The following points highlight the physiological role of microelements in plants. It must always be remembered that a single element may play fundamentally different roles in the metabolic activities of plants.
Some of the microelements present in plants whose physiological role is given here are: (1) Nitrogen (2) Phosphorus (3) Sulphur (4) Magnesium (5) Calcium and (6) Potassium.
1. Nitrogen:
It is the most important constituent of proteins. As protoplasm or the living substance is predominantly proteinaceous, life is impossible without nitrogen. It is also a constituent of many vitamins, auxin and chlorophyll, all of which play fundamental roles in the metabolism of the living cells. Nitrogen also occurs- in many substances, such as plant bases, e.g., purine and pyrimidine, the significance of their presence is fully realised now.
2. Phosphorus:
Like nitrogen, it is a necessary constituent of many important substances, such as nucleoproteins, the phospholipids, high-energy phosphates,-the coenzymes, etc. As high-energy phosphates of sugars they are the main agencies which drive the metabolic cycles of photosynthesis and respiration.
3. Sulphur:
It is a component of many proteins, the amino acids, cysteine, cystine, methionine as also of some vitamins (vitamin B1 and biotin). It is also a constituent of lipoic acid, coenzyme A and other compounds such as the antibiotic penicillin.
It is a constituent of glutathione, the respiratory pigment as well as of mustard oil glycosides, sinigrin and glucobrassicin. The pungent odour of onion and garlic is also due to the presence of a sulphur compound, allyl sulphide.
4. Magnesium:
It is a constituent of chlorophyll molecule; but as it is also essential for non-green plants, this cannot be its only function. We now know that magnesium is one of the constituents of the enzyme pyruvic carboxylase and also a host of other enzymes, particularly kinases. It also helps in the maintenance of the physical structures of subcellular organelles and chromosomes.
5. Calcium:
Calcium occurs as calcium pectate in the middle lamella of adjacent cells. Unlike magnesium, however, it is not indispensable for some fungi and bacteria. But the role of calcium is obviously much more fundamental than as an element of cells.
There is also enough evidence to indicate that protoplasm cannot maintain its living entity in the absence of certain ions and calcium is undoubtedly one of those ions; potassium is another. Calcium is essential for growth including the growth of pollen tubes. It can replace magnesium for the activity of several enzymes.
6. Potassium:
It is the only univalent cation that is generally indispensable for all living organisms. The only concrete hypothesis of the role of potassium in plants is that like calcium the K ions are indispensable for the existence of protoplasm, the living substance.
Although K+ is needed in such large quantities by all plants, so far as it is known, it is not a constituent of any known essential organic substance. An important fact is that K + occurs in plant cells only in the ionic form.
Its chief role seems to be to provide the necessary ionic atmosphere for the protoplasm, though it may also conceivably act as an activator of enzymes. K+ is perhaps a necessary cation for photosynthesis. K+ is necessary as an activator of enzyme ALA-dehydrase in the biosynthesis of chlorophyll.
Rubidium has chemical properties similar to K+ and it has been reported that rubidium could substitute for K+ in the biological processes of a cell. Active absorption of potassium ions by the guard cells of stomata perhaps determine their opening and closing. Apical dominance in several plants appears to be lacking or very weak under potassium deficient conditions of the cells.
Application of Fertilizers:
Although soil contains all essential minerals—both macro- and micro-intensive cultivation of crops results in a removal of such nutrients every year and the reserves of each element get depleted. This has to be replenished by adequate doses of fertilizers— organic or inorganic.
The element required to be added in the largest quantities is N, followed by P and K. Nitrogen is usually applied in the form of (NH4)2 SO4, or urea, CO(NH2)2. Usually NH4+ ions are taken up more avidly than SO4=ions, resulting in the development of acidity in the soil; SO4=ions have a tendency to pick up H+ from water and produce H2SO4.
Soil pH is decreased and this affects the capacity of the roots to take up solutes from the soil solution. Urea is much less harmful in this respect, and that is why it is usually preferred to ammonium sulphate.
It can be taken up directly; another advantage of using urea is that it eucourages microbial activity. NH4 NO3 and NH4HCO3or liquid ammonia are also used in some countries as nitrogenous fertilizers. Acidity is usually checked by liming the soil which increases the pH.
Alkaline conditions are also harmful, since several minerals (phosphates, for example) are precipitated at alkaline pH and are thus rendered unavailable to the plant. Phosphate is usually added as bone meal or superphosphate.
In addition to its important role in plant nutrition, it helps in ion uptake; (NH4)2 SO4 as a fertilizer is more useful when applied along with phosphate than alone. Our soils usually have considerable S and thus application of S is not essential usually.
K, however, is required in large quantities and this is usually applied as muriate of potash. 10 to 30 Kg of P and K per hectare is usually applied depending on the crop grown. Nitrogen is usually applied at 20 to 40 Kg per hectare but 60 or 100 Kg are applied for higher yields.
The high yielding cultivations of wheat and rice require the application of high doses of fertilizers for production of high yields. Balanced fertilizers are now commercially available.
Fertilizers are usually applied in split doses, before sowing, at tillering stage of rice and wheat and just before ear emergence. Adequate irrigation is also important. Nutrients which are absorbed may also be applied in the form of a foliar spray.
Urea is absorbed easily. Some micronutrients may also be applied in the form of a foliar spray, provided they are absorbed. Tracer studies have revealed that all inorganic cations are not translocated equally from the leaves, when applied as a foliar spray; some do not move at all.
Applications of organic manures have several advantages. They are less harmful and do not affect much the pH of the soil. Leaf compost, cow-dung, oil cakes, etc. are usually used by our farmers.
They encourage microbial activity in soil and are decomposed; organic and amino acids are taken up by roots directly. However, the yield produced is usually much less than that obtained with synthetic fertilizers.
The practice of rotation of crops, in which a legume and a non-legume are grown alternately on the same soil, maintains the nitrogen content of the soil at particular level which may support the growth of the non-legume.
After harvesting, the plants are ploughed into the soil for decomposition by micro-organisms. Legumes do not need the application of nitrogenous fertilizers, but P and K have to be applied. They also need Fe and Mo in a little larger quantity than other micronutrients.
Saline soils do not favour the growth of plants, unless such plants are salt tolerant. The halophytes in the mangrove vegetation are adapted to such conditions. Salinity- resistant varieties are now being developed for cultivation in saline conditions. Saline soils can be reclaimed by appropriate chemical treatment but this is an expensive proposition.
Soil is usually eroded by river and sea water as also by non-judicious application of fertilizers. Plants which can grow on sea shore and have extensive root systems are usually used. Ipomea pescaprae and I. bilobata are used as sand-binders. Appropriate chemical treatments are necessary to check the bad effects of the application of excessive fertilizers or industrial chemical wastes.