Plant Physiology - M.M. Musiyenko 2001

Root Nutrition of Plants
The Role of Ion Transport in Metabolism

The Transformation of substances (METABOLISM) should be understood not only as The conversion of some compounds into others, but also as their translocation from the environment into the plant or from one part of the plant to another. For convenience, plant metabolism can be divided into three parts: organic matter metabolism, mineral salt ion metabolism, and Water Metabolism. Let us consider The Role of ion transport in water and organic metabolism.

Plants absorb water using two driving forces: the ROOT pumps water, and the leaves draw it up. The lower driving force is root pressure. At high air humidity, when Transpiration is suppressed, water droplets are exuded at the tips of the leaves. This is guttation. Root pressure is based on the well-known phenomenon of osmosis. The osmotic concentration of xylem sap is many times higher than that of the surrounding medium, which drives the Movement of water into the root Cells. However, the main osmotic component of xylem sap is K⁺ ions, which account for 3/4 of its total osmotic potential. Water and the K⁺ dissolved in it constantly move from the root to the Vessels of the stem and leaves. Consequently, for water uptake to be continuous, potassium must constantly enter the root cells and be actively transported into the vessels.

The upper driving force of water absorption is transpiration through the stomatal apparatus. Once again, K⁺ ions play a decisive role in regulating the stomatal apparatus. At night, when the Stomata are closed, potassium is evenly distributed among all epidermal cells. The first rays of sunlight signal the stomata to open. Membrane ion pumps are activated, pumping potassium into the guard cells from neighboring cells. Within just a few minutes, the concentration of K⁺ in the guard cells, and thus the Osmotic Pressure of their Cell sap, increases 4-5 times. As a result, the guard cells draw in water and swell, causing the stomata to open.

When the light signal is removed, in the dark, the ion pumps shut down, and the excess potassium returns to the adjacent neighboring cells. The turgor of the guard cells decreases, and the stomata close.

Thus, ion transport (in this case, K⁺) is the driving mechanism that regulates both driving forces of plant water metabolism—root pressure and transpiration.

But What is the role of ion transport in organic matter metabolism? The main currency of all cellular biosyntheses is simple CARBOHYDRATES, primarily sucrose and glucose, synthesized during Photosynthesis. From the leaves where they are formed, assimilates must be transported to the heterotrophic Organs of the plant (for example, to the root). Long-distance transport of sugars, as is well known, occurs through the phloem. Phloem vessels are loaded with sugars in the fine endings of leaf Veins. Loading occurs against a significant (at least twofold) concentration gradient and therefore requires a substantial expenditure of metabolic energy.

As it turns out, the loading of sieve tubes with sugars is also linked to the operation of membrane-bound ion pumps (likely H⁺-proton pumps). Interestingly, metabolic energy is spent not on pumping carbohydrates into The Cell, but on pumping H⁺ out of it (the pH of sieve tubes is usually more alkaline than that of other plant cells). It can be assumed that the membrane protein of the proton pump, In addition to the H⁺-binding group, has another group with an affinity for carbohydrates. Therefore, in one cycle within the membrane, it pumps a proton H⁺ out of the cell and pumps a sugar molecule into it.

There is evidence that antiport (involving a proton pump) participates in the membrane transport of carbohydrates at the other end as well, for example, during the loading of sucrose into the vacuoles of storage cells in sugar beet roots. Here, too, the ion pump serves as an intermediary between energy and the flow of carbohydrates across the membrane. This case is even more interesting because we are once again encountering another plant monopoly, as animal cells are unable to transport carbohydrates across the membrane against their concentration gradient.

Subsequently, a series of transformations of carbohydrates and the entire diversity of substances synthesized from them take place with the participation of Enzymes. As is well known, all enzymes function only in an appropriate ionic environment. Changes in its composition significantly affect enzymatic activity, decreasing or increasing it in different PARTS OF THE cell. After all, for maximum activity, each enzyme requires a certain concentration of ions in the medium, the presence of specific activator ions (typically alkali and alkaline earth metals K, Ca, Mg), the absence of inhibitor ions (heavy metals), and a specific pH. For example, membrane ATPase requires the presence of magnesium, potassium, and sodium in a 1:2 ratio and a pH of 8. Changing even one of these parameters is enough to deactivate the enzyme. Clearly, by altering the direction and intensity of ion fluxes, the cell can regulate enzymatic activity and, consequently, The rate of various biosyntheses.

Thus, it follows from the above that ion gradients, ion fluxes, and The activity of ion pumps largely determine not only mineral Nutrition itself (as was previously believed), but also the overall metabolism of the plant, including water and organic matter metabolism.