FACTORS AFFECTING MINERAL SALT ABSORPTION
Absorption of mineral salts by plants is influenced by some external and internal factors. These factors include mainly temperature, light, H+ ion concentration, oxygen concentration, interaction between ions, growth of the plant, and ageing.
- Temperature: Rate of mineral absorption either increase or decrease with corresponding increase or decrease in temperature. In general, an increase in temperature results in an increase in the absorption of salt up to a certain level. At a very high temperature, the rate of mineral absorption declines or completely ceases. The inhibition is due to denaturation of proteins which are directly or indirectly involved in salt absorption. Temperature also regulates the movement of ions, since active absorption of ions depends upon the metabolic energy and activity of ion carriers present in the plasma membrane. The enzymes which are involved in mineral absorption get inactivated at higher temperature.
- Light: Mineral absorption depends on the metabolic energy and the rate of transpiration. Light controls both transpiration and photosynthesis. Transpiration helps in the mass flow of mineral ions into the cell and photosynthesis is responsible for conservation of energy needed for active absorption of minerals ions. Hence, absence of light may even stop the absorption of mineral ions by plants.
- H+ ion concentration: In general, decrease in the pH of soil solution accelerates the absorption of anions whereas an increase in pH favours absorption of cations. However, pH value across the physiological range may damage the plant tissue and inhibit salt absorption.
- Oxygen concentration: Like many other metabolic phenomena, absorption of minerals ceases to function in the complete absence of oxygen. In the absence of oxygen, aerobic respiration of plant stops, and therefore, the availability of metabolic activity also ceases. In the absence of metabolic energy, active absorption of mineral ions is not possible. Not only the availability of soil but its concentration determines the rate of respiration as well as the release of metabolic energy. If the oxygen concentration in the soil atmosphere is not sufficient, the rate of ion absorption decreases.
- Interaction of ions: The absorption of one ion is influenced by the presence of other ions in the medium. Viets (1944) demonstrated that the absorption of potassium is affected by the presence of calcium, magnesium, and other polyvalent cations in the soil solution. Viets observed a duel effect by calcium on the uptake of both potassium and bromine. In the absence of calcium, the uptake of both potassium and bromine was less. But, as the concentration of calcium was increased up to a certain level, the uptake of potassium and bromine was increased. Thereafter, further increase in calcium concentration resulted in sharp decline in uptake of both potassium and bromine. Olsen (1942) observed adverse effect of calcium on the absorption of magnesium.
Epstein (1978) demonstrated the interaction of several ions as competitives for binding sites on carriers. For example, potassium (K), rubidium (Rb) and cesium (Cs) compete with one another for same binding site. The rate of absorption of K+ decreases if Ca+and Mg++ are present in the medium.The presence of Ca++ antagonises the absorption of K+. This phenomenon is called ion-antagonism. For the absorption of cations (K+, Na +, Mg++and NH4+ etc). from the soil, the cell sap should have cations (H+) and similarly, for anions (Cl-, NO3-, SO4–) etc in the soil, the cell sap should have anions (OH-). After the ions are being absorbed, they accumulate on the cell wall and then carried to the cell sap by ion carriers. Thus, interaction between the ions and the activity of ion carriers determines the rate of mineral absorption.
- Growth plants: Growth of a plant induces the growth of roots. Prolific growth of roots increases the number of root hairs and finally, increasing the surface area for absorption of minerals. When the volume of the cell increases, the relative concentration in that cell falls. In order to balance the mineral concentration of the cell sap, more amount of minerals from the soil move rapidly into the cell. Thus, growth of plant influences the rate of absorption of minerals.
- Ageing: As the root matures, it increases the surface area which is favourable for salt absorption but due to heavy suberization, the soil uptake is greatly reduced.
DIFFERENCE BETWEEN ACTIVE AND PASSIVE MODES OF MINERAL OR SALT ABSORPTION
|ACTIVE SALT ABSORPTION||
PASSIVE SALT ABSORPTION
|1. Energy is used to carry out active absorption which comes from metabolic activities, especially respiration.||1. No energy is spent on this type of salt absorption. It is carried out by physical forces.|
|2. Rate of respiration increases during active salt absorption. The increased respiration was termed as salt respiration or anion respiration.||2. Rate of respiration does not increase during passive salt absorption.|
|3. Active salt absorption is often against the concentration gradient.||3. Passive salt absorption is generally along the concentration gradient.|
|4. Active salt absorption often results in the accumulation of salts in plant cell.||4. Passive absorption does not help in salt accumulation.|
|5. It stops in the presence of respiratory poisons and enhanced when respiratory stimulants are present.||5. Passive salt absorption is not much influenced by metabolic poisons and activators.|
|6. Active salt absorption uses carriers for passage of ions across the cell membrane. The ions form a complex with the carrier called carrier-ion complex which is capable of moving across the membrane.||6. Passive salt absorption does not require carriers.|