Mineral Nutrition

Introduction

Every organism in this world needs nutrition. Nutrition essentially means those substances or chemicals that are required by our body for survival. When we eat, our bodies break down the food into essential elements for our body to synthesise. Plants extract these minerals through the soil. Other organisms obtain minerals through their environment (air, water, soil, etc.).

Search for Essential Minerals

• The first scientist to study mineral nutrition was Julius Von Sachs
• He put a plant in a mineral solution, a solution that might or might not contain all the minerals required by the plant
• He studied the effects of the minerals on the plant. If the plant grew and thrived, it meant the plant got all the essential minerals to survive. If the plant displayed some symptoms, the symptom was studied, and the mineral responsible for the symptom was identified
• This is how he compiled a list of essential minerals used by the plant to grow. The technique we used earlier (placing the plant in a mineral solution) is called “Hydroponics”

Criteria for essentiality

Just as we humans eat junk food (food that might be harmful or unnecessary for the body), plants also intake minerals that may not be essential for survival. After we learnt that around sixty elements like gold, selenium and even radioactive strontium were found in plants, we had to build criteria for which minerals are absolutely essential for plants or without which plants will die.

These criteria were:

• The element must be vital for the growth of plants. For example nitrogen is one of the essential elements along with carbon, hydrogen and oxygen for protein synthesis. So if nitrogen is deficient plant growth will be retarded
• The element should also be essential for the plant’s reproduction. Phosphorus is one of the essential elements present in DNA which makes chromosomes. If phosphorus is deficient it will affect the replication of DNA hence reproduction will be affected
• Deficiency of that element cannot be treated by supplementing another element. If magnesium is deficient chlorophyll formation will not take place, and no other element can replace magnesium
• The element is used in the metabolic processes. Nitrogen is one of the most essential constituents of proteins, nucleic acids, vitamins and hormones

The minerals were divided into two main categories:

1. Macrominerals (macronutrients)

Minerals present in more than 10 mmol kg–1 in dry matter (i.e., present in large amounts). These minerals include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium, and magnesium.

2. Micro minerals (micronutrients)

Minerals present in less than 10 mmol kg–1 in dry matter (i.e., present in a less amount). These are iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel.

Few critical functions of some minerals

• Carbon, hydrogen and oxygen make up most of the biochemical structures
• Magnesium is present in chlorophyll, and phosphorus is present in ATP
• Magnesium, zinc, and molybdenum are activators of necessary enzymes RuBisCO, alcohol dehydrogenase, and nitrogenase, respectively
• Potassium is used in the opening and closing of the stomata

Roles of some essential minerals

• Nitrogen: It is absorbed in the form of nitrite, nitrate, or ammonium. Plants use it for protein formation, nucleic acid, vitamin, and hormone formation. It is a mineral found in plants in large amounts
• Phosphorus: It is absorbed in the form of phosphate ions. It is used to form cell membrane, nucleic acid, proteins, ATP, and nucleotides
• Calcium: It is absorbed in the form of calcium ions and is used during cell division for cell wall formation. It helps in the formation of mitotic spindles and cell membranes as well
• Magnesium: It is absorbed in the form of magnesium ions. It is used as an activator for respiration and photosynthetic enzymes. Also, magnesium is a structural mineral present in chlorophyll and helps maintain ribosomal structure
• Sulphur: It is absorbed as a sulphate ion. It is present in cysteine and methionine amino acids. Sulphur is a structural mineral for various coenzymes, vitamins, and ferredoxin
• Iron: It is absorbed in the form of ferric ions. It is a structural element for ferredoxin and cytochromes. It also helps in activating catalase enzymes
• Zinc: It is absorbed in the form of zinc ions. It is used in the formation of the hormone auxin
• Copper: It is absorbed in the form of Cupric ions. It is required for metabolic processes and is used in redox reactions

Deficiency of Elements

• When the concentration of elements reaches a point where plant growth is retarded, this concentration is referred to as ‘critical concentration’
• Due to the deficiency of minerals, the plant shows physical changes referred to as ‘deficiency symptoms’
• These symptoms go away when the required mineral is provided to the plant
• The need for minerals is most during a plant’s initial growth, i.e. when the plant is young
• Chlorosis: The yellowing of leaves due to chlorophyll loss is caused by a deficiency of N, K, Mg, S, Fe, Mn, Zn, and Mo
• Necrosis: Death of leaf tissues is caused due to lack of Ca, Mg, Cu, K
• Inhibition of cell division: Caused due to a lack of N, K, S, Mo
• Delay in flowering: Caused due to a lack of N, S, Mo

Toxicity of micro minerals

• When minerals that are required in low amounts are absorbed in a large quantity, this condition is known as toxicity
• For instance, manganese competes with iron, calcium, and magnesium, and when it is absorbed by the plants in toxic amounts, it can result in many diseases

Absorption and translocation of minerals and solutes

• The free and rapid uptake of ions is ‘passive’ into the apoplast of the plant cell with the help of ion channels and selective pores
• Exit or entry of ions through the symplast of the plant cell takes place ‘actively’ with the help of pumps by energy expenditure
• Minerals are translocated in the plant with the help of the xylem

Nitrogen uptake:

• Since nitrogen is the most abundant mineral present in plants, it’s important to observe how plants absorb it
• Nitrogen has three covalent bonds, and they are fixed by converting this nitrogen into ammonia by ‘nitrogen fixation’
• Nitrogen present in dead decaying plants and animals converts into ammonia by a process called ammonification
• Chemoautotrophs convert this ammonia into nitrite and then nitrate, which plants absorb
• The conversion of nitrate to nitrogen by bacteria is called ‘denitrification’

Bacterial nitrogen fixation

• Free living bacteria (Azotobacter, Rhodospirillum) help convert nitrogen into ammonia by an enzyme ‘nitrogenase.’ But it is a little sensitive to oxygen, so a compound known as leg-haemoglobin helps in preventing the interaction between oxygen and nitrogenase)
• Symbiotic bacteria (Rhizobium, Frankia) found in association with leguminous and non-leguminous plants help in nitrogen fixation by the formation of nodules within these plants
• The ammonia produced by these processes is used by the cells of plants to convert into amino acids

Conclusion

Smallest organism on the earth (prokaryotes) to the most complex organism (mammals), all of us require minerals for survival. Mineral nutrition is the availability of inorganic nutrients that occur naturally. Minerals help in performing all the necessary functions needed for survival. Mineral nutrients are essential elements required for the growth of plants. If these minerals are absent, the plant will suffer from a deficiency and eventually die. These minerals have to be present in an optimum concentration, or else they will compete and interfere with other minerals and create an imbalance in the plant. Plants use nitrates and nitrites to make ammonia, which is finally used to make amino acids, proteins, and hormones. Many bacteria also help the plant for nitrogen assimilation along with environmental conditions like UV rays and lightning. Basic desires for all living beings are macromolecules that include carbohydrates, proteins, fats, minerals, and water for overall growth. When an organism lacks a mineral, the body shows some signs, which we refer to as ‘deficiency  symptoms.’ the deficiency can over come when the deficient mineral nutrient is supplied. Some of these mineral nutrients are synthesised by plants, bacteria synthesise some, some are obtained through water, but our body can not make up these minerals. Our body gets these mineral nutrients through different sources.