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
Potassium (K) increases
Enzyme Activation minutes and is incomplete. crop yield and improves
Enzymes serve as cataAs a result, plants with an quality. It is required for lysts for chemical reactions, insufficient supply of K are numerous plant growth being utilized but not conmuch more susceptible to processes. sumed in the process. They water stress. bring together other molecules in such a way
Accumulation of K in plant roots prothat the chemical reaction can take place. duces a gradient of osmotic pressure that
Potassium “activates” at least 60 different draws water into the roots. Plants deficient in enzymes involved in plant growth. The K K are thus less able to absorb water and are changes the physical shape of the enzyme more subject to stress when water is in short molecule, exposing the appropriate chemical- supply. ly active sites for reaction. Potassium also neutralizes various organic anions and other Photosynthesis compounds within the plant, helping to stabiThe role of K in photosynthesis is comlize pH between 7 and 8...optimum for most plex. The activation of enzymes by K and its enzyme reactions. involvement in adenosine triphosphate (ATP)
The amount of K present in the cell deter- production is probably more important in regmines how many of the enzymes can be acti- ulating the rate of photosynthesis than is the vated and the rates at which chemical reac- role of K in stomatal activity.
When the sun’s energy is used to comtions can proceed. Thus, the rate of a given reaction is controlled by the rate at which K bine CO2 and water to form sugars, the initial high-energy product is ATP. The ATP is then enters the cell. used as the energy source for many other chemical reactions. The electrical charge balStomatal Activity (Water Use)
Plants depend upon K to regulate the ance at the site of ATP production is mainopening and closing of stomates...the pores tained with K ions. When plants are K defithrough which leaves exchange carbon diox- cient, the rate of photosynthesis and the rate of ide (CO2), water vapor, and oxygen (O2) with ATP production are reduced, and all of the the atmosphere. Proper functioning of stom- processes dependent on ATP are slowed ates is essential for photosynthesis, water and down. Conversely, plant respiration increases nutrient transport, and plant cooling. which also contributes to slower growth and
When K moves into the guard cells development. around the stomates, the cells accumulate
In some plants, leaf blades re-orient water and swell, causing the pores to open and toward light sources to increase light intercepallowing gases to move freely in and out. tion or away to avoid damage by excess light, otassium is vital to many plant processes. A review of its role involves understanding the basic biochemical and physiological systems of plants. While K does not become a part of the chemical structure of plants, it plays many important regulatory roles in development.
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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 similar to that described above for stomata.
How potassium works to increase crop yields:
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Increases root growth and improves drought resistance
Activates many enzyme systems
Maintains turgor; reduces water loss and wilting
Aids in photosynthesis and food formation
Reduces respiration, preventing energy losses
Enhances translocation of sugars and starch
Produces grain rich in starch
Increases protein content of plants
Builds cellulose and reduces lodging
Helps retard crop diseases
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
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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.
Crop Quality
Potassium plays significant roles in enhancing crop quality. High levels of available K improve the physical quality, disease resistance, and shelf life of fruits and vegetables used for human consumption and the feeding value of grain and forage crops. Fiber quality of cotton is improved. Quality can also be affected in the field before harvesting such as when K reduces lodging of grains or enhances winterhardiness of many crops.
The effects of K deficiency can cause reduced yield potential and quality long before visible symptoms appear. This “hidden hunger” robs profits from the farmer who fails to keep soil K levels in the range high enough to supply adequate K at all times during the growing season. Even short periods of deficiency, especially during critical developmental stages, can cause serious losses.
Quality factors are addressed in more detail on pages 28 and 29.
