Nitrogen in plants

Plants need large amounts of nitrogen, with their dry matter content amounting to 3 to 4% nitrogen. Despite its abundance, atmospheric nitrogen is not available for plant growth. Most plants obtain it exclusively from the soil, and moreover they obtain it in only two inorganic forms: ammonia and nitrate. Although experiments show that plants grow best under the influence of a mixture of ammonia and nitrate, in a proportion that depends on the plant species, most of the nitrogen is ingested in the form of nitrates.

Due to the rapid microbial conversion of ammoniacal to nitrate in soil and the greater mobility of the latter, radicals are more exposed to nitrates than ammoniacals. In humid or acidic ecosystems this may vary. For example, crops grown under sprawling conditions, such as rice, take up the greatest amount of nitrogen in the form of ammoniacal.

The nitrogen ingested by the roots is metabolized and transported to the upper parts of the plant, usually in the form of the amino group (_NH2). In the aboveground part of the plant the nitrogen of the amino group is converted into amino acids and then into proteins. Thus, the intensity of nitrogen metabolism and the rate of protein synthesis control the entry of nitrogen into the different parts of the plant. In general,  Nitrogen in plants is concentrated in the younger sections, with the highest growth rates. When nitrogen uptake by the roots is insufficient, nitrogen from the older leaves moves to nourish the younger organs of the plant. The proteins in these leaves are hydrolyzed into their amino acids (proteolysis) which in turn are redistributed to the growing ends and younger leaves. Proteolysis in older leaves results in a decrease in chlorophyll content and the appearance of yellowness, which is often a symptom of nitrogen deficiency.

In green plant material, protein nitrogen is by far the largest fraction of compounds with nitrogen content, concentrating about 80-85% of total nitrogen. Many plants (e.g. peas, beans, soybeans) are grown specifically for the production of plant proteins for human and animal food. The growth rate and protein content of plants is significantly influenced by the available nitrogen from the soil and fertilizers.

Other parts of nitrogen in plants are nucleic acids, RNA and DNA, which account for about 10% of total nitrogen, and free amino acids along with the rest of the compounds with amino group content that account for 5%.

In plants, nitrogen prevails in functional compounds such as enzymes and less in building blocks such as fibers. Most plants store energy for seedlings in the form of hydrocarbons (starch and fat), but legumes such as peas, lentils and beans accumulate extra protein stores in their seeds. Wild legume plants have their own ecological importance in nitrogen-poor soils because they do not depend on available soil reserves. To prevent nitrogen deficiency in the seedling before it has its own symbiotic system for fixing atmospheric nitrogen, the seed’s protein reserves act as a temporary source of nitrogen.