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Note: 15N solution was applied to roots of intact plants for 24h. After pre-growth of plants in a standard nutrient solution for 5 weeks, plants were exposed to nutrient solutions for 20 days differing in Na+ and K+ concentrations. Source: From H.M. Helal and K. Mengel, Plant Soil 51:457-462, 1979.

4.2.1.3 Ion Absorption and Transport

4.2.1.3.1 Potassium Absorption

Plant membranes are relatively permeable to K+ due to various selective K+ channels across the membrane. Basically, one distinguishes between low-affinity K+ channels and high-affinity channels. For the function of the low-affinity channels, the electrochemical difference between the cytosol and the outer medium (liquid in root or leaf apoplast) is of decisive importance. The K+ is imported into the cell for as long as the electrochemical potential in the cytosol is lower than in the outer solution. With the import of the positive charge (K+) the electrochemical potential increases (decrease of the negative charge of the cytosol) and finally attains that of the outer medium, equilibrium is attained, and there is no further driving force for the uptake of K+ (15). The negative charge of the cytosol is maintained by the activity of the plasmalemma H+ pump permanently excreting H+ from the cytosol into the apoplast and thus maintaining the high negative charge of the cytosol and building up an electropotential difference between the cytosol and the apoplast in the range of 120 to 200mV. If the plasmalemma H+ pumping is affected (e.g., by an insufficient ATP supply), the negative charge of the cytosol drops, and with it the capacity to retain K+, which then streams down the electrochemical gradient through the low-affinity channel, from the cytosol and into the apoplast. Thus in roots, K+ may be lost to the soil, which is, for example, the case under anaerobic conditions. This movement along the electrochemical gradient is also called facilitated diffusion, and the channels mediating facilitated diffusion are known as rectifying channels (16). Inwardly and outwardly directed K+ channels occur, by which uptake and retention of K+ are regulated (17). Their 'gating' (opening and closure) are controlled by the electropotential difference between the cytosol and the apoplast. If this difference is below the electrochemical equilibrium, which means that the negative charge of the cytosol is relatively low, outwardly directed channels are opened and vice versa. The plasmalemma H+-ATPase activity controls the negative charge of the cytosol to a high degree since each H+ pumped out of the cytosol into the apoplast results in an increase of the negative charge of the cytosol. Accordingly, hampering the ATPase (e.g., by low temperature) results in an outwardly directed diffusion of K+ (18). Also, in growing plants, darkness leads to a remarkable efflux of K+ into the outer solution, as shown in Figure 4.2. Within a period of 4 days, the K+ concentration in the nutrient solution in which maize seedlings were grown increased steadily under dark conditions, whereas in light it remained at a low level of <10 |M (19). The outwardly directed channels may be blocked by Ca2+ (20). The blocking may be responsible for the so-called Viets effect (21), which results in an enhanced net uptake of potassium through a decrease in K+ efflux (22).

Time of day figure 4.2 Potassium concentration changes in the nutrient solution with young intact maize plants exposed to light or dark over 4 days. (Adapted from K. Mengel, in Frontiers in Potassium Nutrition: New Perspectives on the Effects of Potassium on Physiology of Plants. Norcross, GA: Potash and Phosphate Institute, 1999, pp. 1-11.)

Time of day figure 4.2 Potassium concentration changes in the nutrient solution with young intact maize plants exposed to light or dark over 4 days. (Adapted from K. Mengel, in Frontiers in Potassium Nutrition: New Perspectives on the Effects of Potassium on Physiology of Plants. Norcross, GA: Potash and Phosphate Institute, 1999, pp. 1-11.)

4.2.1.3.2 Potassium Transport within Tissues

Opening and closure of K+ channels are of particular relevance for guard cells (23), and the mechanism of this action is controlled by the reception of red light, which induces stomatal opening (24). Diurnal rhythms of K+ uptake were also found by Le Bot and Kirkby (25) and by MacDuff and Dhanoa (26), with highest uptake rates at noon and lowest at midnight. Energy supply is not the controlling mechanism, which still needs elucidation (26). Owing to the low-affinity channels, K+ can be quickly transported within a tissue, and also from one tissue to another. This feature of K+ does not apply for other plant nutrients. The low-affinity channel transport requires a relatively high K+ concentration in the range of >0.1 mM (17). This action is mainly the case in leaf apoplasts, where the xylem sap has K+ concentrations > 1 mM (27). At the root surface, the K+ concentrations may be lower than 0.1 mM, and here high-affinity K+ channels are required, as well as low-affinity channels, for K+ uptake.

The principle of high-affinity transport is a symport or a cotransport, where K+ is transported together with another cationic species such as H+ or even Na+. The K+-H+ or K+-Na+ complex behaves like a bivalent cation and has therefore a much stronger driving force along the electrochemical gradient. Hence, K+ present near the root surface in micromolar concentrations is taken up.

Because of these selective K+ transport systems, K+ is taken up from the soil solution at high rates and is quickly distributed in plant tissues and cell organelles (28). Potassium ion distribution in the cell follows a particular strategy, with a tendency to maintain a high K+ concentration in the cytosol, the so-called cytoplasmic potassium homeostasis, and the vacuole functions as a storage organelle for K+ (29). Besides the H+-ATPase, a pyrophosphatase (V-PPase) is also located in the tonoplast, for which the substrate is pyrophosphate. The enzyme not only pumps H+ but also K+ into the vacuole, and thus functions in the cytoplasmic homeostasis (Figure 4.3). This mechanism is an uphill transport because the vacuole liquid is less negatively charged than the cytosol. In Table 4.3, the typical pattern of K+ concentration in relation to K+ supply is shown (30). The cytosolic K+ concentration remains at a high level almost independently of the K+ concentration in the nutrient solution, whereas the vacuolar K+ concentration reflects that of the nutrient solution.

4.2.1.3.3 Osmotic Function

The high cytosolic K+ concentration required for polypeptide synthesis is particularly important in growing tissues; the K+ in the vacuole not only represents K+ storage but also functions as an indispensable osmoticum. In most cells, the volume of the vacuole is relatively large, and its turgor is essential for the tissue turgor. The osmotic function is not a specific one as there are numerous

Cytosol

Pyrophosphate

2 Phosphate

Vacuole

Tonoplast figure 4.3 Pyrophosphatase located in the tonoplast and pumping H+ or K+ from the cytosol into the vacuole.

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