Uptake and Translocation of Mineral Nutrients

Plant species require essential mineral nutrients to conduct all of their functions. These mineral nutrients are the chemical elements required by the plant. The soil water is the main source of minerals to the plants. Some minerals exist in the form of positively charged cations and some in the form of negatively charged anions.

Mineral ions are absorbed by plants mainly through the young roots. There are nearly 17 different minerals required by the plant for its growth and development which are called essential elements. Among them, carbon, hydrogen, nitrogen, oxygen, sulphur, potassium, calcium, magnesium and iron are required in large quantities and hence these elements are called macronutrients. Elements like zinc, manganese, boron, copper and molybdenum are required in small quantities and hence they are called micro-nutrients.

Mineral absorption is the natural process by which all of the essential minerals enter the cellular material of the plant, usually through the same pathway as water. Mineral nutrients are taken up by both the roots and the leaves.

The process of mineral nutrient uptake or transport is carried out by plant cells in two different ways:

• Passive absorption: It is the movement of mineral ions into the root cells along the concentration gradient by diffusion without the direct use of metabolic energy. It is also known as physical absorption
• Active absorption: It is the direct expenditure of metabolic energy along with mineral absorption. Ions from the cell’s outer space move into the inner space during active mineral absorption, which normally occurs against the concentration gradient or electric potential gradient. As a result, it requires metabolic energy, which is obtained either directly or indirectly from the cell’s metabolism. Active absorption is a selective process

Because the concentration of minerals in the soil is minimal and they are present as ions that cannot cross the cell membrane, they cannot be transported passively in the roots. These are actively carried through the roots, using ATP as a source of energy. Minerals are carried by particular proteins found in the root hair membrane. Plasma membranes of endodermal cells include transport proteins that regulate the type and number of solutes that reach the xylem. These proteins are called carriers or transporters or transmembrane proteins present in the bilipid layer of plasma membrane. These carrier proteins bind to the ions and unload them in the cytoplasm and return to bind with ions again. The carrier molecule transports 100-1000 ions per second in a cell. The active absorption is slower than the passive absorption. Membrane potential plays an important role in the transport of solutes across the membrane. Membrane potential is the difference in electric charges of the inner and outer surface of the membranes. The factors which affect metabolism also affect the active absorption. Active absorption increases with increase of respiration.

Factors affecting Mineral Absorption

Plants’ mineral salt absorption is influenced by a variety of environmental/ external and internal factors.

Temperature

the rate of mineral absorption increases with increase in temperature up to 35^oC and then declines sharply. Below 2^o C is not suitable for mineral absorption. At higher temperatures, enzymes involved in mineral absorption become inactive.

Light

Light increases the rate of mineral absorption. Both transpiration and photosynthesis are influenced by light. Transpiration helps in the bulk flow of mineral ions into the cell, while photosynthesis preserves the energy required for active mineral ion absorption. As a result, the absence of light may even prevent plants from absorbing mineral ions.

H+ ion concentration: High pH range increases the anion uptake while low pH range accelerates the absorption of cations.

Oxygen concentration

A decrease in the oxygen level of soil decreases the rate of mineral absorption.

Ion interaction

Absorption of one ion is affected by the presence or absence of other ions. For example, the absorption of K+ ions by roots is inhibited by Ca+, Mg++ etc… The low concentration of Ca influences the absorption of Br and K+ by roots

Plant growth

The rate of mineral absorption increases during the growth of the plant as the surface area of absorption increases.

Ageing

Ageing increases the volume of root which has some effect on the rate of absorption of minerals.

Significance of Mineral Absorption

• Provides strength to plants
• Essential for photosynthesis
• Helps in chlorophyll formation
• Rapid growth
• Improving the quality of fruits
• Improves root growth and seed production
• For building proteins and other biomolecules
• Reduction of diseases in plants

Conclusion

The pressure-flow hypothesis is a mechanism used for the translocation of food from source to sink. The process of photosynthesis in plants led to food production in leaves in the form of glucose. It is then converted to sucrose which is moved to the companion cells and then the live phloem sieve tube cells by active transport. The water moves in the phloem from the xylem due to the hypertonic condition created in the phloem by the process of osmosis. Phloem sap moves to the areas of lower pressure due to the build-up of osmotic pressure. At the sink, the atmospheric is reduced. In order to move sucrose out of the sap active transport is needed which will use sugar and it gets converted to energy, starch, or cellulose. When sucrose moves out of the sap led to decrease in the osmotic pressure and water moves out from the phloem.

Minerals may enter roots by Active absorption, which uses energy and by passive absorption which does not use any energy. Transport of mineral nutrients in plants may be multidirectional or unidirectional. Substances that are to be transported are plant growth regulators, organic nutrients, mineral nutrients, and water. For small distances, substances move by diffusion, and for longer distances, substances move through with the help of the vascular system in plants. Transport of nutrients may be affected by internal factors and external factors as well.