The hydroponic root zone is a microcosm of the real world below ground. Confined and restricted by volume, it requires a high degree of control for maximum efficiency. Plants grown in soil and in hydroponics have the same requirements for water, oxygen and nutrients, but the frequent rate of replenishment of these in soilless culture is one of the main advantages of hydroponic production.
While root growth and health may go largely unobserved by many indoor gardeners, the proper functioning of roots is vital to plant productivity, and hydroponic methods can place specific demands on root system physiology.
Soil vs. Hydroponic Root Systems
In most soil systems, plant roots have the freedom to grow and stretch out as they need to. Many plants will send tap roots many feet deep into the soil in search of nutrients and water, while other species may develop shallow, but highly branched, root systems. This is due to the fact that root morphology depends not only on genetics, but also on soil and water constraints in the root zone.
In hydroponics, where the available root volume is highly restricted, root density is 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.
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 is an ongoing process. Over time, root density increases as new roots are produced. The regeneration of new roots is essential for normal plant development, as the majority of nutrients are absorbed through younger root tissues.
Roots respond to gravity, and to touch when they contact a solid surface. For example, in a restricted growing container, roots tend to head downwards and form a mat in the lower regions of the growing substrate. Eventually, as roots continue to grow, the point will be reached where extreme root binding occurs—even in plants receiving a regular supply of nutrients—and overall plant growth is restricted. The size of the container must allow for this continual root growth.
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, as without enough oxygen to complete the respiration process, roots will suffocate. Plants will exhibit a strategy called oxytropism, where roots will avoid growing in oxygen-deprived areas such as water-logged soils, overwatered hydroponic substrates and stagnant nutrient solutions.
Some plants require large amounts of oxygen within the root zone, particularly when growing in the protected, warm conditions provided year-round with indoor gardens. A restricted root zone is limited in how much oxygen it can hold, so it relies heavily on oxygen replenishment. This can be carried out by dissolved oxygen in the nutrient solution, or by oxygen percolation down into the root zone during irrigation.
If roots require more oxygen than what they can get through replenishment, their function begins to slow, as does the uptake of water and nutrients.
Eventually, a lack of oxygen can cause root-cell death and increase the risk of root diseases such as pythium. The more restricted the root zone volume, the greater the replenishment rate of oxygen must be. In hydroponics, this can be achieved in a number of ways. Some grow mediums contain larger pores than others and allow oxygen to diffuse faster down into the root zone. Second, nutrient solutions carry dissolved oxygen, so increasing the dissolved oxygen content of the solution via aeration and making sure the root zone is not oversaturated with water will ensure more oxygen is available for root uptake.
In many hydroponic systems, plants can be grown in separate containers or slabs of substrate, yet 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 neighbor as compared to plants growing alone. It is thought 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, as it appears that the roots of some plant species can sense the roots of neighboring plants and respond accordingly. Further studies might help us determine how plants grown side-by-side may influence the growth of each other in hydroponic systems.
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. Research has shown that even less than 30 minutes of root zone heat buildup can have a negative impact on many crops, which cannot be countered by having a low daily temperature average. Just a few minutes a day of root zone temperatures of more than 86°F will slow the growth of heat-sensitive crops such as lettuce and parsley.
Chilling the root zone and nutrient solution is one solution to this unique aspect of plant physiology. Nutrient solution chilling allows the tops of heat-sensitive crops like lettuce to withstand higher-than-optimal temperatures. This method of root-zone chilling assists plants in a number of ways. Cool nutrient solutions hold more dissolved oxygen for root uptake, which means oxygen starvation is less likely to occur. Cooling the roots well below ambient air temperatures also allows for higher assimilation rates by reducing both photoinhibition and stomatal closure, which typically occur once the plant becomes heat-stressed. The positive effects of nutrient chilling seem to be largely the result of changes in the production of plant growth hormones (abscisic acid and cytokinins) in the root tissue, which control a wide range of plant responses.
Salinity and Phytotoxicity
While salinity and phytotoxicities can be serious issues in both soil and soilless production, hydro growers can largely avoid these issues, allowing crop growth in areas where soil salinity and toxicity would otherwise prevent cultivation. When salinity, or EC, is too high, root cells lose moisture and often die. There are wide differences in tolerance to high EC and salinity among plant types, even those that have similar growth requirements. Tomatoes, for example, can tolerate high salinity through changes in root physiology, which prevents salt damage, whereas salinity-sensitive crops like lettuce and strawberries are easily damaged when EC becomes higher than optimal.
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.
Root Exudates and 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 mucilage, and releasing ions that influence pH. As positive ions (cations Ca2+, K+, Mg2+, etc.) are removed from the nutrient solution, hydrogen ions (H+) are released from the root system, equalizing the ratio of anions to cations in the root zone and thereby lowering the pH of the solution. When crops begin an active growth phase, anions (NO3, etc.) are taken up and increase the pH through the release of hydroxyl ions (OH-) into the solution.
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 some 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. Certain exudates released by roots are used by plants to attract and select certain micro-organisms in the rhizosphere. These microbes can then 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, when plant roots sense an attack by pathogenic microbes, they release certain exudates called phytoalexins (defense proteins) and other unknown compounds, engaging in a process of underground chemical warfare. For example, the roots of sweet basil plants have been shown to release rosmarinic acid, an antimicrobial compound, in response to attacks by phytophthora cinnamomi, a soil-borne water mold that leads to root rot. This is just one of many such defense responses we have yet to fully understand.
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.