Essential Mineral Elements
Plants require all 112 elements for mineral nutrition of them. Most of the mineral elements present in soil are absorbed by roots of the plant. But all are not essential. There are 17 elements are known to be essential for the growth of all higher plants. In the absence of each essential element, plants develop deficiency symptoms.
Criteria for Essentiality of Elements
The essential elements (or essential nutrients) are chemical elements that are absolutely needed by plants for their growth and development. The roots absorb about 60 elements from the soil. To determine which one is an essential element, the following criteria are used; (i) an essential element is absolutely necessary for supporting normal growth and reproduction, (ii) the requirement of the element is very specific and it cannot be replaced by another element, (iii) the element is directly involved in the metabolism of the plant. Example: Magnesium is said to be an essential element because it is essential for the formation of chlorophyll molecule. Its deficiency causes yellowing of leaves.
Types of Essential Elements
Essential elements may be required in small amounts or large amounts. Accordingly they have been grouped into two categories;
Macro elements/Macro nutrients
Micronutrients or trace elements are needed in very small amounts (less than 10mmole Kg-1 of dry matter).
Macronutrients are required in relatively large amounts by plants (in excess of 10 mmole Kg-1 of dry matter).
Examples: Manganese, Boron, Copper, Molybdenum, Iron, Zinc and Chlorine are required in very small quantities.
Examples: Carbon, Hydrogen, Oxygen, Phosphorous, Potassium, Calcium and Magnesium, Nitrogen, Sulphur.
Sources of Essential Elements for Plants
Most of the essential elements are taken from soil, and some from the atmosphere.
Carbon: Taken as CO2 from the atmosphere (air).
Oxygen: Absorbed in the molecular form from air or from water. It is also generated within a green plant during photosynthesis.
Hydrogen: Released from water during photosynthesis in the green plant.
Nitrogen: Absorbed by the plants as nitrate ion (NO3–) or as ammonium ion (NH4+) from the soil. Some organisms like bacteria and cyanobacteria can fix nitrogen from air directly.
Potassium, calcium, iron, phosphorus, sulfur, magnesium: absorbed from the soil in the ionic forms e.g. K+, Ca2+, Fe3+, H2PO4–/ HPO42–.
Symptoms of Mineral Deficiency in Plants
There are different symptoms for deficiency of different elements. When a deficient mineral is provided to the plant, the symptoms disappear. But if the deficiency continues, it may lead to the death of the plant.
Some common deficiency symptoms are:
Chlorosis: is an abnormal condition of green plants in which the stems and leaves turn pale green or yellow. The yellowing is due to reduction in the levels of the green chlorophyll pigments. It may be caused by a number of factors. It is caused by the deficiency of elements like K, Mg, N, S, Fe, Mn, Zn and Mo.
Necrosis: is the localized death of tissue of leaves. The death of tissues may spread to the entire leaf. The deficiency of calcium results in necrosis of young meristematic regions, such as root tips or young leaves, particularly leaf tissue is caused by deficiency of K, Ca and Mg.
Deficiency of N, K, B, S and Mo causes stunted growth because of inhibition of cell division. Deficiency of N, S and Mo delays flowering.
Stunting refers to retarded growth. The stems appear condensed and short and caused by the deficiency of N, P, K, Zn and Ca.
Premature fall of leaves and buds is caused by deficiency of K, P.
Delay in flowering is caused due to deficiency of N, S and Mo.
Uptake of Mineral Elements
Plants take in mineral nutrients, including some which may not be essential to them, through roots. Process of intake of nutrients from the soil is called mineral absorption. Mineral salts from soils are not absorbed as whole molecules but as ions. Soluble mineral salts are present in soil water as cations and anions. The uptake of mineral ions by the roots may be passive or active;
Passive absorption: the inner space refers to cytoplasm and the vacuole and entry of ions into the outer space does not require expenditure of metabolic energy is called as passive absorption. It is the initial and rapid phase and ions are absorbed into the outer space of the cells apoplast.
Active absorption: On the other hand, entry into and/or exit from the inner space, usually requires metabolic energy and is called as active absorption. It is the second phase of ion uptake. The ions are taken in slowly into the inner space the symplast of cells.
The movement of ions is termed as flux.
The movement into the cell is influx and the outward movement is called efflux.
The minerals ions absorbed by the root system are translocate through the xylem vessels to other parts of the plant.