Horray for Roots!
by Jody Padgham

This article was first printed in the Sept/Oct 2009 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.

Have you been pondering roots lately? Most of us haven’t spent a lot of time thinking about the role that the root system plays in a plant’s life. Bud Markhart, Professor of Organic Horticulture at the University of Minnesota, has spent a lot of his time not only understanding roots, but really appreciating their critical importance in the life of plants. His enthusiasm for roots was obvious as he led a workshop session held in February 2009 at the MOSES Organic Farming Conference in La Crosse.  Bud helped everyone understand and appreciate the function and value of a plant’s root system. We’d like to share some of his thoughts with you here.

Before zeroing in on roots, Bud starts with a little basic horticulture lesson. All living organisms need to acquire resources. While animals directly consume packets of resources, plants must collect their resources from a dilute environment.  The fact that this collection is from dilute resources is a key factor in how plants are designed. The leaves and above-ground part of the plant collect sunlight, carbon dioxide (CO2), and oxygen (O2). The roots take in water, minerals, and oxygen.

Water Collection is Key
In order for a plant to take in the amount of CO2 it needs it must have a huge surface area of leaves to perform photosynthesis. A large surface area, however, also allows a lot of water loss to occur through transpiration, the evaporation of water through the plant’s pores. The majority of plants consume a terrific amount of water. For example, a typical Iowa corn plant uses 500 liters of water during its life from seed to harvest. Collecting all of this water is a primary function of the root system. Losing more water than they can take in is the most prevalent cause of plant death around the world. Providing this valuable water-collection mechanism, well functioning root systems are critical to the success of the plant.

Beyond this basic water collection, roots have several other functions.  They provide an anchor for the plant in the soil, they adsorb things, they store things, but they also act as a filter to keep things out of the plant. In the process of  “doing their job” roots consume resources- in fact, no matter what the root shape or structure, over 45% of the photosynthate created by a plant is used by the roots.

A Little Basic Physiology
The root tip is the key root element where adsorption occurs. The outside end of the root tip is covered with a slimy group of cells called the root cap. This slime on the root cap “oils” the root tip and eases the root system’s way through the soil. Directly behind the cap are the reproducing cells, where cell division occurs. The development of cells here is what “pushes” the root through the soil. Elongation cells are next, followed by differentiation cells. Here is where root hairs are formed- single cells of the epidermis, each with only one nucleoli. These root hairs are the primary way that the plant increases surface area, which is needed to increase adsorption.

Looking at a cross section of a root, one will see the outer epidermis surrounding the cortex, which contains the central, large transport cells. The layer inside these two is the endodermis, which is the tissue that acts as a filter to keep unwanted stuff out of the plant. These endodermal cells have very thick walls, which stop unwanted things from going through. Items are forced through the cell wall membrane which acts like a filter or strainer to keep unwanted things such as bacteria, viruses or extra nutrients out.

Back to Water, and the SAPC
The whole plant functions using a “soil, air, plant continuum” or SAPC. Within the SAPC, the leaves lose water, the stem transports water, and the roots adsorb and filter water. However, water doesn’t just flow peacefully up and down within the plant. Transpiration from the leaves pulls water through the root hairs, into the roots and up- it is not pushed. As water transpires away from the leaf, a molecular hole is left and water is drawn up via capillary action through transport tubes to replace it.  A resistance is created due to the root’s filtration activity and the soil’s natural holding capacity. There is a struggle between the pulling of the leaf and the holding of the roots. If the leaves are stressed by too much water need, and the tension becomes too great, the columns of water can snap and break, causing gaps in the water flow, or embolisms. Once broken, the water will no longer flow up to where it is needed in the leaves. This process, called “cavitation,” can actually be measured, as the breaking of the columns causes a “pop” that can be recorded by equipment. The more water stressed a plant is, the faster the cavitation rate. Measurements of the rate have been used to help track irrigation needs.   Once the water column is broken, the capillary tubes will no longer function until recovery occurs, which may happen at night as evaporation rate lowers and the air spaces can be filled. Plants that grow really fast will develop larger xylem vessels, which cause these tubes of water to break more easily, under less tension.

Mineral Nutrition
Another key root function is the taking up of the fertilizers, minerals, and nutrients that the plant needs to grow. This uptake of minerals is an active process that uses energy from the plant. Cells in the root produce energy just as our own bodies produce energy- through the burning of carbohydrates by the mitochondria. In order for this to happen smoothly, root cells need to be healthy and have plenty of oxygen and carbon available. Key features affecting nutrient uptake include: adequate concentration in soil solution, adequate water content/flow in soil, adequate surface area of root complex, adequate oxygen in rhizophere and adequate Microbes.

Diffusion is the most important mechanism for getting nutrients into the root. Diffusion is more or less the random movement of elements back and forth- elements will randomly get close enough to the root and be adsorbed. Diffusion works great at short distances, and not at all well over long distances. As long as a root is contacting minerals in the soil, there is a chance of their being adsorbed. There is a zone of adsorption within which diffusion will be able to work and nutrients taken up. These zones can be very small and the mechanism very slow.

To compensate for the inefficiency of the diffusion mechanism, plants have adapted by developing huge amounts of root surface area, or large “root complexes.” These root complexes are made up of branch roots, root hairs and important associations with microbes in the soil. This last mechanism, in which symbiotic relationships are developed with mycorrhizae, bacteria, fungi and other microorganisms in the soil, is incredibly significant and mistakenly overlooked by the chemical farming industry, Bud notes. Microbes help by attaching to the root and providing more surface area for adsorption. They also provide ways to solubilize nutrients and make them more easily adsorbed. However, these microbes do ask a price- they need a place to live and reproduce, and also take energy from the plant’s resources. They use carbohydrates, fertilizers and photosynthates from the plant, and in return help increase nutrient adsorption. Microbes provide a great way for plants to expand the adsorptive surface area.

Healthy Growth is Critical
By now it is clear to see how critical roots are to water and nutrient adsorption, but Bud takes a moment to emphasize that the presence of roots is not only key, but that they must be actively growing roots.

The root tip is the active zone of elongation. The root hairs, where the adsorption takes place, is right behind the tip. Behind the active adsorption area is an area called the maturation zone. Mature roots do not adsorb water or nutrients. As the root tip grows and moves through the soil, a portion of the root behind matures and stops bringing in resources. As long as the root tip is growing all is well, but if the tip stops growing for some reason, the maturation process does NOT stop. If this happens, the zone of adsorption will be reduced and the plant can die. Things that contribute to the slowing or stopping of root tip growth include not enough water, soil compaction, soil too hot, soil too cold, floods, too much fertilizer and other causes. We need to optimize root growth in order to optimize adsorption of water and nutrients for optimal plant growth. We can keep roots healthy by keeping the soil healthy, with a balance of nutrition, the right water holding capacity and the right aeration. Ideally soil is made up of 50% dirt, 25% water and 25% air. Compaction is a roots’ “worst nightmare” Bud notes, as it takes away air spaces for both air and water storage and makes it harder for roots to push through to access nutrients. The best way to ensure optimum root growth, Bud reminds us, is to have high levels of organic matter to balance the soil particles. 

Some root systems are better at doing their work than others, and Bud closes the session with details of interesting work he is doing grafting herbaceous annual plants onto root stock of disease resistant or otherwise superior root stock. Success has been found using this method with tomatoes, and Bud sees the sample technology as being useful for larger scale vegetable growers. For more on this research and more, visit Bud’s website at the University of Minnesota horticulture department, http://horticulture.cfans.umn.edu/Bud_Markhart.html

Jody Padgham has been with MOSES since 2002. She is the organization's Financial Manager, and the editor of the Organic Broadcaster newspaper. Jody raises poultry and sheep organically on a 60-acre farm in west-central Wisconsin.