Biological systems are a complex web of chemical interactions that generate energy, sustain homeostasis and promote reproductive success. In the two previous installments of Biology Brief, we covered the importance of phosphorus and calcium. In this column, we have the unique opportunity to link three important nutrients together and explore how plants acquire nutrients from their environment. Before we dive in to the important roles of potassium, let’s look at the interplay among phosphorus, calcium and potassium.
Phosphorus is constituent of the macromolecules known as phospholipids. These unique molecules are comprised of long-chain fatty acids attached to a phosphate-substituted sugar. This unique molecular configuration creates the lipid bilayer (see Biology Brief: “Phosphorus and Cannabis,” CBT - August 2017) that segregates the tightly controlled internal cell system from the external environment. Calcium binds to the phospholipid bilayer (i.e., cell membrane) and maintains its superstructure, membrane permeability, ionic balance, and-ultimately-homeostasis (see Biology Brief: “Got Calcium?” CBT - October 2017).
So, what is potassium’s role and how do plants gather resources?
Plants do not gain nutrients passively from their environment (i.e., through diffusion); instead, plants obtain nutrients through active processes (i.e., enzymes or ion channels). Emanuel Epstein, who conducted his graduate studies with some of the most notable plant biologists, dedicated his life’s work to understanding how plants acquire nutrients from their environment. In Dr. Epstein’s book “Plant Physiology 4th Edition,” he describes his conclusion from his graduate work that the movement of ions across root-cell-membranes violated diffusion principles. That finding led him to the idea that nutrient uptake rate might follow Michaelis-Menten kinetics (i.e., mathematical description of enzyme-catalyzed reactions).
Upon investigating that idea with barley roots under controlled conditions, Dr. Epstein concluded that the movement of nutrients into roots did indeed follow Michaelis-Menten principals, which indicated that a mediator was responsible for nutrient acquisition.
That research lead to the identification of numerous membrane-bound proteins that facilitate the transport of ions from the root zone to root tissues. Ultimately, nutrients (i.e., ions) are concentrated inside root tissue, and water is drawn into the roots through normal osmotic processes (i.e., directional water flow from high concentration to low concentration). Potassium is acquired with a high degree of efficiency due to its many functions; therefore, it is highly concentrated in root cells and acts as the major ion promoting water flow into the roots.
The bottom line: Phosphorus is a constituent of the cell membrane. Membrane structure and permeability is maintained by calcium. Research has shown that ions are concentrated inside of root cells via protein-mediated processes. Potassium is extremely important to plant physiology and is obtained in a preferential manner; its concentration is principally responsible for a plant’s ability to acquire water.
Physiological Roles of Potassium
Potassium is a macronutrient because its concentration in plant tissues is between 1 percent and 3 percent. (See “Plant Physiology 4th Edition.”)
Ujwala Ranade-Malvi describes potassium in the paper “Interaction of Micronutrients with Major Nutrients with Special Reference to Potassium” as a non-constitutive (i.e., mobile) ion that is second in importance only to nitrogen. That paper lists a wide variety of potassium-mediated physiological functions; essentially, potassium acts as a metabolic regulator that is leveraged to maintain the plant’s water budget. Therefore, potassium is an extremely important plant nutrient, and we will do our best to describe its major roles in plant physiology; however, due to the breadth of functions that potassium plays in the plant cell, it will be impossible to cover everything.
Potassium and Water Relations
After water is drawn into the roots via osmosis, it travels through the roots to the leaves through capillary action. This process is driven by water losses in the leaves.
Water is lost at the leaf level through a structure called stomata. Stomata are opened during the day to acquire carbon dioxide for photosynthesis. Pumping potassium into and out of the guard cells of the stomata will determine whether the aperture of the stomata is open or closed. Potassium is pumped into the stomata’s guard cells, which causes water to move into those cells and the stomata’s aperture to open, and allows the plant to obtain carbon dioxide for use in photosynthesis.
If it is dark (remember carbon dioxide is only necessary when photosynthesis is occurring) or water is limited, potassium is pumped out of the stomata’s guard cells, water moves out and the aperture closes. Kalavati Prajapati and H.A. Modi, in their paper “The Importance of Potassium in Plant Growth–A Review,” indicate that this process occurs quickly (in minutes) when potassium is sufficiently supplied to plants, but could take hours (or be impossible) when potassium is deficient. This suggests that potassium is an extremely important ion to water conservation and drought resistance in plants.
The bottom line: Potassium is pumped into guard cells to open the stomata, which allows the plant to acquire carbon dioxide for use in photosynthesis. Potassium is pumped out of guard cells to close stomata during times of darkness or limited water availability. Potassium deficiency can lead to slow or incomplete stomata response, which can lead to significant stress during times of limited water availability.
Potassium, Enzyme Activation and Energy Generation
The most common line among authors who have written about potassium’s role in enzyme activity is: Potassium has been identified as an activator of 60-plus enzymes. This statement is notable because it suggests that plant nutritional research has identified a near global role for potassium in plant physiology. As you can imagine, it would be impossible to cover every enzyme in this article, but we can provide an overview to suitably characterize potassium’s importance to enzyme activity.
Potassium is an important component to enzyme activity because it acts as an activator that binds to specific sites on the catalyst’s surface (i.e., enzyme), which changes enzyme shape and promotes its activity. Furthermore, enzymes are sensitive to changes in salinity and pH; potassium concentration maintains cytosolic tone (i.e., salinity) and neutralizes basic metabolic byproducts. In their book “Better Crops with Plant Food,” John M. Van Brunt and John H. Sultenfuss suggest that the total amount of available potassium will dictate the efficiency of potassium-based enzyme systems, due to potassium’s multiple roles in determining enzyme activity.
Potassium is thought to play a role in every step of the protein synthesis pathway from transcription of DNA to translation of messenger RNA (mRNA). While it is beyond the scope of this column to look in depth at specific roles of potassium, it is safe to say that activation of enzymes, pH balance and maintenance of cytosolic ion content are the important roles potassium plays. All the books and authors cited in this article note the importance of potassium in the protein-building process. Many research studies note build-up of protein precursors (i.e., amino acids, amides, nitrate, etc.) in the tissues of potassium-deficient plants. Additionally, potassium is involved in the synthesis of starches from the sugars produced during photosynthesis. Prajapati and Modi (“The Importance of Potassium in Plant Growth–A Review”) note that precursors to starch production accumulate in the leaves of plants growing in potassium-poor conditions. This suggests that starch synthase (i.e., the enzyme responsible for starch production) is activated by potassium.
Finally, potassium plays a critical role in the energy-generating processes occurring in the plant cell. Photosynthesis is a process mediated by numerous enzymes, and many of those enzymes are activated by potassium. However, potassium is also the major ion that maintains the electrical balance at the production sites of Adenosine Triphosphate (ATP, an “energy-carrying molecule found in the cells of all living things,” as defined by Britannica.com). Potassium-deficient plants exhibit slow rates of energy generation, which affects downstream processes such as protein synthesis, material transportation mechanisms and productivity.
The bottom line: Potassium activates multiple enzymes in numerous metabolic processes including protein/starch synthesis and energy generation. Additionally, potassium plays an important role in modulating the intercellular environment by maintaining ionic tone, buffering pH and promoting favorable electrochemical conditions at reaction sites.
Potassium is thought by many to be second only to nitrogen in plant nutritional importance. It is a non-constitutive ion that is mobile in plant tissue, and it takes part in a mosaic of plant physiological processes. Potassium is normally the largest constituent of plant-fertigation programs where recommendations range from 250 ppm to 400 ppm.
A notable feature of potassium acquisition by plants that was not described in this column is that many authors agree that plants acquire 80 percent or more of their total potassium prior to seed generation. This is of note in the cannabis sector because cannabis production principally focuses on flower production. Therefore, it is important to provide plants plenty of potassium at all growth stages.