Uptake and Transport of Mineral Nutrients

Introduction

Plants use mineral nutrients as chemical elements for proper functioning. The growth and development of plants depend majorly on the available nutrients in the soil. Mainly, plants obtain carbon dioxide from the atmosphere, but the rest of the nutrients are absorbed and transported to different parts by the roots. The transport of water and minerals in plants is done via two main plant tissues, i.e., the xylem and the phloem. This article will study the uptake of mineral nutrients and the translocation of ions.

Essentially, 13 chemical elements are absorbed by the roots, among which six of them are macronutrients (magnesium, nitrogen, calcium, phosphorus, potassium, sulphur) and the rest are micronutrients. Macronutrients are responsible for the following functions:

• Growth and development of plants
• Acting as an ingredient for chlorophyll formation and photosynthesis
• Reducing diseases and helping in root growth and seed formations

Uptake of mineral nutrients

This is a natural process where plants absorb the cellular nutrients and the chemical elements enter the plant through the same pathway as water. This process takes place in two ways: passive absorption and active absorption.

Passive absorption

The process where absorption of minerals occurs without direct use of metabolic activity. Generally, these are minimal because:

• Highly charged particles are present in the soil 
• The concentration of minerals in the roots is higher than in the soil

Active absorption

This is the process where minerals are absorbed from the soil to the cellular pathway of the uptake of mineral ions against the concentration gradient. Here, metabolic activity (energy in the form of ATP) is needed to transfer minerals. Since all the nutrients cannot be transported passively, active transportation provides necessary help by using energy stored as ATP. Active uptake of minerals by roots mainly depends on the expenditure of metabolic energy.

The specific protein present in root hair cells helps transport minerals by actively pumping ions from the soil into the cytoplasm of the epidermal cell.

Endodermal cells control the type and quantity of solutes that reach the xylem tissue. These specific cells have certain proteins that allow only selective solute to pass through the membrane.

Translocation of ions

From the roots, nutrients and minerals are transported throughout the plant body by means of transpiration pull. Generally, the minerals from older parts of the plants get transported to young areas like apical and lateral meristems, young leaves, growing flowers, fruits etc. The elements that get transported are potassium, nitrogen, phosphorus, sulphur etc. These are transported by the process of active diffusion and active uptake by cells of the plant. 

Transport in phloem

The phloem is responsible for food transport from leaves to the various parts of the plant where it is required or stored. The parts of the plant where food is stored or needed are called source and sink. Source and sink vary in different seasons. For example, in early spring, the roots might become the source and the buds become the sink.

Source – synthesises the food

Sink – stores or needs food

The direction of movement of food in phloem can be upwards or downwards, i.e., bidirectional. The sap of the phloem mainly consists of water and sucrose.

Mass flow or pressure flow hypothesis

-The process or mechanism in plants for translocation of food from a source to sink is called the pressure-flow hypothesis. 

-By the method of photosynthesis, food is produced, which is mainly glucose. 

-This converts to sucrose and moves to the companion cells and phloem sieve tubes by the process of active transport.

– The water moves from the xylem to the phloem because of the hypertonic solution created in phloem cells by the process of osmosis.

– The osmosis creates osmotic pressure, due to which phloem sap moves to the areas bearing low pressure. The pressure gets reduced at the sink. 

-The sucrose is needed to move out of the phloem sap by the process of active transport so that the cells could use the necessary sugar, which will eventually get converted to energy, starch. -Once the sucrose moves out of the phloem sap, the osmotic pressure reduces and water moves out.

Toxicity of micronutrients

The need for micronutrients is always low. However, a moderate decrease can cause deficiency and a moderate increase can cause toxicity. There is a range of concentrations at which the elements are optimum. Any kind of mineral ion that reduces the dry weight of tissues by 10 percent is considered toxic. Toxicity varies from plant to plant and it is challenging to identify toxicity in simple ways.

For example, a prominent syndrome of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins. However, it is essential to know that manganese also competes with iron and magnesium for binding enzymes and inhibits calcium-like translocation in the shoot apex.

Deficiency of essential nutrients

If the supply of necessary elements becomes limited, plant growth starts slowing. The concentration of the nutrient or element below which the plant shows retardation in growth is called critical concentration. Since each of the chemical elements has a specific function in the cellular mechanism of a plant, a deficiency of these elements shows morphological changes in the plant’s body. These deficiency symptoms disappear when the elements are provided adequately. However, if the deprivation continues, it might lead to the death of the plant.

Some kinds of deficiency seen in plants are:

Chlorosis: loss of chlorophyll resulting in yellowing of leaves (lack of sodium, potassium, magnesium, sulphur etc.)

Necrosis: Death of leaf tissue (lack of calcium, magnesium, copper, potassium)

Conclusion

Plants acquire a wide variety of mineral inorganic nutrients from the soil, air and water. Among 105 elements discovered so far, only 21 elements are required for the growth and development of plants. These elements consist of proteins, fats, nucleic acid, etc, which essentially take part in a plant’s metabolic activity. Deficiency of each of these elements might lead to impaired cell division, necrosis, stunted growth etc. the xylem and the phloem are the tissues responsible for the proper functioning of a plant. The xylem controls water transport in a unidirectional manner and phloem controls food transport in a bi-directional manner.

Passive or active transport is responsible for the digestive mechanism of plants.