Authored by Rianne Wagner and Shawn Conley
Potassium (K) is often called the “quality element” in crop production because it is essential for photosynthesis, nutrient translocation, and biological nitrogen (N) fixation in soybeans. Deficiencies in K impair these physiological processes, leading to significant yield losses. Understanding potassium’s physiological function is crucial for making informed fertilizer decisions, especially when application costs are high. For example, University of Wisconsin Madison Extension guidelines note that when Bray-1 extractable soil test K is in the optimum range (121–150 ppm), the probability of a yield response to K fertilization at the crop removal rate is only 50/50 (Havlin et al., 2013; Laboski & Peters, 2012). This article examines these core processes and explores the specific roles potassium plays in soybean growth and productivity.
In a Bean Pod
- K deficiency induced stomata and mesophyll resistance limits CO2 diffusion in the plant, resulting in a photosynthetic decline in K deficient crops
- Reduced energy production: Low potassium levels prevent carbon dioxide from moving easily through leaf pores (stomata), limiting the plant’s ability to produce energy through photosynthesis.
- K is essential for the maintenance of turgor pressure in meristematic tissues, leading decreased cell expansion and meristematic growth under K deficiency
- Stunted growth: Potassium helps maintain the water pressure needed for new cells to expand, so a shortage directly slows down new plant growth.
- Reduced photosynthetic assimilate production and transport in K deficient crops decreases the assimilate supply available to sink tissues, ultimately diminishing the yield and quality of the crop
- Lower yield and crop quality: Because potassium deficiency limits the production and transport of energy in the plant, growing parts (flowers, pods, and seeds) don’t get enough vital resources, reducing final yield and quality.
- K deficiency restricts the plant factors controlling nodulation and N fixation in soybean, leading to a reduction in biological N fixation
- Reduced biological nitrogen fixing: In crops like soybeans, potassium deficiency hinders the formation and productivity of root nodules, cutting down the plant’s ability to convert atmospheric nitrogen into plant available N.
The Role of Potassium in Photosynthesis
Photosynthesis, arguably one of the most important plant functions, relies on K to operate. Potassium plays crucial roles in the processes of both the plant stomata and mesophyll, which directly affect photosynthetic capacity and activity (Huber, 1985). In the stomata, K fluxes in guard cells are responsible for stomatal opening and closure, regulating gas exchange and water loss in the plant (Sardans & Peñuelas, 2021). Insufficient K levels induce a resistance to these stomatal movements, decreasing the levels of photosynthesis per leaf unit area due to limitations on the gas exchange necessary for the photosynthetic process (Pettigrew, 2008). However, stomatal resistance does not solely explain the photosynthetic decline in K deficient crops, especially under extreme deficiency conditions. Plant chloroplasts in mesophyll tissue require adequate K concentrations to establish and maintain alkaline conditions in the stroma lamella, which is necessary for photosynthesis to proceed at maximum rates (Huber, 1985). When insufficient levels of K are present, the pH of the stroma is altered (mesophyll resistance), limiting the Rubisco biosynthesis and activity (Sardans & Peñuelas, 2021). The combination of both stomata and mesophyll resistance limits carbon dioxide diffusion in K deficient soybean crops (Imtiaz et al., 2023), reducing the production of photosynthetic assimilates.
Potassium Transport Across Membranes
Similar to how K assists in the process of stomatal opening and closing, the nutrient is essential for the maintenance of turgor pressure in meristematic tissues (Pettigrew, 2008). Adenosine 5′ -triphosphate phosphohydrolase (ATPase) activity in the membrane of meristematic cells results in a difference between the electrical potential of the cell and outer medium, which induces the passive uptake of K+ and an accompanying accumulation of malate in the cell (Mengel, 1985). The accumulation of solutes inside the cell lowers the water potential, leading to water entry and increased turgor pressure in meristematic cells (Nieves-Cordones et al., 2016). This increased turgor pressure is necessary for cell expansion, the driver of cell growth. In the case that cells are undersupplied with K+, cell turgor levels decrease, resulting in decreased cell expansion and meristematic growth (Mengel, 1985).
As the most abundant cation in the phloem, K+ greatly affects the phloem-loading process, which is the initial step in the long-distance transport of photosynthetic assimilates from source (primarily leaves) to sink (flowers, pods, and seeds) tissues. Solute movement in the phloem follows a hydrostatic pressure gradient created from concentration differences in solutes such as K+ (Tränkner et al., 2018). During phloem loading, ATPase and K+ membrane channels create a K+ gradient across the plasma membrane, generating the energy necessary to actively transport sucrose from the apoplast of source tissues into the sieve tube cells of the phloem (Duke & Collins, 1985; Sardans & Peñuelas, 2021; Tränkner et al., 2018). This transmembrane K+ gradient is known as a “K battery” that acts as a mobile energy source, providing energy for other transport processes (Sardans & Peñuelas, 2021; Tränkner et al., 2018). Under K deficient conditions, the transport of photosynthetic assimilates to sink tissue via the phloem is decreased, leading to increased carbohydrate concentrations in the leaf tissue and reduced concentrations in the roots (Cakmak et al., 1994). Restricted photosynthetic assimilate transport, coupled with reduced production, decreases the assimilate supply available to sink tissues and ultimately diminishes the yield and quality of K deficient soybean crops (Pettigrew, 2008).
Potassium’s importance in solute transport extends to the xylem as well. Osmotic water flow in the xylem is controlled by the secretion of ions creating a lower water potential in the xylem vessels, with K+ being the key ion responsible for this process (Mengel, 1985). Potassium also supports the transport of nitrate through the xylem. The K+ ion is the major accompanying counter cation for nitrate uptake and transport to the shoot system, cycling up the xylem with nitrate and back down the phloem with malate (Blevins, 1985). Once malate reaches the roots, it undergoes a chemical reaction that produces bicarbonate, which can then be exchanged for nitrate. As nitrate uptake and translocation within the plant are dependent upon adequate K concentrations, K+ is one of the dominant ions in soybean xylem sap (Israel & Jackson, 1982). However, when plant K concentrations fall below sufficiency levels, soybean’s ability to transport both water and solutes via the xylem is negatively impacted.
The Role of Potassium in Biological N Fixation
Soybeans, like all other legumes, have the unique ability to acquire a majority of their required N through a symbiotic relationship with rhizobia. This process, known as biological N fixation, is indirectly affected by K, as the nutrient influences plant factors that control soybean nodulation and N fixation (Duke & Collins, 1985). As K fertilization increases soybean yield, the plant’s demand for N increases, which is primarily supplied through N fixation. To meet the crops’ increased need for N, K increases soybean nodulation, nodule productivity (i.e., moles of N fixed per unit time per unit mass of nodule), or both (Divito & Sadras, 2014). This increase in nodulation and/or nodule productivity is largely due to the increased translocation of photosynthate (Duke & Collins, 1985), as K controls the transmembrane potentials that contribute to solute transport (Sardans & Peñuelas, 2021). In the case of K deficiency, the plant factors controlling nodulation and N fixation become restricted, leading to a reduction in biological N fixation. As such, proper K fertilization is essential to ensure that N fixation can adequately support soybean growth.
Ultimately, potassium serves several critical functions within the soybean plant, directly impacting photosynthesis, cellular expansion, nutrient transport, and biological nitrogen fixation. Balancing potassium fertility with the farm’s bottom line safeguards these vital biological processes, helping growers protect crop health while maximizing soybean seed yield and quality.
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