In order to do better understand the relationship of “root to shoots,” looking at the concepts of water/transpiration/osmosis, root absorption, root/shoot relationship, and root mass in hydroponics is a good place to start.
Water, transpiration, and osmosis
Nutrient uptake depends largely on water flow from the substrate to the roots and then up to the shoots. Interestingly, of the total water absorbed by the plant, only 5% is utilized in biochemical processes; the rest is loss via transpiration—the loss of water through the leaves due to a concentration gradient between air surrounding the leaf and the leaf itself.
While transpiration may be viewed unfavorably, it is responsible for cooling the plants (absorbing solar energy via photosynthesis produces heat) and, most importantly, moving water and nutrients to the leaves where they are needed. Transpiration also creates water demand in plants. As the leaves lose water, transpiration creates a suction or pull that brings water and nutrients up through the xylem—imagine this process as a person sipping from a straw.
Another concept, osmosis, is the net movement of water driven by solute concentrations across a membrane that is semi-permeable and selective. If the solute concentration is higher on one side of the membrane, water will move to create an equilibrium between intracellular (inside a cell) and extracellular (outside the cell) solute concentrations. This movement of water creates a pressure either inward or outward.
For example, think of a balloon filled with solute (NaCl). Next, imagine that balloon is surrounded by pure water. Since the concentration of salt inside the balloon is high, water will move into the balloon and place pressure that will inflate the balloon. Without outward rectification, the balloon would pop; thus, salts must be transferred out to reduce this water (osmotic) pressure.
Now that we have discussed a specific role of water, let’s examine how that applies to nutrient absorption.
How Plant Roots Work
Roots branch out both laterally and vertically, and most plants will produce roots hairs (smaller, finer branches of roots) that further increase the surface area by several magnitudes. Roots provide two important functions for the plant. First, they act as structural anchors, fortifying the plant into the soil or rooting substrate.
Secondly, they take up both water and nutrients to distribute throughout the plant. Absorption of water and nutrients is greatest in the new and younger sections of roots and is less active in the older sections. The specific mechanisms of this absorption is unclear, but it is known that ions are moved by both passive and active processes.
Passive transport occurs when ions exist at a higher concentration and move to the lower concentration without the cost of energy. Active transport occurs when ions are moved against a concentration gradient, generally by a carrier protein. The function and types of these specific transporters is beyond the scope of this article; however, temperature can easily be explored and should be considered when considering effective ion absorption.
Absorption of water and nutrients is kinetically driven. That is to say, temperature is paramount for effective uptake. Ideal substrate temperatures should be between 68 and 86oF. Although the uptake of ions is regulated, water and ion absorption will increase as temperatures rise.
Over time, this accelerated uptake could lead to mineral toxicity. Also, if the rooting substrate is below 68oF and the growing conditions favor high transpiration, then the plant will be unable to absorb enough water to meet its biological water demand and wilting will occur. This is why monitoring both the growing conditions and the nutrients within the rooting substrate is paramount to acceptable growth.
The relationship between shoots and roots
While separate organs of the plant, roots and shoots are dynamically interconnected. Leaves manufacture carbohydrates that provide the carbon needed to construct and grow roots. They are also the source for respiration-generated adenosine triphosphate (ATP), which is the energy “currency” needed for mineral absorption and other biological processes.
The roots, in turn, uptake water and nutrients that the plants need to maintain turgor pressure and that assist with photosynthesis, as well as with promoting vegetative growth, flowering and fruiting. Excessive vegetative (shoot) growth will facilitate carbohydrates away from roots; likewise, excessive root formation will pull away too many carbohydrates, thus reducing shoot development. As a result, it is important to understand the ramifications of root production in hydroponics.
Root mass and hydroponics
The roots in hydroponic systems do not have to work as hard at obtaining water and nutrients as its traditional soil-bound brethren. Indeed, moist, fertile conditions like those utilized in hydroponics favor extensive root formation. While root pruning may seem like the logical solution, it might only lead to further proliferation of roots.
Also, if you severely reduce the root mass, you will be reducing the absorption of both water and nutrients to the shoots causing reduced overall growth. Application of chemical hormonal sprays might control excessive root formation, but they’re probably not practical or economical. In the end, it makes more sense to not invasively manipulate root growth. Instead, hydroponic growers should establish practices to balance root and vegetative formation.
Proper growing conditions (water status, temperature, humidity, etc) coupled with regular monitoring of nutrient status (especially nitrogen in nitrate form, phosphorus, sulfur and iron) will go a long way in promoting better growth and reducing excess nutrients that cause proliferation of the roots.
While understanding specific biochemical reactions or in-depth plant physiology might not be interesting to some growers, understanding certain basic principles is integral in promoting optimal growth. Hydroponics involves soilless growing in a water-based system. Therefore, the role that water plays, especially in nutrient acquisition, for plants is vitally important to a hydroponic grower.
Hopefully this article has highlighted a few key points and now that you are “rooted” with information, go out there and start growing!