ABSTRACT
Nitrogen is the most important element for plant growth and development. In the cell, it is present as constituent of proteins, amino acids, nucleic acids, coenzymes, vitamins, chlorophylls, hormones, and functional groups of enzymes. It is a major part of chlorophyll and the green color of plants. It is a key constituent of nucleic acids that serve as genetic material and regulate vital processes like growth, reproduction, and heredity. Being a principal macronutrient, N has a special place in plant nutrition, and plants require it in large amounts. Though N is the most abundant element in our atmosphere, majority of the plants cannot utilize it directly in the elemental form. An adequate supply of N in the soil is essential to maintain proper growth and yield of crops. The availability of N in the soil is due to N2 xation by symbiotic and nonsymbiotic bacteria; mineralization of organic matter, manure, and animal waste products; or application of inorganic N fertilizers. Though the natural sources can make signicant contributions to soil N levels, with the advent of modern agricultural practices, to
24.1 Introduction .......................................................................................................................... 453 24.2 Nitrogen Sources, Their Uptake, and Assimilation .............................................................. 454
24.2.1 Sources of Nitrogen .................................................................................................. 454 24.2.2 Absorption and Assimilation of Nitrogen................................................................. 456
24.2.2.1 Nitrate Transport Systems.......................................................................... 456 24.2.2.2 Nitrate Transporters ................................................................................... 457 24.2.2.3 Reduction of Nitrate ................................................................................... 457
24.3 Nitrogen Absorption and Assimilation under Different Stresses ......................................... 459 24.3.1 Salinity ...................................................................................................................... 459 24.3.2 Water Stress ..............................................................................................................465 24.3.3 Light ..........................................................................................................................468 24.3.4 Heat ........................................................................................................................... 470 24.3.5 Chilling ..................................................................................................................... 471 24.3.6 Metal Toxicity ........................................................................................................... 472 24.3.7 Ultraviolet B Radiation ............................................................................................. 474
24.4 Concluding Remarks ............................................................................................................ 474 References ...................................................................................................................................... 475
meet the needs of high-yielding varieties of crops, inorganic N fertilizers have become major input to the soil in order to achieve required optimum yields. To meet the increasing food demand for the growing population, keeping pace with high N requirements of the improved varieties of crops, in the near future, excessive uses of different forms of N fertilizers are anticipated.
