Deficiency Symptoms of Essential Elements
There are different symptoms for deficiency of different elements. When a deficient mineral is provided to the plant, the symptoms disappear. But if the deficiency continues, it may lead to the death of the plant.
Appearance of deficiency also depends on the mobility of the element in the plant. Some elements are actively mobilized in plants and are exported to young developing tissues. Deficiency of such elements first appears in the older tissues. For example; the deficiency symptoms of nitrogen, potassium and magnesium are first seen in the senescent leaves. This happens because these elements are mobilized to younger leaves.
Some elements are relatively immobile in plants. These elements are not transported out of the mature organs. Deficiency of such elements first appears in younger parts of the plant, e.g. sulphur and calcium.
Some deficiency symptoms in plants are; chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds and inhibition of cell division.
Chlorosis: Loss of chlorophyll is called chlorosis. This results in yellowing of leaves. Chlorosis is caused by the deficiency of N, K, Mg, S, Fe, Mn, Zn and Mo.
Necrosis: Death of tissue; particularly leaf tissue; is called necrosis. Necrosis is caused by the deficiency of Ca, Mg, Cu and K.
Deficiency of N, K, S and Mo causes stunted growth because of inhibition of cell division. Deficiency of N, S and Mo delays flowering.
Toxicity of Micronutrients
If a mineral ion concentration in tissues reaches to a level that it reduces the dry weight of tissues by about 10%, the mineral then becomes toxic. It is difficult to identify the symptoms of toxicity. Sometimes, excess of an element may inhibit the uptake of another element. For example; the symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins. Manganese competes with iron and magnesium for uptake. Manganese also inhibits calcium translocation in shoot apex. Hence, excess of manganese results in deficiency of iron, magnesium and calcium. So, the apparent symptoms of manganese toxicity are in fact the deficiency symptoms of iron, magnesium and calcium.
Mechanism of Absorption of Elements
Absorption of minerals takes place in two main phases. In the first phase, passive absorption takes place through apoplast pathway. In the second phase, absorption takes place through symplast pathway. The first phase involves passive transport (facilitated diffusion), while the second phase involves active transport. After that, minerals are transported through xylem.
Nitrogen is available in limited amount in soil. Plants have to compete with microbes for this form of nitrogen. Hence, nitrogen is a limiting nutrient for plants.
Lightning and ultraviolet radiations provide energy to convert gaseous nitrogen into oxides of nitrogen (NO, NO2 and N2O). Atmospheric nitrogen oxides also come from industrial combustions, forest fires, automobile exhausts and power stations.
Decomposition of organic nitrogen of dead plants and animals leads to the formation of ammonia. This process is called ammonificaiton. Most of this ammonia is converted into nitrate by soil bacteria, while some of the ammonia vaporizes and re-enters the atmosphere.
Conversion of ammonia into nitrate; by soil bacteria takes place in following steps:
Biological Nitrogen Fixation (BNF): In this process, the atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. This can be shown by following equation:
N2 + 6H+ + 6e− → 2NH3
This process is coupled with the hydrolysis of 16 equivalents of ATP. This is also accompanied by the co-formation of one molecule of H2.
In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase or glutamate synthase pathway. Many nitrogen-fixing organisms exist only in anaerobic conditions; because the enzymes responsible for nitrogenase action are highly susceptible to destruction by oxygen.
Symbiotic Biological Fixation of Nitrogen
The plants of the legume family (Fabaceae) are the major contributors towards nitrogen fixation. The root nodules of these plants harbor the Rhizobium bacteria. These bacteria produce nitrogen compounds which help the plant to grow properly. When the plant dies, the fixed nitrogen is released into the soil. Thus nitrogen becomes available for other plants.