Mineral Nutrition
1. Essential Mineral Elements for Plants
- In 1860, Julius von Sachs, a prominent German botanist, demonstrated, for the first time, that plants could be grown to maturity in a defined nutrient solution in complete absence of soil. This technique of growing plants in a nutrient solution is known as hydroponics.
- Most of the minerals present in soil can enter plants through roots. In fact, more than sixty elements of the 105 discovered so far are found in different plants. Some plant species accumulate selenium, some others gold, while some plants growing near nuclear test sites take up radioactive strontium.
- Only a few elements have been found to be absolutely essential for plant growth and metabolism. These elements are further divided into two broad categories based on their quantitative requirements-Macronutrients, and Micronutrients.
- Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole \[K{{g}^{-1}}\]of dry matter). The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium. Of these, carbon, hydrogen and oxygen are mainly obtained from \[C{{O}_{2}}\] and \[{{H}_{2}}O\], while the others are absorbed from the soil as mineral nutrition.
- Micronutrients or trace elements, are needed in very small amounts (less than 10 mmole \[K{{g}^{-1}}\] of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
- In addition to the 17 essential elements named above, there are some beneficial elements such as sodium, silicon, cobalt and selenium. They are required by higher plants.
- Nitrogen is the essential nutrient element required by plants in the greatest amount. It is absorbed mainly as \[N{{O}_{3}}^{-}\] though some are also taken up as \[N{{O}_{2}}^{-}\] or \[N{{H}_{4}}^{+}\]. Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells. Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins and hormones.
- Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either as \[HP{{O}^{2-}}_{4}\] or \[{{H}_{2}}P{{O}^{-}}_{4}\] ). Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.
- Potassium is absorbed as potassium ion \[({{K}^{+}})\]. In plants, this is required in more abundant quantities in the meristematic tissues, buds, leaves and root tips. Potassium helps to maintain an anion-cation balance in cells and is involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.
- Plant absorbs calcium from the soil in the form of calcium ions (\[C{{a}_{2}}^{+}\]). Calcium is required by meristematic and differentiating tissues. During cell division it is used in the synthesis of cell wall. It accumulates in older leaves. It is involved in the normal functioning of the cell membranes. It activates certain enzymes and plays an important role in regulating metabolic activities.
- Magnesium is absorbed by plants in the form of divalent \[M{{g}^{2+}}\] It activates the enzymes of respiration, photosynthesis and are involved in the synthesis of DNA and RNA. Magnesium is a constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure.
- Plants obtain sulphur in the form of sulphate (\[S{{O}^{2-}}_{4}\]). Sulphur is present in two amino acids - cysteine and methionine and is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.
- Plants obtain iron in the form of ferric ions (\[F{{e}^{3+}}\]). It is required in larger amounts in comparison to other micronutrients. It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes. It is reversibly oxidised from \[F{{e}^{2+}}\] to \[F{{e}^{3+}}\] during electron transfer. It activates catalase enzyme, and is essential for the formation of chlorophyll.
- It is absorbed in the form of manganous ions\[(M{{n}^{2+}})\]. It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism. The best defined function of manganese is in the splitting of water to liberate oxygen during photosynthesis.
- Plants obtain zinc as \[Z{{n}^{2+}}\] ions. It activates various enzymes, especially carboxylases. It is also needed in the synthesis of auxin.
- It is absorbed as cupric ions (\[C{{u}^{2+}}\]). It is essential for the overall metabolism in plants. Like iron, it is associated with certain enzymes involved in redox reactions and is reversibly oxidised from \[C{{u}^{+}}\] to \[C{{u}^{2+}}\] .
- Boron is required for uptake and utilisation of \[C{{a}^{2+}}\] membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.
- Molybdenum is a component of several enzymes, including nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.
- Chlorine is absorbed in the form of chloride anion (\[C{{I}^{-}}\]). Along with Na+ and \[{{K}^{+}}\], it helps in determining the solute concentration and the anioncation balance in cells. It is essential for the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.
- Chlorosis is the loss of chlorophyll leading to yellowing in leaves.
2. Nitrogen Cycle
- Plants compete with microbes for the limited nitrogen that is available in soil.
- The process of conversion of nitrogen (\[{{N}_{2}}\]) to ammonia is termed as nitrogenfixation. In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (\[NO,\,\,N{{O}_{2}},\,\,{{N}_{2}}O\]). Industrial combustions, forest fires, automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides. Decomposition of organic nitrogen of dead plant s and animals into ammonia is called ammonification. Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria.
- Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus. The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter. These steps are called nitrification. These nitrifying bacteria are chemoautotrophs.
- The nitrate thus formed is absorbed by plants and is transported to the leaves. In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids. Nitrate present in the soil is also reduced to nitrogen by the process of denitrification. Denitrification is carried by bacteria Pseudomonas and Thiobacillus.
- Very few living organisms can utilise the nitrogen in the form \[{{N}_{2}}\], available abundantly in the air. Only certain prokaryotic species are capable of fixing nitrogen. Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation. The enzyme, nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes. Such microbes are called \[{{N}_{2}}^{{}}\]- fixers.
- The nitrogen-fixing microbes could be free-living or symbiotic. Examples of free-living nitrogen-fixing aerobic microbes are Azotobacter and Beijemickia while Rhodospirillum is anaerobic and Bacillus free-living. In addition, a number of cyanobacteria such as Anabaena and Nostoc are also free-living nitrogen-fixers.
- Several types of symbiotic biological nitrogen fixing associations are known. The most prominent among them is the legume-bacteria relationship. Species of rod-shaped Rhizobium has such relationship with the roots of several legumes such as alfalfa, sweet clover, Sweet pea, lentils, garden pea, broad bean, clover beans, etc. The most common association on roots is as nodules. These nodules are small outgrowths on the roots. The microbe, Frankia, also produces nitrogen-fixing nodules on the roots of nonleguminous plants (e.g., Ainus). Both Rhizobium and Frankia are freeliving in soil, but as symbionts, can fix atmospheric nitrogen.
