types of non-woody roots are:

1. Root hairs on non-woody roots are extensions of single epidermal cells. Common on seedlings, root hairs grow to maturity in a few days. They function for a few weeks and then begin to die.

On mature trees, they are usually not abundant. When they do form, they do so when soil conditions are optimum for absorption of water and elements. I have found root hairs growing in non-frozen soils beneath frozen soils in winter.

2. Mycorrhizae are the other type of non-woody roots. Mycorrhizae are organs made up of tree and fungus tissues that facilitate the absorption of phosphorus-containing ions and others essential for growth.

The fungi that infected developing non-woody roots to form mycorrhizae were very "biologically smart." Rather than competing with other microorganisms in the rhizosphere for exudates from the tree, the mycorrhizal-forming fungi went right to the source inside the tree. And, even more to their advantage, many of the mycorrhizal fungi grew thread-like strands of hyphae-long, vegetative tubes of fungi-out from the mycorrhizae. This inside and outside presence gave the fungi a distinct advantage over other microorganisms in the rhizosphere.

The tree gains efficiency with mycorrhizae in several ways.

A block of frozen soil several inches deep was lifted away to reveal these mycorrhizae and strands of litter-decomposing fungi. Note the cavities surrounding the mycorrhizae.

 

1. With their extended hyphae, mycorrhizae not only greatly extend the absorbing potential into the soil, but the hyphae may connect with other hyphae on other trees. In this way, the mycorrhizae serve to connect trees of the same or a different species. This leads to the conjecture that the natural connections that developed over long periods in the natural forest may have some survival value. That is why forest types are often named for the groups of species commonly found growing together. For example, we speak of the birch-beech-maple forest, or the pine-oak forest. From a practical standpoint, when trees are planted in cities and parks, there may be great survival advantages by planting groups of trees made up of the species that are normally found together in natural stands.

2. The mycorrhizae have been shown to provide some resistance against root pathogens. It may be that the pathogens would have difficulties in building their populations in the rhizosphere dominated by the mycorrhizal fungi.

Perhaps the most important feature of the mycorrhizal fungi is that their boundary material is mostly chitin.  Chitin is slightly different from cellulose by the replacement of some cellulose atoms by a chain of atoms that contain a nitrogen atom. This slight change in some way makes chitin a material better suited for absorption of elements. Remember that the fungus hyphae gain all their essentials for life by absorption through their boundary substance.

There are other advantage,, to the chitin and the tube-like hyphae that ramify the soil in the rhizosphere and beyond. When the hyphae die, they add a nitrogen source for other organisms. Also, when the hyphae are digested, they leave tunnels in the soil that are about eight to 10 microns in diameter. For the bacteria, these small tunnels may mean the difference between life and death. The bacteria quickly colonize the tunnels. The survival advantage here is that the major threats to their survival are protozoa that are usually much larger than 10 microns. So the hungry amoebae are not able to get at the bacteria inside the eight-micron tunnels.

A common treatment for compaction is to fracture the soil and add water,. The fracturing allows air to penetrate the soil, but does not provide any eight-micron tunnels for the bacteria. The only way to bring back the tunnels is to bring back the fungi in well-composted wood and leaf mulch, as nature does, or by inoculating the mulch with mycorrhizal fungi.

Who Was First?

I do not know if the fungi were the first to grow into the root to get first chance at exudates or whether it was the bacteria. Regardless, bacteria and their close relatives, the actinomycetes, also infect non-woody roots to form organs that serve for the fixation of atmospheric nitrogen. Fixation means that the nitrogen that makes up almost 80 percent of our air is converted to a soluble ionic form by the action of the bacteria and actinomycetes within the nodules on the roots. (Some free-living soil bacteria can also fix nitrogen.) An enzyme called nitrogenase is the catalyst for the reaction that will take place only under very exacting conditions. There must be soluble molybdenum and iron and no free oxygen available. These conditions are present within the nodules. Here again, the microorganisms benefit the tree by providing a source of soluble nitrogen, and, in turn, the bacteria and actinomycetes get first chance at exudates. Even more importantly, the nodules protect them from foraging protozoa.

Infections that result in benefits to both parties are called mutualistic. When the benefits are greater than the sum of the parts, the association is called synergistic.

Species of legumes commonly have bacterial nitrogen-fixing nodules and mycorrhizae. The mycorrhizae facilitate absorption of elements, and the nodules provide a nitrogen source. Many species of trees have actinorhizae, which are the nodules formed by the root infections by actinomycetes. Species of Alnus have very large nodules. The actinorhizae are common on tropical and subtropical trees, and especially on trees that have adapted to soils low in available elements essential for life.

On some subtropical and tropical trees, such as the macadamia, multi-branched clusters of non-woody roots called proteoid roots form. The proteoid roots alter the rhizosphere by acidification processes that facilitate the absorption of phosphorus-containing ions. When I examined the roots of dying macadamia nut trees in an orchard in Hawaii, I could not find proteoid roots, yet only a few days earlier I had found them on macadamia nut trees growing in the wild. I learned later that the orchard where trees were dying was heavily fertilized on a regular, basis with phosphorus.

Another type of nodule forms on species of cycads. These nodules harbor blue green algae, or cyanobacteria, that have the ability to fix atmospheric nitrogen.

My point is that many different synergistic associations have developed in, on and about non-woody roots that provide elements, not an energy source. These associations are of extreme benefit to all connected members. At the same time, the conditions that provide for the associations are very delicate and exacting. It does not take much to disrupt them.