Potassium (K+) uptake is highly selective, and K+ ions are very mobile in plants at all levels. That is mobile within individual cells, within tissues, as well as in long-distance transport via the xylem and phloem. Selective integral membrane proteins (transporters and cation channels) facilitate uptake and transport of K+ throughout the plant and its movement across the plasma membrane. Potassium concentrations in the cytosol and chloroplasts are regulated to be around 100– 200 mM. In the cytosol and chloroplasts K has important specific metabolic functions which cannot be replaced by other cations such as Na+. Note that there’re exception for natrophilic species like sugar beet (Beta vulgaris) a crop that benefits from high Na concentrations and this crop is able to use Na as an osmiticum as well. Whereas cytosolic K is tightly regulated, vacuolar K concentrations may vary between 10 and 200 mM and can even reach 500 mM in guard cells of stomata.
Potassium plays a key role in the following physiological processes. It plays a key role in plant–water relations, as its highly mobile and it is not metabolized, movement in and out the vacuole and cell can regulate osmotic potential of cells and tissues. Thus, a key function of K+ is osmoregulation and therewith turgor pressure-driven solute water balance of plants. At the level of tissues or individual cells, similar mechanisms are responsible for cell extension and several types of movement. For example, stomata movement, where K plays a major role in turgor changes in the guard cells. Another type of movement is the reorientation of leaves in response to light signals. An example is leave blades which are folded in the dark and unfolded in the light, or reorientation towards the light source. These responses can prevent damage by excess light or increase light interception. The underlying mechanism of these movements is again the reversible turgor change in dedicated tissues called the motor organs (or pulvini). Not only K is involved in this osmoregulation also Cl- and malate2+ play an important role in inducing water flow from cell to cell.
In addition to osmoregulation, potassium also plays a role in enzyme activation and synthesis. That is a large number of enzymes are either completely dependent on or are stimulated by K+. Potassium can moreover activate membrane-bound proton-pumping ATPases. These activated ATPases facilitate K+ transport from the external solution across the plasma membrane into the root cells. Because of this regulation capacity the K is has a dual role in cell extension and osmoregulation, it is an activator and the osmotic. Potassium deficient leaves show reduced photosynthesis. The main reason for this reduction is hampered stomatal regulation. Additionally K is the main counter-ion to the light-induced H+ flux across the thylakoid membranes and for the creation of a transmembrane pH gradient necessary for the synthesis of ATP (photophosphorylation). Furthermore, at low leaf K concentration both RuBP carboxylase activity and photorespiration is decreased.
In the phloem potassium play an important role in both the loading of sucrose and the rate of the mass flow-driven solute transport in the sieve tubes. It is, furthermore, well documented that K-deficient plants are more prone to abiotic and biotic stresses. This is nicely illustrated by the plant injury of K-deficient plants under high-light intensity, drought, low temperature, iron toxicity and pest and disease pressure.
After nitrogen, potassium is the nutrient required in the largest amount by plants. The K requirement for optimal plant growth is in the range of 20–50 g kg-1 in vegetative parts, fleshy fruits and tubers. In natrophilic species K+ requirements are lower, because they can replace K+ by Na+ . When plants don’t get enough K plant growth is retarded. Under mild deficiency plants will transport K+ from older leaves and stems to feed the younger extending leaves where K is most needed. If deficiency gets severe these older leaves become chlorotic and necrotic, especially when they are exposed to high light conditions. Under deficiency lignification of vascular bundles is also impaired which is together with turgor loss a reason for increased lodging susceptibility in K-deficient plants. Low potassium levels will affect the nutritional and post-harvest quality. Especially fleshy fruits and tubers with their high K requirement are affected by low K. Low K can, for example cause ripening disorders (‘green back’) in tomato and in potato tubers many quality criteria are depend on a sufficient K tissue concentration.
If growers want to increase plant organ K concentration this is simply done by increasing K fertilisation rate. Unfortunately, higher K fertilizer dose does not influence grain and seeds K concentration as in most cases it is regulated to be around 3 g kg-1. At high K supply rates ‘luxury consumption’ occurs, this deserves attention as it can induce Mg and Ca deficiency symptoms.