High requirement by most crops
A catalyst in many reactions
Activates many enzymes associated with photosynthesis, protein and carbohydrate metabolism
Plays a pivotal role in stomatal function and other water relations such as osmosis
Essential for flowering
Improves fruit quality
Enhances disease resistance
Potassium does not combine in plants with other nutrients and exists only as the ion K+. Its mode of action is mainly catalytic. Many enzymes depend on potassium for their activation. It is essential in the translocation of metals and to the balance of ions in the plant. Potassium plays an important role in water relations and it is responsible for much of the osmotic “pull” of water by the roots. Increased presence of potassium in the vicinity of the guard cells that surround the stomata regulates stomatal opening by increasing turgor pressure. Potassium can influence the photosynthetic process and 50% of the total K in leaves is found in the chloroplasts. Adequate K is needed for the synthesis of adenosine triphosphate (ATP). ATP is essential in energy storage and transfer reactions and provides (along with ADP) the energy required by all biological processes. The process by which glucose is converted into starch utilizes the enzyme, starch synthetase, which requires potassium for its activation. Potassium has an important role in fruit formation and can influence fruit quality.
Potassium is absorbed by roots from the soil solution as the potassium ion K+. Only a small portion of total soil K is available to plants.
Because of K mobility to young leaves, deficiency symptoms first occur on older leaves. Older leaves turn yellow, the tips and margins dry and die prematurely. There is a tendency for retarded growth and increased susceptibility to disease. In deficient plants, the response to water stress is increased since potassium influences osmotic regulation and water uptake. There is a tendency for poor stomatal closure which increases the rate of transpiration and results in reduced photosynthesis.
CO2 assimilation and conversion into sugar is greatly reduced.
Can induce Calcium (Ca) and Magnesium (Mn) deficiency.
LINKS
http://www.uwex1.edu/ces/pubs/pdf/A2521.PDF
http://www.back-to-basics.net/efu/pdfs/potassium.pdf