Maintaining Healthy Root Systems

While root growth and health commonly go ignored by many indoor gardeners, the healthy functioning of roots is vital to plant productivity. Hydroponic methods enable specific demands on the root systems' physiology.

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Soil vs. Hydroponic Root Systems

In most soil systems, plant roots have the freedom to grow and stretch out far as they need to. Most plants will send tap roots deep into the soil in search of nutrients and water, while other species develop shallow, but highly branched, root systems. This is due to the fact that root morphology depends not only on genetics, but also on the soil and water constraints from the root zone.

In hydroponics, the available root volume is highly restricted. This results in root density to be considerably higher than in most soil-based systems. However, with water, nutrients and oxygen delivered directly to the root surface on a frequent basis, hydroponic plants don’t need to expend as much energy on growing roots for long-distance foraging.

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While we may feel secure in the fact that our plants are being provided plentiful supplies of root-essential oxygen, moisture and nutrient ions, there are other root physiology factors in soilless systems that need some consideration. Root function is dependent on a wide range of interrelated factors.

Just as in soil, hydroponic roots can be affected by temperature, microbial populations, competition, pathogens, salinity, toxicities and root exudates.

Root Growth

Root growth is an ongoing process. Over time, root density increases, and new roots are produced. Regeneration of new roots is essential for plants development, as younger root tissues absorb the majority of nutrients.

Eventually, as roots continue to grow, the point will be reached where extreme root binding occurs, yes, even in plants receiving a regular supply of nutrients.

Always take into consideration the size of your container needs to allow for continued root growth.

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Root Oxygenation

Oxygen is a vital component of root physiology. The superior oxygenation of the root zone and nutrient solution that hydroponics provides helps improve root health. Without enough oxygen to complete the respiration process, roots will suffocate.

Some plants require large amounts of oxygen within the root zone, particularly when growing in protected, warm conditions provided year-round by indoor gardening. A restricted root zone is limited in how much oxygen it can hold and relies heavily on oxygen replenishment.

Eventually, a lack of oxygen can result in root-cell death which highly increases the risk of root diseases such as pythium. The more restricted the root zone volume, the greater the replenishment rate of oxygen must be.

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Root Competition

Many hydroponic systems allow plants to be grown in separate containers or slabs of substrate, however some plants are often grown side-by-side to allow the roots of both plants to intermingle.

Some studies have found that plants produce more root mass when sharing space with a neighbour compared to plants growing alone.

It is suggested that this triggers plants to enhance their competitive ability for nutrients. However, root overgrowth may occur in this situation at the expense of reproductive growth, and such findings may be species-specific.

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Root Temperature

The temperature of a root zone strongly affects shoot growth. In fact, root zone temperature plays more of a role in growth and development than the temperature of the air surrounding the plant because the root tissue sends numerous, non-hydraulic messages to the shoot, which influences the way the shoot responds to its environment. So, with many plant functions under the control of what goes on down in the roots, root zone temperature becomes an extremely important factor to monitor.

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Salinity and Phytotoxicity

When salinity, or EC, is too high, root cells lose moisture and often die. There are wide differences in tolerance to high EC and soil salinity among plant types, even those that have similar growth requirements.

Toxicities occur when root cells are damaged or destroyed by compounds such as high levels of certain trace elements or, more commonly in hydroponics, plasticizers leaching from unsuitable materials in contact with the nutrient solution or roots. When root cells are damaged, plant pathogens such as pythium often invade the site of the damage, leaving growers wondering what the initial cause of the root damage was.

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Root Exudates & Microbial Relationships

Root systems are able to change the environment directly surrounding them by secreting a wide range of organic compounds, known as root exudates, and releasing ions that influence pH.

In the past, root exudates in hydroponics were a cause for concern. In the early days of soilless culture it was believed these organic compounds would rapidly build up in the limited root zone and restrict plant growth.

This turned out not to be the case in most circumstances. Instead, it is now believed that the vast array of compounds plant roots excrete can account for between 5 and 21% of the photosynthetically fixed carbon, which is a significant cost to the plant.

For that reason, the plant must obtain some benefit from secreting compounds into its rhizosphere, and this is something researchers are investigating in great detail. Root exudates consist not only of organic compounds such as amino acids, organic acids, sugars and a wide range of carbohydrates, phenolics, lignins, fatty acids, sterols, enzymes, mucilage and proteins, but also of released ions and inorganic acids.

Relationships with beneficial microbe populations in the rhizosphere occur in hydroponics just as they do out in the field, with diverse and beneficial microbe species found in a wide range of different soilless systems.

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Certain exudates released by roots are used by plants to attract and select certain micro-organisms in the rhizosphere. These microbes work via different mechanisms, to influence plant health and growth. For example, root exudates act as signals that encourage and initiate a relationship or symbiosis with rhizobia and mycorrhizal fungi, as well as rhizo-bacteria, which is beneficial for both microbes and plants.

Finally, if plant roots sense an attack of pathogenic microbes, they release certain exudates called phytoalexins (defense proteins) which initiates a process of underground chemical warfare.

While ongoing research on soilless systems continues to unveil new and exciting findings, we have yet to understand how to fully harness the power of the hydroponic root system.

With a vast array of biochemical processes, ranging from nutrient and water absorption, to the production of essential plant growth hormones and interaction with microbes, root physiology is something every grower needs to consider when designing and running a hydroponic system.

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