Role of Mineral Elements in Plants

In this article we will discuss about the role of mineral elements in plants.

The mineral elements, when present as ions or as constituents or organic molecules, perform several important functions in plants in a number of different ways (Table 9-3).

They are important constituents of protoplasm and cell wall e.g., sulphur occurs in proteins, phosphorus in nucleoproteins and adenosine phosphates, magnesium in chlorophyll, and calcium pectate in the middle lamella. Mineral salts dissolved in the cell sap partially influence the osmotic pressure of the cell.

The mineral salts which are absorbed from the soil also affect the pH of cell sap as well as cytoplasm. Phosphate and carbonate systems are two of the important plant buffer systems. They originate in substances absorbed by the plant from its environments.

The cation components of plant buffer systems, other than H⁺ ion are such mineral elements like potassium, calcium, sodium and magnesium. A buffer system is a mixture of a weak acid and its salt e.g., carbonic acid+sodium carbonate.

It enables the cytoplasm and the cell sap of the cell to resist changes in pH due to the loss or addition of H⁺ or OH^– ions. Cations and anions influence the permeability of the cytoplasmic membranes. For instance, calcium and other di-and trivalent cations have a decreasing effect on the permeability of the cytoplasmic membranes whereas monovalent cations have an increasing effect.

Furthermore, the specific ions in contact with the cell also influence the mechanism of accumulation of ions in plant cells. Ions of many mineral elements have a toxic effect on protoplasm, causing its disintegration even in very dilute concentrations. Some of these ions which have toxic effect are aluminium, arsenic, boron, copper, lead, manganese, mercury and molybdenum.

Antagonistic Effect:

Sometimes, the effect of one ion can be reversed by another ion and it is called antagonism effect. Some amount of antagonism does exist between any pair of salts. For instance, sodium chloride increases the permeability of cytoplasmic membranes to various solutes whereas calcium chloride added to the medium has a decreasing effect.

Similarly calcium ions also reduce the toxicity of copper ions. In vivo these and other elements perform balancing functions.

Catalytic Effects:

Certain minerals like iron, copper and zinc take part in catalytic systems and are prosthetic groups of certain enzymes. Iron is constituent of the cytochromes. Other minerals like magnesium, manganese and cobalt act as activators or inhibitors in one or more enzymic systems.

In addition, silicon, aluminium, selenium, chromium, tin and iodine may be present as trace elements and may not have specific role in plant metabolism. For the purposes of discussion and easy understanding nutrient elements can be classified into four groups.

These are given below:

Group 1:

N and S, in reduced form are covalently bonded constituents of plant organic matter.

Group 2:

P, B, and Si, they occur as oxyanions; phosphate, borate or silicate.

Group 3:

K, Na, Mg, Ca, CI, they have roles in osmotic and ion balance and have specific functions in enzyme conformation and catalysis (e.g., metal loprotein complexes).

Group 4:

Fe, Cu, Mo, Zn, they are present as structural chelates or metalloproteins. The first three elements take part in redox reactions.

Table 9-5 gives a functional classification of the mineral nutrients needed by the higher plants.

adopted from Clarkson and Hanson (1980) in Annual Review of Plant Physiology, 31,239.

Carbon, Hydrogen and Oxygen:

They are secured from the air and soil in the form of carbon dioxide and water, respectively. The three gases are components of the protoplasm, cell wall and most of the organic constituents of the plants.

Nitrogen:

Excepting the legumes, plant obtains their nitrogen supply mainly in the form of nitrate (NCT₃) from the soil but also as ammonium ions (NH₄). Legumes, which possess Rhizobium bacteria in their nodules, have the ability to utilize molecular nitrogen. In the plant cell the nitrate ions are reduced to NH₂ group through series of enzymatic reactions.

Nitrogen is an essential component of proteins, protoplasm, enzymes and also chlorophyll. It is also a constituent of purines, pyrimidines, porphyrins and coenzymes.

Whereas purines and pyrimidines are components of nucleic acids (RNA and DNA), the porphyrin structure is present in chlorophylls and the cytochrome enzymes. It also occurs in coenzymes which are essential for the functioning of enzymes. In addition vitamins also contain nitrogen. Plants supplied with excessive nitrogen are usually dark green in colour, abound in foliage but usually have a feebly developed root system.

For instance potato plants subjected to high nitrogen have small tubers but profuse shoot growth. Perhaps sugar translocation to the tubers and roots is affected in some way. Furthermore, excessive quantity of nitrogen also reduces flowering and even seed formation in several crops.

However, short day plants given abundant nitrogen flower faster. Wheat plants supplied with high nitrogen doses have shown increased susceptibility to rust. In a subsequent chapter we shall discuss details of nitrogen metabolism.

Deficiency Symptoms:

Nitrogen deficiency results in yellowing of leaves due to the loss of chlorophyll first in the old leaves and lastly in the young ones. The symptoms appear last in the young leaves because of the mobility of nitrogen since they obtain nitrogen translocated from the old leaves.

In the severe cases the old leaves become yellow and fall off while the young leaves remain attached for a long time but turn pale green due to the development of anthocyanin, making the leaf petioles and veins purple. Due to the less availability of nitrogen, leaf development is poor and plants become stunted due to low protein synthesis.

Phosphorus:

Soil is a major source of phosphate ions (H₂ PO₄^‑) and in the organic compounds, it occurs in the oxidized form.

It is an important constituent of every living cell and enters into the composition of phospholipids, nucleic acids, nucleo-proteins, co-enzymes like NAD and NADP, and even ATP. It occurs in abundance in the meristematic tissues and, storage organs e.g., seeds and fruits.

The co­enzymes like NAD and NADP are important in oxidation-reduction reactions and several processes, like photosynthesis, glycolysis, respiration, and fatty acid synthesis are dependent upon their action.

The application of phosphate fertilizers to the soil alters the nitrogen balance in plants and therefore, the role of nitrogen and phosphorus in plant metabolism is interrelated in several ways.

Deficiency Symptoms:

Plants are stunted but are often dark green in colour. The petioles of leaves or fruits develop necrotic areas. Phosphorus deficiency results in premature leaf fall and purple anthocyanin pigmentation due to decreased protein synthesis and accumulation of sugars in the vegetative organs.

Because of the high mobility of phosphorus to the growing tissues in plants, the old leaves are first to show phosphorus deficiency symptoms. The less availability of phosphorus may also distort leaves shape. Plant maturity may also be delayed.

Calcium:

Calcium is absorbed from the soil as calcium nitrate or calcium sulphate. It is relatively immobile in plants. Old leaves abound in calcium compared with the young ones. It is a constituent of middle lamella and occurs as calcium pectate.

It helps to cement the wall of the cells together. Sometimes, cell vacuoles contain insoluble crystals of calcium oxalate such as raphides and sphaeroraphides. Calcium is essential for the formation of cell membranes and lipid structures.

In small amounts calcium is essential for affecting normal mitosis and may be concerned with chromatin or mitotic spindle organization.

Its deficiency causes abnormalities of chromosome structure and even mitosis. It activates a number of enzymes including α-amylase and plays an important role in nitrogen metabolism by participating in the reduction of nitrates. It also influences the permeability of cytoplasmic membranes and depresses the entry of sodium and potassium.

The role of calcium in the hydrolysis of starch into sugar and its subsequent transport is also shown. Thus deficiency of calcium results in the accumulation of starch in leaves.

Similarly, under calcium deficiency number of mitochondria decrease as reported in wheat roots. Calcium deficiency leads to the death of meristematic regions. Chlorosis occurs along the margin of young leaves becoming necrotic. The tips of young leaves become hooked, cell walls become rigid and brittle.

Magnesium:

Magnesium plays significant role in photosynthesis and carbohydrate metabolism. It is a constituent of chlorophyll molecule and prevents the interveinalchlorosis of the old leaves. It activates numerous enzymes of carbohydrate metabolism, nucleic acid (DNA and RNA) synthesis from nucleotide polyphosphates.

These reactions involve phosphate transfer and ATP becomes linked to the enzyme surface through magnesium as an intermediate carrier and facilitates bond breakage. This element helps in the maintenance of structure of ribosomes. It activates enzymes involved in the synthesis of nucleic acid and also several other enzymes including certain transphosphorylases, dehydrogenases, and carboxylases.

It also acts as an important binding agent in ribosomal particles. Magnesium plays in important role in respiratory mechanism by regulating phosphate metabolism in plants. Deficiency in magnesium causes interveinalchlorosis first in the old leaves and then in the young leaves.

Potassium:

It occurs mainly as soluble inorganic salt or salts of organic acids in the cells and is highly mobile in plants. Young and actively growing regions such as buds, leaves and root tips have abundant of K⁺. This element activates enzymes involved in the synthesis of certain peptide bonds during protein synthesis.

Therefore, potassium deficiency causes low level of protein but high accumulation of amino acids and amides or its also activates enzymes of carbohydrate metabolism. It is essential for chlorophyll development and catalyzes normal carbohydrate break down during respiration.

Potassium deficiency results in weakening of leaves, chlorosis, rolling of leaves, stunted growth and shortening of internodes.

Sulphur:

Sulphur is absorbed from the soil as sulphate ions and reduced to-SH group in the formation of sulphur-bearing amino acids such as cystine and methionine. It is also essential for the synthesis of sulphur-bearing vitamins like biotin, thiamine and coenzymes.

Sulphur is also present as sulphydryl groups in many enzymes. Sulphur deficiency results in general chlorosis followed by anthocyanin pigmentation. Plants deficient in sulphur show chlorosis of the young leaves first. Severe deficiency results in chlorosis of all the leaves since sulphur is immobile in the plant.

Amino acids and other nitrogen containing compounds accumulate in the tissue and proteolytic activity also increases. Plants look stunted and flowering is delayed. Sulphur is converted into organic compounds through adenosine derivative called 3′- phosphoadenosine- 5′- phosphosulphate (PAPS). This compound is produced at the expense of ATP. The sulphur moiety of PAPS is then reduce and incorporated into organic molecules.

Iron:

It is in the ferric state (Fe⁺⁺⁺) that iron is absorbed but metabolically, it is active in the ferrous state. It is not a constituent of chlorophyll but is important for its synthesis. It is the component of metallo-flavoproteins and component of iron porphyrin proteins such as cytochromes, peroxidase and catalase. Iron is also an important part of ferredoxin and nitrite reductase.

Iron-deficient plants exibit interveinal chlorosis in the leaves. The maximum effect is in the young leaves while the mature leaves may not show chlorosis at all. Iron is immobile. Lack of iron may inhibit protein synthesis.

Many scientists believe that iron is an essential activator for enzymes catalyzing reactions involved in chlorophyll synthesis. If iron salt is applied in a soluble form to the chlorotic leaves, green colour develops at the places where the salt has penetrated the leaves.