Root zone temperature and plant health
By Geary Coogler, BSc Horticulture
The temperature story
To begin with, there are two main parts of a plant, the roots and the shoots, and one main intersection known as the crown. Although they are made of similar material, have similar components and similar engineering, the function of the roots is, basically, the opposite to the function of the rest of the plant. The crown serves as the switching centre that facilitates the change in function. But the basic chemistry is universal, and although our focus here is on the root zone, the other two parts are affected as well.
The basic purpose of the root system is to take up water and chemical elements that the plant needs to function and which are available in the soil around the roots. The roots’ other purposes include anchorage, support and storage, which in some plants may be more important functions than taking up water and nutrients. Roots absorb water and nutrients through the basic process of osmosis: water moves across a membrane into the cells of the plant because of differences in their respective ion concentrations. However, most elements are often actively pumped into the plant cells, and this requires energy.
The part of the plant above the surface is called ‘shoot’ and is able to regulate its temperature through transpiration. The temperature range in the shoot can therefore be larger and it can change faster. The part of the plant below the surface is called root zone and is not able to regulate its temperature at all. The temperature range is therefore smaller and the roots need to stay cooler.
Root protective systems
The root also has to guard itself against an excess of certain substances and the loss of substances already taken up. To do this, it has evolved protective systems and barriers that do just this. Obviously, the roots are not there to harvest the power of sunlight. But in fact, they do consume large amounts of the energy that the plant absorbs from the sun.
Using this energy involves respiration – the process of taking in oxygen (O2) and using it to convert the carbohydrates (made through photosynthesis in the top zone of the plant) into energy for use in local processes.
The roots have no need for carbon dioxide (cO2). Respiration gives off heat, and it also requires a minimum temperature to start and continue; if the temperature gets too high, the reactions go haywire. If that happens, the roots will take in water but then they will do their best to retain it and so the process of transpiration (which provides water to the whole of the plant and cools it) will be disrupted. Instead, the roots transfer the excess heat generated from respiration (latent heat) to the surrounding medium. Dense mediums such as soil, sand, and even water, have a large temperature buffer which means that the temperature fluctuations over 24 hours will be minimal under normal conditions.
Energy and carbohydrates
The basic function of the top zone of the plant is to produce energy from the sun and fruiting structures so that the plant will pass on its genes. As part of this process, it produces energy in the form of complex building blocks known as carbohydrates, and some of these are passed down to the roots so that they can continue to function and grow, providing the plant with the water and nutrients that it needs.
The tissues are designed to allow water and elements to move as quickly as possible through the tissue of the plant to each and every cell.
Complex systems such as transpiration - simple in their basic concept but wonderfully complex in design - have evolved to move raw materials and finished products upwards, provide a ridged support to the structure of the tissue, facilitate the collection of solar energy and convert simple elements into complex organic molecules. The chemical reactions involved in cell metabolism and function are the same in both sections of the plant – both above and below the ground.
When the lights go on, the air temperature rises and as a result the soil temperature will rise too. It takes a while before the soil warms up (just as it will cool down slowly after the lights are turned off). But it’s not only the air that influences the soils temperature. The material, depth (volume) and moisture level also changes its ability to give off or retain heat.
Chemical reactions
Other reactions specific to converting light energy into chemical energy also occur and some of the cells in the shoot of the plant are like chemical factories that produce more when reactions occur faster. The temperature issues remain the same as in the roots; these chemical reactions will occur too quickly as the temperature increases, and they will slow down as it decreases. When coupled with the extra incoming heat from the light energy, it becomes critical that the plant has a system for regulating its temperature and transferring excess heat out into the air, which is subject to much larger temperature fluctuations than a denser medium such as soil.
The tissue in the top zone of the plant also consume oxygen at a more or less constant rate, night or day, and they take in carbon dioxide during the light period to assemble the basic building blocks of life, carbohydrates. The upper section of the plant has to do all this in a temperature range that is subject to significant fluctuations over a 24-hour period, sometimes 10°c or more and occurring very rapidly.
A scanning electron micrograph (SEM)
of a section through a rootlet taken from
a flowering plant. The vascular bundle
consists of xylem (four green circles, centre)
and phloem tissue (blue). The xylem
transports water and mineral nutrients
from the roots to the rest of the plant
while the phloem transports
carbohydrates and plant hormones.
Crown of the plant
The crown of the plant is the junction between the root tissue and the shoot tissue. In some plants, these crowns are clearly defined and ridged, while in others they are less clear. This area of the plant is like a massive telephone switching station that must take the incoming osmotically generated pressurized flow of water and nutrients from the roots, and feed it into a vacuum system that is pulling the flow up and out through the transpiration sinks (areas of negative pressure) in the leaves, effectively changing the physics of the flow.
Chemical reactions occur, the temperature fluctuates, there is a change in the systems used for temperature control, and oxygen is used in large quantities. A crown exists at the interface of the medium and the air, and if it goes too far into one or the other (planting too high or too deep), problems will arise.
Maintaining temperature
The temperature in the top zone has to be right for the chemical reactions to occur. The top zone itself can slow down transpiration or increase it as needed to maintain a certain temperature in the production tissues.
When the lights go on, temperatures are low and there is less need for cooling. As the day progresses, the energy and temperature in the air and plant tissues increase, as does the rate of transpiration, which then falls back again as the day comes to an end. These temperatures can, for example, start at around 18°c and reach a peak of 29°c before falling back, an 11-degree difference over half a day. In the root zone, these temperatures may vary between 18°c and 19°c – only a 1°c difference, but the roots must function well enough in that constant temperature range to provide everything that the top zone needs, and then does not need, as it goes through the rapid daily temperature change.
Using this knowledge
plants took millions of years to evolve to the circumstances in which they had to survive and propagate themselves under natural conditions. Soil temperature and characteristics vary according to latitude and composition. Plants have evolved according to the needs they faced in specific locations. Soil, whether natural or artificial, varies in its ability to lose or retain heat based on material, depth (volume) and moisture level. In very porous material, the temperature will fluctuate rapidly, as it will in dry material. But temperature fluctuations decrease as the material becomes denser or if it contains more moisture, and this is increasingly the case deeper into the soil profile. But soil will undergo fewer temperature fluctuations than air under all these conditions.
However, if the medium is confined to a bucket, a raised bed, or some other container, then these fluctuations will become more rapid and intense and the temperature profile will become closer to that of the air surrounding them. The medium loses its ability to serve as a temperature control for the roots under these conditions, resulting in an under-performing root system which cannot supply the top zone with all it needs. Shallow-rooted plants work with wider temperature fluctuations closer to average day/night air temperatures, whereas deeper-rooted plants have to handle smaller fluctuations and cooler temperatures than this average.
Regulating plant temperature
The plant’s root system does not regulate its own temperature, and once temperature in the medium strays outside the optimum zone for reactions to occur, it can no longer supply the rest of the plant with the optimum level of water and nutrients. This is the case whether the temperature is too high or too low. The greater the fluctuation in temperature in a 24-hour period, the more stressed the root system will become, and the more problems a plant will have both physically and pathologically, and it will become increasingly susceptible to pathogens and insects. Placing any root system in a medium above ground will increase the surface area from which heat can be gained or lost.
plants become dormant when the root system stops most of its functions, whether this is a result of too cool or too hot conditions. This is true in container plant nurseries located in warm, sunny areas. In the summer, the containers warm up due to the surrounding air temperature and the plants go into a second period of dormancy even though they are being watered and fed for maximum production and growth. Even the temperature of the irrigation water or nutrient solution will increase or decrease the root function, and any sudden large temperature change will shock the roots. Good growers will warm or chill the water to be in the correct range before irrigating.
Close monitoring
Temperature is extremely important for plants to grow and flourish, but all the factors involved are much more complicated than we can go into here. The root and shoot systems have a different set of needs when it comes to temperature: one can function with larger and faster temperature fluctuations, while the other needs a much smaller, cooler, and stable range. Good plant growers will take this into account. A weak or poorly functioning root system will slow top zone development because it will not be able to perform the chemical reactions required by slowing the uptake of nutrients. Not all nutrients will be affected and some will be faster than others, which can show up as individual deficiencies.
The root system will develop and function best when kept within a specific temperature range and a good grower will monitor this closely, just as they would monitor and regulate air temperature. All parts of the plant are interconnected and nutrient issues can occur in plants that are being fed properly if the root zone temperatures stray outside the correct range for too long. In the end, there are two different and entirely separate environments that a plant lives in, and a good grower will pay close attention to both.
