This article throws light upon the top ten roles of potassium in plants. The roles are: 1. Translocation 2. Meristematic Growth 3. Maintaining Water Status 4. Photosynthesis and Translocation of Photosynthates 5. Enzyme Activation 6. Protein Synthesis 7. Light-Driven and Seismonastic Movements 8. Phloem Transport 9. Replacement of Potassium by Sodium 10. Cation-Anion Balance.
Role # 1. Translocation:
Potassium is an essential for all living organisms. In plant physiology it is the most important cation not only in relation to its content in plant tissues but also with respect to its physiological and biochemical functions. The most characteristic feature of potassium is the high rate of absorption by plants resulting its Luxury consumption.
This rapid and high rate of uptake of K+ is dependent on high permeability of plant membranes to potassium which possibly occurs from ionophores located in the membrane facilitating diffusion mechanism. This high permeability of membranes to K+ leads to a greater efflux of K+ from the cell, if the cellular metabolic activity is suppressed.
Potassium retention in the plant cell depends mostly on the negative potential of the cell, with lowering negative potential i.e. by affecting respiration, the potassium retention capacity decreases. Besides this passive uptake of K+ driven by ATPase activity, an active uptake of K+ also occurs at low K concentrations.
Due to its high permeability of plant membranes, various physiological processes like meristematic growth, water status, photosynthesis and long distance transport etc. are affected. The direction of K+ transport is usually towards the younger tissues, redistribution frequently occurring from older to younger plant parts.
The maximum amount of K+ is taken up by plants during its vegetative growth stage. For cereal crops like rice, wheat, maize etc. such bulk uptake of K occurs from tillering to panicle initiation stage.
The high uptake rate of K+ indicates that it is a strong competitor in the uptake of other cation species. However, the uptake and retention of K+ in plant cells are competitively affected by the presence of other cations like H+, Ca2+, Mg2+ and Na+.
The concentration of K+ in the cytoplasm is in the range of 100 mM which is 5 to 10 times higher than that of K+ concentration in the vacuole. Potassium is also present in the phloem sap.
As the solutes of the phloem sap can be trans located both upwards (acropetal) and downwards (basipetal) in the plant, long distance transport of K+ takes place very easily. Plant organs preferentially supplied with phloem sap such as young leaves, meristematic tissues and fleshy fruits are usually high in potassium.
Role # 2. Meristematic Growth:
Potassium is involved in meristematic growth. The growth process is initiated by a plasma lemma located ATPase which pumps H+ ions out of the cytoplasm into the apoplast. The acidification of the apoplast results in a loosening of cell wall constituents and in the activation of hydrolyzing enzymes.
Such loosening of cell wall constituents is a pre requisite for cell expansion. The release of H+ ions depends more on the potassium present in the apoplast since the concomitant uptake of K+ results in a depolarization of the plasma lemma potential which in turn release H+ ions brought about by the ATPase.
In addition, potassium also influences the synthesis of phytohormones which are involved in the growth of meristematic tissues. As for example, the effect of cytokinins on the growth of cucumber cotyledons has been observed to be enhanced by potassium. Potassium ions also act directly as solutes changing the osmotic potential in the plant cells and thereby turgor, and as carriers of charges also the membrane potential.
Role # 3. Maintaining Water Status:
Potassium plays an important role in maintaining water regime of plants, uptake of water in plant cells and tissues is frequently the consequence of active uptake of potassium.
The loss of water from plants has been found to be decreased with well supplied potassium to soils resulting from the reduced rate of transpiration which also depends on the osmotic potential of the mesophyll cells as well as opening and closing of stomata. So potassium plays a significant role in stomata opening and closing.
It has been found that the potassium content in the open condition of stomata is higher than that of the same content of guard cells from closed stomata. Under light conditions the guard cells produce abundant ATP in photosynthetic phosphorylation supporting active uptake of K+ with adequate energy.
Therefore, potassium is accumulated in the guard cells in large quantities and the resulting high turgor pressure causes opening of the stomata. The mechanisms for closing and opening of stomata totally depend on the potassium flux. In addition, potassium plays a key role in osmoregulatory process cell extension and various types of movement through variation of water status in plants.
Cell extension involves the formation of a large central vacuole occupying 80-90% of the cell volume and it (cell extension) has two requirements:
(i) An increase in cell wall extensibility and
(ii) accumulation of solute for creating an internal osmotic potential.
In most of the cases, an accumulation of potassium in cells is necessary for both stabilizing the pH in the cytoplasm and increasing osmotic potential in the vacuoles.
Role # 4. Photosynthesis and Translocation of Photosynthates:
It is evident that potassium has a role on the rate of CO2 assimilation. Potassium did not directly involve and influence photosystem I or II, but potassium promoted synthesis of the enzyme ribulose bisphosphate carboxylase. Potassium also decreased the diffusive resistance for CO2 in the mesophyll.
Potassium is the dominant counterion to the light induced H+ flux across the thylakoid membranes and for the establishment of the trans membrane pH gradient necessary for the synthesis of ATP (photo phosphorylation). The effect of valinomycin (ionopheres which makes bio membranes “leaky” for passive K+ flux resulting decrease in CO2 fixation) can be compensated for by high external K+ concentrations.
The rate of decrease in photosynthesis in plants grown under drought conditions is much less severe when adequate potassium is supplied. However, such ameliorating effect of potassium is associated with higher leaf K concentrations.
An increase in potassium content in leaves increases rate of photosynthesis and RuBP (ribulose bisphosphate) carboxylase activity as well as photorespiration and these effects may be due to a stronger depletion of CO2 at the catalytic sites of the enzyme.
With an increase in potassium content in leaves dark respiration decreases with the simultaneous increase in photosynthesis. Higher rates of respiration are a typical characteristics of potassium deficiency. Potassium nutritional status may also affect photosynthesis in leaves via its function in stomatal regulation.
Potassium stimulates photosynthetic O2production which supports the view that potassium has a direct influence on e– transport in the photosynthetic e– transport chain.
It is believed that the movement of K+ from the thylakoid spaces to the stroma of chloroplasts which occurs in the light should depolarize the thylakoid membrane and as a result favour e– flow in the transport chain. Potassium is directly involved in phloem loading.
Potassium increases the translocation of photosynthates. It not only promotes the translocation of recently synthesized photosynthates but also has a favourable effect on the mobilisation of stored materials. Potassium is responsible for both phloem loading and unloading and hence has controlling influence on phloem transport.
Role # 5. Enzyme Activation:
Potassium also plays an important role for the activation of various enzyme systems in plants. Potassium and other monovalent cations (Rb+, Cs+, NH4+, and Na+) activate enzymes by inducing conformational changes in the enzyme protein. All macromolecules ate highly hydrated and stabilized by rigidly held water molecules forming an electrical double layer.
At monovalent salt concentrations of about 100-150 mM (milli moles), maximum suppression of thickness of this electrical double layer and optimisation of the protein hydration occur. This concentration range agrees well with the K concentrations in the cytosol and stroma of plants adequately supplied with potassium.
However, in general, potassium induced conformational changes of enzymes increase the rate of catalytic reactions, (Vmax) and in some cases also the affinity for the substrate (Km).
The activity of starch synthase is also highly dependent on monovalent cations, of which K+ is the most effective. The enzyme catalyses the transfer of glucose to starch molecules:
ADP-glucose + Starch DADP + glucosyl-starch.
Besides, due to poor supply of K to plants, low molecular weight compounds viz. amino acids and sugars may accumulate in plants and this may be due to an inadequate energy (ATP) supply. An energy shortage may lead to delay in protein synthesis which in turn indirectly affects enzyme activity because of a lack in enzyme proteins.
Another function of potassium is the activation of membrane bound proton-pumping ATPases. Such activation facilitates the transport of K+ from the external solution across the plasma membrane into the root cells as well as making potassium an important nutrient element for cell extension and osmoregulation.
Acute potassium deficiency in plants leads to the formation of various toxic amines such as putrescine and agamatine from the decarboxylation of arginine as follows:
Agamatine is then converted into carbamyl putrescine which is hydrolysed to putrescine and carbamic acid as follows:
The formation of such compound is an enzymatic process occurs at low cellular pH levels which are largely influenced by the potassium supply to plants. However, due to synthesis of putrescine in K-deficient plants, the plants are not able to show visual symptoms of potassium deficiency in-spite of having K demand in crops.
Such synthesis (putrescine) mechanism is inhibited by high K+ concentrations and stimulated by low cellular pH. Considering the dominant role of K+ in the maintenance of high cytoplasmic pH, enhanced synthesis of putrescine as divalent cation is a reflection of a homeostasis of the cytosol pH. In potassium deficient plants the putrescine concentrations may account for up to 30% of the deficit in potassium equivalents.
Role # 6. Protein Synthesis:
Potassium is necessary for protein synthesis in higher amounts as compared to the activation of enzymes which is maximum at about 50 mM K+. The synthesis of RuBP (ribulose bisphosphate), carboxylase, a chloroplast protein, is inhibited under potassium deficiency and responded rapidly with the supply of potassium to plants.
However, a 10 mM concentration of K+ in the external solution is sufficient to maintain a more than 10 times higher K+ concentration in the chloroplasts which is necessary for higher rates of protein synthesis. Plants supplied with adequate amounts of potassium contribute more towards protein synthesis than that of the plants deficient for potassium.
Role # 7. Light-Driven and Seismonastic Movements:
In leaves of various plants especially in Leguminosae, leaves re-orientate their laminae photonastically in response to light signals under both non-directional light signals (leaf blades folded in the dark and unfolded in the light) and directional light signals (leaf blades re-orientate towards the light source). These photo-nastic responses either increase light interception or allow avoidance of damage by excess light.
Such movements of leaves as well as leaflets are carried out by reversible turgor changes in specialised tissues, the motor organs (pulvini). The turgor changes result shrinking and swelling of cells in opposing regions of the motor organs and K+, CI– and malate2- are major solutes involved in osmoregulation and volume change and hence movement of leaves and leaflets.
In leaflet movement, the driving force of K+ influx is a plasma membrane bound H+– ATPases and thus the movement of leaflet can be stopped by anaerobiosis or vanadate. In the leaf’ movement, the environmental signals i.e. light activate calcium (Ca2+) channels in the membranes and thereby increase cytosolic free calcium (Ca2+) concentrations.
However, in contrast to guard cells, in the motor organ the extensor and the flexor regions respond to these signals in opposite ways.
An identical mechanism is responsible for the movement of leaves and other plant parts in response to a mechanical stimulus. In the seismonastic reactions a rapid long-distance transport of the “signal” from touched leaflet to other leaflets also occurs and this signal is an action potential, travelling in the phloem to the motor organs and inducing phloem unloading of sugars like sucrose in the motor organ.
Localised high concentrations of sugar might contribute to the ion channel-mediated changes in turgor pressure in a given region of the motor organ necessary for the movement of leaflets.
Role # 8. Phloem Transport:
Potassium also plays an important role in both loading of sugars and solute transport in the sieve tubes through mass flow.
The function of potassium with regards to phloem transport of sugars and other photosynthates is related to the maintenance of high pH in the sieve tubes (symplasm) for sucrose loading and the contribution of K+ to the osmotic potential in the sieve tubes and hence the rate of transport of photosynthates from source to sink.
It is evident that in potassium deficient plants the rate of export of photosynthates is much lower as compared to plants well supplied with potassium where the rate of transport is much higher. In legumes well supplied with potassium, the root nodules have a greater supply of sugars which enhances the rate of nitrogen fixation and export of bound nitrogen.
Role # 9. Replacement of Potassium by Sodium:
In less specific processes such as raising cell turgor some replacement of K+ by Na+ is possible. The extent of such substitution, however, depends much on the uptake potential for Na+ which varies with the plant species, being higher in natrophilic (sodium-loving) plants. In natrophilic plants, sodium contributes to the osmotic potential of the cell and thus has a positive role on the water status of plants.
However, the beneficial effects of Na+ on plant growth are observed quite prominently in plants growing on inadequate K+ supply. In the lower range of K+ concentrations, Na+ increased grain yields, whereas at the higher K+ concentration Na+ results a slight yield depression.
Under field conditions, sodium deficiency of plants has not yet been observed. In general, plant species can be grouped into four in relation to their growth response to sodium.
High amount of potassium can be replaced by sodium without any effect on plant growth e.g. sugar beet, turnip and various grasses (C4 types).
Smaller amount of potassium can be replaced by sodium without decrease in growth of plants e.g. cabbage, radish, cotton, pea, wheat and spinach.
Substitution only takes place on a limited extent where sodium has no specific effect on plant growth, e.g. barley, millet, rice, oat, tomato and potato.
Plants in this group cannot substitute potassium by sodium, e.g. maize, rye, soybean and beans.
However, a tentative relationship among various groups of crops in relation to their replacement ability is depicted by figure 21.14.
Role # 10. Cation-Anion Balance:
In charge compensation, potassium is the dominant cation for counter-balancing immobile anions in the cytoplasm, chloroplasts, and also for mobile anions in the vacuoles, xylem and phloem. The accumulation of organic acid anions in plant tissues is the consequence of K+ transport without an accompanying anion into the cytoplasm e.g. root or guard cells.
Such cation-anion balance as affected by potassium also influences the nitrate metabolism in plants. As a result of reduction of NO3â€”N in leaves, the rest of the potassium requires the stoichiometric synthesis of organic acids for charge balance and pH homeostatis, some portion of such recently synthesized potassium malate may be re-trans-located to the roots for subsequent utilization of K+ as a counter ion for NO3– â€”N within the root cells and for xylem transport. In legumes having ability to form nodules, such recirculation of K+ may behave towards similar function in the xylem transport of amino acids.