Potassium (K) is not an integral part of any major plant component, but it does
play a key role in a vast array of physiological processes vital to plant
growth, from protein synthesis to maintenance of plant water balance. Potassium
deficiency is characterized by reduced plant growth and a yellowing and/or
burning of the leaf edges. Since potassium is mobile in the plant, the symptoms
appear on the older leaves first. Another indication of potassium deficiency is
reduced straw or stalk strength, which results in lodging problems, reduced
disease resistance, and reduced winter hardiness of perennial or winter annual
crops.

Enzyme Activation
Enzymes serve as catalysts for chemical reactions, being utilized but not consumed in the process. They bring together other molecules in such a way that the chemical reaction can take place. Potassium "activates" at least 60 different enzymes involved in plant growth. The K changes the physical shape of the enzyme molecule, exposing the appropriate chemically active sites for reaction. Potassium also neutralizes various organic anions and other compounds within the plant, helping to stabilize pH between 7 and 8 ... optimum for most enzyme reactions.The amount of K present in the cell determines how many of the enzymes can be activated and the rates at which chemical reactions can proceed. Thus, the rate of a given reaction is controlled by the rate at which K enters the cell. 

Stomatal Activity (Water Use)
Plants depend upon K to regulate the opening and closing of stomates ... the pores through which leaves exchange carbon dioxide (CO2), water vapor, and oxygen (O2) with the atmosphere. Proper functioning of stomates is essential for photosynthesis, water and nutrient transport, and plant cooling. When K moves into the guard cells around the stomates, the cells accumulate water and swell, causing the pores to open and allowing gases to move freely in and out. When water supply is short, K is pumped out of the guard cells. The pores close tightly to prevent loss of water and minimize drought stress to the plant. If K supply is inadequate, the stomates become sluggish - slow to respond - and water vapor is lost. Closure may take hours rather than minutes and is incomplete. As a result, plants with an insufficient supply of K are much more susceptible to water stress. Accumulation of K in plant roots produces a gradient of osmotic pressure that draws water into the roots. Plants deficient in K are thus less able to absorb water and are more subject to stress when water is in short supply. 

Photosynthesis
The role of K in photosynthesis is complex. The activation of enzymes by K and its involvement in adenosine triphosphate (ATP) production is probably more important in regulating the rate of photosynthesis than is the role of K in stomatal activity. 
When the sun's energy is used to combine CO2 and water to form sugars, the initial high-energy product is ATP. The ATP is then used as the energy source for many other chemical reactions. The electrical charge balance at the site of ATP production is maintained with K ions. When plants are K deficient, the rate of photosynthesis and the rate of ATP production are reduced, and all of the processes dependent on ATP are slowed down. Conversely, plant respiration increases which also contributes to slower growth and development. In some plants, leaf blades re-orient toward light sources to increase light interception or away to avoid damage by excess light, in effect assisting to regulate the rate of photosynthesis. These movements of leaves are brought about by reversible changes in turgor pressure through movement of K into and out of specialized tissues.

Transport of Sugars
Sugars produced in photosynthesis must be transported through the phloem to other parts of the plant for utilization and storage. The plant's transport system uses energy in the form of ATP. If K is inadequate, less ATP is available, and the transport system breaks down. This causes photosynthates to build up in the leaves, and the rate of photosynthesis is reduced. Normal development of energy storage organs, such as grain, is retarded as a result. An adequate supply of K helps to keep all of these processes and transportation systems functioning normally. 

Water and Nutrient Transport
Potassium also plays a major role in the transport of water and nutrients throughout the plant in the xylem. When K supply is reduced, translocation of nitrates, phosphates, calcium (Ca), magnesium (Mg), and amino acids is depressed. As with phloem transport systems, the role of K in xylem transport is often in conjunction with specific enzymes and plant growth hormones. An ample supply of K is essential to efficient operation of these systems. 

Protein Synthesis
Potassium is required for every major step of protein synthesis. The "reading" of the genetic code in plant cells to produce proteins and enzymes that regulate all growth processes would be impossible without adequate K. When plants are deficient in K, proteins are not synthesized despite an abundance of available nitrogen (N). Instead, protein "raw materials" (precursors) such as amino acids, amides and nitrate accumulate. The enzyme nitrate reductase catalyzes the formation of  proteins, and K is likely responsible for its activation and synthesis. 

Starch Synthesis
The enzyme responsible for synthesis of starch (starch synthetase) is activated by K. Thus, with inadequate K, the level of starch declines while soluble carbohydrates and N compounds accumulate. Photosynthetic activity also affects the rate of sugar formation for ultimate starch production. Under high K levels, starch is efficiently moved from sites of production to storage organs
 
 
 

 
 
Listed below are the principal uses of potassium in the form of compounds: 

· Potassium bromide, KBr, is extensively used in the manufacture of photographic plates, films, and papers. 
· Potassium chlorate, KClO3, is a powerful oxidizing agent. It is used in explosives, matches, weedkillers, fireworks, and disinfectant. 
· Potassium chloride, KCl, is the most abundant of potassium compounds. It is perhaps best known as a no-sodium table salt substitute. It is also an ingredient in many chemical fertilizers and is used in the manufacture of other chemicals. 
· Potassium hydroxide, KOH, is often called caustic potash. It is used in the manufacture of soaps and detergents. It is a good drain cleanser because it combines with grease (that clog drains) to form water soluble soaps. 
· Potassium carbonate, K2CO3, is used in the laboratory as a drying agent and industrially in the manufacture of soft soap, hard glass.
· Potassium iodide, KI, is used in medicine, particularly in the treatment of goitre resulting from iodine deficiency. 
· Potassium nitrate, KNO3, is used in chemical fertilizers, in gunpowder and match heads. 
· Potassium permanganate, KMnO4, is a purple solid soluble in water. It is used in volumetric analysis as an oxidizing agent. It is also used as a bactericide and a disinfectant. 
· Potassium sulfate, K2SO4, is used as a fertilizer, particularly for tobacco plants, and in the chemical industry in the preparation of alums. 
· Potassium superoxide, K2O, is used in the respiratory equipment because it efficiently generates fresh oxygen while removing carbon dioxide. 
· Potash A generic and commercial name for naturally occurring potassium salts. Its name is derived from pot ashes denoting the ancient method of leaching wood ashes for their potassium carbonate content and concentrating the extract in iron pots.
 

 Sources:

The Agronomy Guide, 1989-1990, page 9.
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Author:  Douglas Beegle, Associate Professor of Agronomy
Department of Agronomy, Penn State
November 1988
PENpages Number:  08801326

Hydroponics World: State of the Art in Soilless Crop Production, Adam J. Savage Ph.D., Editor
Knotts' Handbook For Vegetable Growers