Plant Potassium Cycle
Kellen Fullenwider-Programmer, Ian Lowe-Researcher, Zopal Bayarmagnai-Writer
Potassium is considered the most common univalent cation in the cytoplasm of plants. “It forms only weak complexes and does not compete strongly with other cations in metabolic reactions. As a consequence of these physical and chemical characteristics, K is uniquely suited to serve as the primary inorganic cellular osmoticum and, through the reversible formation of complexes with soluble and insoluble anions, as a cytoplasmic pH buffer” (Brown). Some of the important functions of K in plants include osmoregulation, turgorrelated processes, and counterbalancing of ions.According a study conducted b Emmanuel Epstein and colleagues at the University of California, there are two mechanisms that are important to the nutrient uptake by plants. First, high-affinity system operates at low external concentrations of K, which was saturable and was described by Michaelis-Menton kinetics. The second mechanism which operated at higher concentrations of K was claimed to be present either in the same or in different membranes. There are three gene families with a total of 34 individual genes that contribute to K uptake and transport. There are numerous K transporters differing in their affinity for K and the special mechanisms by which they facilitate uptake of K. These are divided into high-affinity carriers (HKTl) and low affinity channels.
The main functions of K include its role as an osmoticum for turgor-driven processes, a counterion for charge balance related to H+ and solute transport, and as a stabilizer of enzymes and proteins. The cell expansion and stomatal opening entail the active transport of large amounts of K into the vacuole. “Potassium facilitates the loading of sucrose into phloem by providing the required osmotic potential in source tissues and through its role as a counterion for sucrose and H+ transport into phloem sieve tubes” (Brown).
Potassium availability affects numerous processes in cultivated and natural ecosystems. Potassium is considered a major element for plants because it is involved in osmotic balance and photosynthesis (Barre). Also, potassium is a limiting nutrient in many agricultural systems and potassic fertilization is widely used in the world. Soil K dynamics and its availability is important to study for agronomical and ecological processes.
There are two main K pools and soils called the exchangeable pool and non-exchangeable pool. Exchangeable K+ ions are adsorbed on soil organic matter. But non-exchangeable pool is much larger, which is about 90-99% of total K in many soils. Non-exchangeable K, on the other hand can contribute about 80-100% of the K supply available to plants (Barre). Soil clay mineral layers that contain anhydrous K+ ions are known as illite. These layers are 2:1 layers, which is based on mica structure of 1 nm basal spacing. X-ray diffractometry can be used to calculate the abundance of these layers. In general, the largest part of the anhydrous potassic illite layers is involved with smectite layers in mixed layer structures, which are called interstratified or mixed layered minerals. Illite and smectite mixed layer minirals are stable in temperature soils condition (Barre).
In many studies, the release of interlayer K from soil clays was quantified. For example, a study done by Velde and Peck involved direct observations of clay mineral modifications induced by K depletion. But extraction by plants of interlayer anhydrous K+ ions allowed these layers to behave as smectites. This led to significant changes in XRD patterns, in which the K loss from the clay minerals was not quantified in these experiments. The progress in X-ray diffraction techniques and X-ray data processing provide some hope to observe faster and more suble clay mineral modifications.
Many studies show that “non-exchangeable” K+ ions fixed in 2:1 clay mineral interlayers contribute to plant nutrition. The experiment on the X-ray observation of the minerals in the clay fractions done by Pierre Barre and Bruce Velde showed that X-ray techniques allow observing short-term modifications induced by interlayer K addition or depletion through plant action on a whole soil clay assemblage. They estimated the cg position from complex X-ray patterns. This indicator, which was based on X-ray data is strongly correlated with the clay K fixation or removal (Barre). They established a quantitative relationship between X-ray observations and interlayer K Dynamics. Also, their results showed that interlayer K chemically explained as non-exchangeable is to some extent available to plants. Furthermore, the reversibility of anhydrous K layer formation plays an important role in ecosystem functioning to 2:1 soil clay minerals.
Pierre Barre and Christophe Montagnier also did a study on clay minerals as a soil potassium reservoir. Their data showed that soil 2:1 clay minerals behave as a K reservoir. They used X-ray measurements of materials formed under field conditions to fill and empty the reservoir. They said that this reservoir will have a key role for K cycle in soils. It could also supply short term K plant needs and preserve long term ecosystem productivity by decreasing K leaching (Barre). “Our work also validates the use of the centre of gravity position to qualitatively and quantitatively study clay mineral modifications in soil dominated by illite and mixed layer illite-smectite clay minerals which open many perspectives for soil K cycle understanding from roots to ecosystem scale” (Barre).
Works Cited
Barre, Pierre. "Soil-plant potassium transfer: Impact of plant activity on clay minerals as seen from X-ray diffraction." Plant Soil 292(2007): 137-146, 213-220.
Brown, Patrick. "Potassium and Other Macronutrients."Encyclopedia of Plant and Crop Science. 2004.
Internet Cites
http://www.cfaitc.org/Commodity/pdf/Potassium.pdf
http://www.nature.com/nature/journal/v370/n6491/abs/370655a0.html
http://www.springerlink.com/content/3050184665536p27/