Since Lebieg's research on the chemical composition of plant matter, potassium has been classified as an essential nutrient for plants. This was confirmed by subsequent detailed studies on its role in the life processes of plant cells. Within the group of alkali metals, this element occupies a unique position in terms of its importance in the growth and development of all higher and lower plants. Therefore, plants contain significant amounts of this element in their tissues. Potassium ranks eighth among the elements constituting the Earth's crust, accounting for almost 2.5%. Potassium salts are found in rocks, which, through mechanical and chemical weathering, gradually fragment these minerals, releasing K+ ions into the soil solution. Small amounts of potassium are also found in the atmosphere, for example, in particles of sprayed soil and in smoke and droplets of seawater carried by the wind.
- Potassium Uptake by Plants.
Potassium reaches the root surface primarily by diffusion. Direct potassium uptake by plants, i.e., the amount of K+ ions flowing per unit of time across the cytoplasmic membrane of the root cell, is determined by:
• the potassium concentration in the phloem, which determines the shoot's quantitative demand for this nutrient;
• the K+ cation concentration in the vacuole, which determines the rate of ion transport from the root cell to the xylem;
• the rate of potassium ion movement from the root apoplast through the cytoplasmic membrane to the cytoplasm.
In solutions with very low potassium concentrations, when the K+ ion concentration in the root apoplast does not exceed 0.5 mmol, potassium is actively taken up using carriers, according to the HAS (high affinity transport system) curve, against the electrochemical gradient.
In solutions with high K+ concentrations, above 0.5-1.0 mmol, K+ ion transport across the cytoplasmic membrane follows the LATS (low affinity transport system) curve, passively, via protein ion channels, based on electrochemical potential differences, with significantly lower carrier efficiency. Therefore, plant potassium nutrition is determined more by environmental conditions than by their nutritional needs. The rate of potassium uptake is particularly high in
young plants, but over time, potassium concentration in cells decreases due to redistribution from older to younger organs. - The Role of Potassium in Plants.
Potassium performs multiple physiological functions in plants, primarily due to two properties of this element: the rate at which K+ ions selectively pass through biological membranes and the activation of numerous enzymes. The former determines its beneficial effect on water management; this element influences the hydration state of cellular plasma colloids, while the latter determines its role in carbohydrate and nitrate metabolism. Potassium is known to activate over 60 enzymes. In most cases, its effect on enzymes is specific, meaning it cannot be replaced by other alkaline elements. This is directly related to the volume of hydrated ions and the degree of toxicity of these elements. Thus, cations with a small volume in the hydrated state (Rb + and Cs + ) and dimensions similar to K + have a greater ability to activate enzymes than cations with a large volume in the hydrated state (Na + and Li + ). However, Rb + and Cs + cannot play a major role because the concentrations required to activate enzymes would be toxic to the plant.
- The Interaction of Nitrogen and Potassium in Plant Fertilization.
Nitrogen and potassium are essential macronutrients, but they perform distinct physiological functions in plants. Nitrogen, a component of proteins that build cell structures and enzymes, occurs in plants in mineral form, in the form of NO3 ions, only in small amounts. Potassium, a coenzyme and regulator of cell hydration, is present in the cell sap only in ionic form. Insufficient supply of this nutrient increases cytoplasmic viscosity, restricting cell growth and, consequently, the entire plant. It has been proven that balanced nitrogen and potassium fertilization increases the effectiveness of each. Potassium is a cation that assists in nitrate transport in the xylem, thus significantly affecting the plant's nitrogen balance. The rate of potassium accumulation in plants, however, depends largely on the level of nitrogen fertilization. At high nitrogen doses, plants absorb more of both nitrogen and potassium. Adequate nitrogen supply results in the production of more leaf-forming cells, with greater volume. - Potassium-calcium-magnesium interactions and interactions with anions.
Other cations have a significant impact on potassium uptake by plants, especially at high concentrations. The mechanism of this interaction is not always fully understood. It is known, however, that these interactions are very complex, involving both direct and indirect ion interactions. Cations that can reduce potassium uptake most often include calcium, along with magnesium and sometimes sodium.
The antagonism between Ca and K was formerly referred to as the "calcium-magnesium ratio law." It is now accepted that this is a rather general phenomenon, involving competition between ions for carriers located in the plasma membrane. An excess of one ion can inhibit the binding of another ion to the carrier, thus reducing its uptake. Competition between other cations and potassium is believed to occur only when potassium concentrations are low. In fact, if potassium concentration is sufficient, the effect of calcium on K+ ion uptake is very small. Therefore, too low or too high a concentration of Ca2+ ions does not favorably affect potassium uptake by plants.
An optimal Ca:K ratio must exist for the uptake of both nutrients and plant growth to occur properly. Similarly, the interaction with magnesium ions increases at low potassium concentrations. However, at higher potassium concentrations, magnesium uptake decreases only slightly. Among the anions, in addition to the NO3 ions discussed earlier, chloride ions also favorably affect potassium uptake. Sulfate ions (SO4 2-) hinder K uptake by plants, and similarly to sulfate ions, HCO3 ions also inhibit potassium uptake by plant tissues.
- Symptoms of K deficiency and excess.
Due to the high mobility of this element, deficiency symptoms begin on older leaves as small black dots, which over time may enlarge into visible holes. Chlorosis of the oldest leaves, progressing to necrosis, may also occur. Lesions may primarily affect the leaf tip or marginal tissues. Most often, excess symptoms are associated with inhibition of the plant's uptake of nitrogen and calcium, and are characteristic of N and Ca deficiency.
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-Xinxiang Xu, Xin Du, Fen Wang, Jianchuan Sha, Qian Chen, Ge Tian, Zhanling Zhu, Shunfeng Ge and Yuanmao Jiang
“Effects of Potassium Levels on Plant Growth, Accumulation and Distribution of Carbon, and Nitrate Metabolism in Apple
Dwarf Rootstock Seedlings” Front. Plant Sci., 23 June 2020.