Essentiality of Mineral Elements | Plants

In this article we will discuss about the Essentiality of Mineral Elements required by a Plant. After reading this article you will learn about: 1. Criteria of Mineral Elements 2. Chemical Analysis of Mineral Elements 3. Detection.

Criteria of Mineral Elements:

An-element is considered as essential:

(a) If without it, the plant cannot complete its life cycle

(b) Action of the element must be specific, no element be replaced by other elements

(c) Essentiality is confirmed if the element is shown to be directly involved in the nutrition of the plant, i.e., to be a necessary component of an essential metabolite or at least required as an activator of an essential enzyme, of course, there are certain exceptions.

An element may be present in insufficient amounts. Its function may be partially replaced by another element. This sparing action may be an essential part of the survival of certain organisms under adverse conditions.

Chemical Analysis of Mineral Elements:

Chemical analysis of plant body reveals the presence of a large number of mineral elements, of course, the amount and number of elements may differ from plant to plant and from place to place. Table 5.1 lists all of the elements known to be essential for higher plants.

The elements found in plants are grouped as follows according to their function.

(i) Framework Elements:

Carbon, hydrogen and oxygen are the most important plant nutrients from the standpoint of bulk, and constitute 90 per cent or more of the dry matter of common plant materials. These elements make up cellulose and lignin, the protective waxes, the reserve foods and most of the protoplasm of plants. These are called the framework elements. These elements are normally furnished free by air and rainfall.

(ii) Protoplasm Elements:

In addition to carbon, hydrogen and oxygen, the proteins and other compounds which make up protoplasm, plants require nitrogen, phosphorus and sulphur. These are absorbed as anions and nitrogen and phosphorus are most important fertilizer elements. Calcium is known to occur in certain cell wall constituents and magnesium in chlorophyll although both elements appear to have other uses.

Two tentative groupings may be made on the basis of probable functions:

(a) Balancing Elements:

The basic elements, absorbed as cations, are in some way concerned with maintenance of ionic balance in the plant. Magnesium, calcium and potassium come in this group and sodium should be included for the plants growing in the saline environment.

Chlorine is found to function as anions in the maintenance of this balance. There is considerable evidence that potassium and magnesium may function in the following groups also, acting as inorganic catalysts rather than as enzyme formers.

(b) Catalytic Elements:

Elements like molybdenum, copper, zinc, magnesium, iron, boron and chlorine are used by plants in small quantities and have been assigned catalytic or enzyme forming roles. Because of the small requirements, these elements have been termed infinitesimal nutrients or micronutrients.

The elements which are required comparatively in greater amounts which is equal to or higher than 1000µg^-1 of dry matter are termed major elements or macro-elements or macronutrients. The minor elements or microelements or trace elements, on the other hand, are required by plants in very minute amounts equal to or less than 100µg g^-1 of dry matter.

Detection of Mineral Elements:

There are three methods to determine the mineral requirements of plants.

These are:

(i) Plant analysis

(ii) Solution culture or hydroponics

(iii) Solid medium culture.

(i) Plant Analysis:

The plant material is dried in an oven at a temperature of 70-80°C. At higher temperature some of the sulphur and nitrogen-containing compounds may be converted into gaseous form and may be lost. The dried sample is then powdered with a porcelain mortar and pestle. This powdered plant material is subjected either to wet digestion or to ash analysis.

(a) Wet Digestion:

In this method, to a small quantity of concentrated sulphuric acid the powdered sample is taken and heated on a low flame. The materials dissolve and a clear solution is obtained.

(b) Ash Preparation:

Ash is prepared keeping the powdered sample to high temperatures (600°C) in a muffle furnace. All the volatile and non-volatile organic compounds are burnt into gases, leaving a white powder called ash. Carbon, hydrogen and oxygen are given off as CO₂, water and oxygen. Nitrogen cannot be detected with this method since it escapes as ammonia or nitrogen gas. Chemically, ash consists of oxides of metals.

The ash content of different plants and tissues varies from 1 per cent to 4 per cent of fresh weight. The ash content is maximum in halophytes and xerophytes and minimum in hydrophytes. Ash is dissolved in warm dilute hydrochloric acid or nitric acid. Detection and quantification of the elements, present in the solution obtained by either of the methods, are done by some chemical, physical and physicochemical methods.

The improved methods are atomic absorption spectrometry, optical emission spectrometry, etc. Atomic absorption spectrophotometers are now used to measure metals and some non-metals. In optical- emission spectrophotometry, the elements are vaporized at temperatures above 5000 K to excite electrons from their ground states to higher-energy excited states.

These electrons return to their ground states by emitting the absorbed energy at wavelengths different for each element. Wavelengths are measured and the energy content is quantified by the spectrophotometer.

Analysis of Plant Ash:

Plant ash is analysed to detect the mineral elements present in the plant. Ash is prepared keeping the dried plant material to high temperatures (about 600°C) in a muffle furnace.

The ash consists of oxides of metals. About 1 g of ash sample is taken and dissolved in 20ml of 20 per cent warm HNO₃. After a few minutes, it is filtered if necessary and then transferred to a measuring cylinder and the volume is made up to 100 ml with distilled water.

The solution may be filtered at this step also and with the filtrate the following tests are performed:

a. Test for Sulphate:

To an aliquot of the ash solution, a few drops of 5 per cent barium chloride (BaCl₂) solution is added. A white crystalline ppt. of barium sulphate is formed.

SO₄^2- + BaCl₂ → BaSO₄ + 2Cl^–

The ppt. is insoluble in dilute mineral acids.

b. Test for Calcium:

To about 10 ml of ash solution, a few drops of ammonium hydroxide are added to make the solution alkaline, and finally saturated ammonium oxalate solution is added to it. A white calcium oxalate ppt. is formed.

Ca²⁺ + (NH₄)₂C₂O₄ → CaC₂O₄ + 2NH₄⁺

c. Test for Magnesium:

1. The calcium oxalate ppt. formed in the above experiment is filtered off and the filtrate is concentrated by heating and then cooled in running water. To the cooled solution containing NH₄OH, 10 per cent disodium hydrogen phosphate is added. A white crystalline ppt. of magnesium ammonium phosphate is formed.

d. Test for Iron:

1. The acidic ash solution is taken in a test tube and heated. To it, 2 per cent potassium Ferro cyanide solution is added. A Prussian blue colouration or precipitation appears. The colouration is due to the formation of ferric Ferro cyanide complex.

Fe³⁺ + K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃ + 4K ⁺

2. To a portion of ash solution, 5 per cent ammonium thiocyanate or potassium thiocyanate is added. The solution turns blood red due to the formation of ferric thiocyanate.

If ammonium bi-fluoride solution is added to the mixture, the red colour disappears.

e. Test for Phosphorus:

A portion of the ash solution is taken in test tube and heated. To this, excess ammonium molybdate is added. The temperature of the mixture should not exceed 40°C. A canary-yellow ppt. of ammonium phosphomolybdate is formed.

PO₄^3-+(NH₄)₂MoO₄ →(NH₄)₃[PMo₁₂O₄₀]

f. Test for Copper:

The sample solution is heated and then neutralized with NH₄OH and then one or two drops of acetic acid is added. To the mixture, potassium Ferro cyanide solution is added. A chocolate-brown ppt. of cupric Ferro cyanide is formed.

Cu²⁺ + K₄ [Fe (CN)₆] → Cu₂ [Fe (CN)₆]

g. Test for Zinc:

A small portion of the ash solution in a test tube is neutralized with NH₄OH. One or two drops of acetic acid is added to make the solution slightly acidic. Then to it uranyl acetate solution is added along with a little NaCl. A yellow ppt. of sodium zincouranyl acetate is formed.

h. Test for Chlorine:

To a portion of ash solution, AgNO₃ solution is added. A white ppt. of silver chloride is formed which is insoluble in dilute acid but soluble in ammonia.

Cl^-1 + AgNO₃ → AgCl + NO₃^–

i. Test for Molybdenum:

To a portion of the acidic ash solution, excess of ammonium phosphate solution is added and heated. The temperature should not exceed 40°C. A canary-yellow ppt. is formed due to the production of ammonium phosphomolybdate (NH₄)₃[PMo₁₂O₄₀l complex

MoO₄^2- + (NH₄)₃PO₄ → (NH₄)₃|PMo₁₂O₄₀]

(ii) Hydroponics:

In 1860 W. Pfeffer, Julius Sachs and W. Knop grew plants in this way and is referred to as hydroponics or solution culture. This method provides an excellent means for controlling the quantity and relative proportions of mineral salts given to a plant in an experiment.

In this method mineral salts are dissolved in double glass-distilled water. Every time only one element is left out from the solution and the plant is grown on it. If, without that particular element, the plant shows some deficiency symptoms and if those symptoms vanish on supplying the missing element this element is considered to be essential.

There are two advantages for using solution cultures in mineral nutrition studies:

(i) Water is an excellent solvent for the mineral salts, and

(ii) Water can be easily freed from contaminations.

In spite of such advantages, the technique has some disadvantage as follows:

(i) There is a need for root aeration that is not sufficient in solution.

(ii) There is a need to replace the solution every day or two for maximum growth because the solution composition changes as certain ions are absorbed more rapidly than others.

(iii) This selective uptake also causes pH changes.

In solution culture, there are many sources of contamination like the reagents, the water, the containers and the dust in the surrounding atmosphere. It is not possible to eliminate totally these contaminating influences, but they can be kept to a minimum. Most of the difficulties encountered in mineral nutrition studies are associated with trace element contamination.

(iii) Solid Medium Culture:

To avoid some disadvantages of liquid cultures, solid medium cultures are generally used by many physiologists. As a solid medium for roots highly purified silica sand or crushed white quartz sand or gravel is generally easier to work with because those are very low in available trace elements. In this method, the roots can easily anchor the solid substratum and no supporting device needs to be provided to the plant.

Nutrient solutions are applied in three different ways:

(i) By pouring over the solid medium (called slop culture),

(ii) Dripping onto the solid medium at suitable intervals (called drip culture) and

(iii) By forming solution up from the bottom of the container (called sub-irrigation culture), in all these techniques, the solutions that are added, drain out through an outlet in the bottom of the container.

In the sub-irrigation technique re-circulating solutions that flow through the solid medium around the roots, are used.

The unabsorbed solution flows down into a reservoir in which the pH and solution composition can be monitored and adjusted automatically and then the solution is pumped up to drain down again bathing the roots. The pumping apparatus is attached to a timing mechanism that gives periodic irrigation to the sand, quartz, or gravel.

The slop culture is the easiest method to operate but exposure of roots to constant amount of essential elements and water cannot be controlled. In drip culture the amount of solution being added should be equal to the amount of solution drained off.

So this system allows for a continuous and more or less constant nutrient and water supply. The sub-irrigation system which operates automatically, is the most desirable of the three systems, but it is most costly and needs sophistication.

Several formulations of nutrient solutions have been developed and named after the workers like Knop, Hoagland, Evans, Shive, Sachs, etc. The most important formulations are Hoagland’s solution and Shive’s solution (modified by H.A. Evans). The recipes are listed in Table 5.2.

These are the general formulations having necessary elements in concentration to allow good growth of many higher plants, but a solution ideal for one species may not be ideal for another species.