BIOLOGY Volume 2 - A Textbook of General Biology - 2004

13. TRANSPORT IN PLANTS

13.6. Mineral Salt Uptake and Transport in Roots

For proper Nutrition, plants require not only the CARBOHYDRATES produced during Photosynthesis, but also various mineral nutrients. The Functions of these elements are detailed in Table 7.7. In higher plants, mineral nutrients are absorbed from the soil or surrounding Water by the roots¹. Maximum absorption occurs within the ROOT Hair zone. The Role of mycorrhizae in this process is discussed in Section 7.10.2.

To understand the mechanisms of mineral ion Uptake and Transport, several key points must be kept in mind.

1. Mineral elements essential for plant growth occur as salts. In aqueous solution, salt molecules dissociate into freely moving ions.

2. Ions can cross cellular membranes via various pathways. One such pathway is Active Transport. This process requires energy in the form of ATP generated during Respiration, and it can drive the movement of ions against their concentration gradient (Section 5.9.8).

3. Extending inward from the root epidermis is a continuous network of Cell walls known as the apoplast. Water and any dissolved solutes can freely penetrate this system from the soil.

Fig. 13.19 shows the potassium ion uptake by young cereal roots that had been pre-washed in pure water. After 90 minutes, a respiration inhibitor—potassium cyanide—was added to the solutions.

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Fig. 13.19. Potassium ion uptake by a young cereal plant in an aerated solution.

13.18. a) Describe how potassium ion uptake proceeds at 0 °C and 25 °C.

б) Explain the effect caused by potassium cyanide (KCN).

в) Explain why roots must be thoroughly washed before being placed in a solution containing potassium ions.

Fig. 13.19 demonstrates that ion uptake is clearly divided into two phases. The first phase lasts approximately 10–20 minutes, during which uptake is relatively rapid. As potassium ions come into contact with the root epidermis, they enter The Cell walls and move inward through the apoplast via mass flow driven by Transpiration, or via diffusion. This phase is largely Temperature-independent, as the uptake rate is nearly identical at both 25 °C and 0 °C, indicating that it is a passive process.

The second phase is temperature-dependent and is absent at 0 °C, a temperature at which metabolic and respiratory rates are very low. The inhibition of K⁺ ion uptake by potassium cyanide indicates that this process relies directly on respiration. During this second phase, potassium ions enter the root Cells across the Plasmalemma via active transport.

Similar results can be obtained using isolated Tissues. Such experiments typically employ plant storage Organs, such as carrot root tissue. The data presented in Fig. 13.20 confirm that ion uptake is dependent on respiration and is inhibited by potassium cyanide.

Fig. 13.20. Respiration rate and potassium chloride uptake in carrot root discs. (Based on Robertson and Turner, 1945.)

Thus, Ion uptake into the root is driven by two processes: 1) passive uptake, where ions move via mass flow and diffusion through the apoplast; and 2) active transport, in which ions are transported into cells against their concentration gradient using energy generated by respiration.

13.19. Fig. 13.20 shows that the respiration rate in excised carrot discs increases when they are transferred from pure water into a potassium chloride solution.

By comparing the graphs, explain why this occurs.

13.20. Why does potassium chloride uptake cease following The addition of KCN?

13.21. If an experiment similar to the one in Fig. 13.19 is conducted using phosphate uptake, it is found that 16% of the phosphate absorbed by barley roots is lost within the first few minutes after the roots are transferred back to pure water. Explain this result.

13.22. Can ions reach the xylem by moving exclusively through the apoplast?

Active transport is a selective process dependent on respiration, whereas diffusion is non-selective and requires no Energy Expenditure. As a result of passive absorption, all Cells of the root primary cortex are bathed in a solution whose composition is similar to that of the soil solution. This creates a vast surface area for ion absorption.

Ions moving through the apoplast reach only the endodermis, where their further advancement is blocked by the Casparian strips (sec. 13.5.2). To cross this barrier, ions must do so via diffusion or active Transport Across the plasmalemmas of endodermal cells, entering their Cytoplasm and possibly vacuoles. In this way, the plant controls which Mineral Substances ultimately reach the xylem.

13.23. How can radioisotopes and autoradiography be used to demonstrate that the endodermis acts as a barrier to the movement of ions through cell walls?

Ions can also travel via the symplast. Once inside the cytoplasm of a single cell, they continue to move further through plasmodesmata without crossing any membranes. The continuous symplastic pathway extends from the root hairs all the way to the xylem. Fig. 13.18, A illustrates all possible pathways of ion transport within the root.

The final stage of mineral salt translocation across the root is the release of ions into the xylem. To achieve this, they must sooner or later leave the Cell Cytoplasm through the plasmalemma. This occurs either by diffusion or via active transport.