There are many aspects of crop and plant production that determine the success of all the effort you put in, but one of the aspects of growing that is most often overlooked is the temperature of the root zone. After all, the roots are out of sight, and besides, what could you do about root temperature anyway? Surely the plants will be fine if they are all at the same temperature. Right? No, wrong actually, and here is why.
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.
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.
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.
Complex systems such as transpiration 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.
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.
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.
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.
In the root zone, these temperatures may vary, for example, 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. 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. 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. 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.
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. Good plant growers will take this into account.
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.