Preface Majority of agricultural soils are deficient in nitrogen. In India, approximately 77 per cent of the soils have been classified as low and 23 percent as medium. These soils are unable to maintain an adequate supply of nitrogen without fertilizers to match the crop requirements. Well-planned and concerted efforts have made ‘green revolution’ realizable by increasing grain production at a rate faster than the increase in population. Three major inputs that have made it possible are: seeds of high yielding varieties of crops, fertilizers and increased irrigation facilities. Nitrogen assimilation is the key function in plant cells, and this mineral has played a major role in the success story of green revolution. But, uneven, excessive and in-efficient use of N fertilizers by the major cereals has led to the pollution of groundwater and eutrophication of the surface water posing a severe problem for human health as well as ecosystem. Enormous losses in the form of emission of NOx also lead to changes in nitrogen-carbon balance. Pollution caused by fertilizers (and manures) is being referred to as a ‘chemical time bomb’ but, to sustain food production for the ever increasing population their demand increases by several fold every decade. The consumption of total fertilizer nutrients was 22.4 m tonnes during 2006-07, and by the end of 2011-12 the projected demand is 28.76 m tonnes of urea alone. The major challenge facing the agriculture therefore, is to increase the food production with minimum adverse impacts on the environment. At present yield is balanced with constituents towards environment and sustainable agriculture and further improvements in yield can be through selection as well as gene technology. Research on uptake of nitrogen, its acquisition, assimilation and storage is therefore, essential.

Abstract Nitrate uptake from the soil is the first step in nitrate acquisition and utilization by plants. Nitrate uptake and its release into cells is mediated by nitrate transporter(s) located into the plasma membrane of the root. Three transporters have been identified by kinetic measurements in plant roots. These are constitutive high affinity nitrate transporter (cHATS), inducible high affinity nitrate transporter (iHATS) and low affinity nitrate transporter (LATS). Two of these display saturable kinetics; a low capacity constitutive system and a high capacity inducible system. In addition, a non-saturating low affinity, high capacity system becomes apparent only at higher external NO-3 concentration. Genes that encode representatives of each class of transport system have been identified and fall into two families: NRT1 and NRT2. Members of these families are induced in response to nitrate in the environment and are regulated by internal signals including nitrogen metabolites and shoot demand for nitrogen. This chapter deals with the physiology and molecular biology of nitrate transporters in higher plants.

Abstract Nitrate reductase (NR) catalyzes the reduction of nitrate to nitrite, which is the first step in the nitrate assimilation pathway, but can also reduce nitrite to nitric oxide (NO), an important signaling molecule that is thought to mediate a wide array of developmental and physiological processes in plants. Accordingly, the amount of NR protein and its catalytic activity are closely controlled in the cell. This chapter focuses on the posttranslational regulation of NR activity, which has been known for some time to involve reversible protein phosphorylation. The hinge 1 region of NR contains the serine-534 regulatory site, which when phosphorylated can bind a 14-3-3 protein resulting in inhibition of NR activity. This fundamental mechanism underlies the rapid modulation of NR activity in leaves in response to light/dark transitions, and the activation of NR that occurs in response to hypoxia (e.g., as with soil flooding). Recent results suggest that the basis for hypoxia-induced NR activation, which is thought to be essential for plant survival, may involve reactive oxygen species (ROS), such as H2O2, that are produced in response to many plant stressors including hypoxia.

Abstract Nitrogen is the most important element in terms of nutrition for plants, nitrate being the most preferred form. Nitrate not only acts as nutritional element but also as a signal to modulate metabolism and plant architecture. Nitrate sensing mostly happens in the root tip through a hitherto unknown mechanism, but its response can be demonstrated on an organism-wide scale. Functional genomic studies revealed over a thousand nitrate-responsive genes, involved not only in N and C metabolism, but also in various other physio-logical processes. Identification of nitrate response element/s common to all these genes could pave the way to unravel the mechanism of nitrate signaling, but findings in this direction have remained inconclusive so far. Several trans-acting factors have been implicated in N signaling and response, but none of these have been convincingly demonstrated to be specific to this response. Nitrate uptake, which is sometimes associated with nitrate sensing, is also highly regulated process and involves multiple transport systems like HATS, LATS and dual affinity transporters.

Abstract Nitrogen is the most essential nutrient for all plants and is also the major limiting factor in plant productivity. High yielding varieties of all crops are responsive to nitrogen but its ever-increasing use has shown detrimental impact on the environment along with extremely low use efficiency. Nitrogen use efficiency (NUE) in crop plants depends on various internal and external factors, which are dealt in detail. There has been a significant interest in genetic engineering of the crops to improve NUE. The use of biotechnological interventions by manipulating genes of the nitrogen utilization pathway to improve NUE has not been very successful. But transgenics/mutants with modified capacities for nitrate uptake, assimilation and remobilization have enhanced our understanding of the genetic control of NUE. In both cellular, and at whole plant level the mechanisms involved in N remobilization from the senescing organs towards the grain have recently gained importance but their understanding is still preliminary. Recent evidences have shown that grain filling during later stages of crop growth is supported by N recycling.

Abstract The abiotic stresses viz. drought, high temperature, water stress, salinity, heavy metal contaminations etc. cause variable effect on nitrogen (N) mineralization, its uptake and transport, assimilation and remobilization, and subsequently the growth and productivity of plants. The affect is generally negative, however, the magnitude of decrease in various processes is dependent on genotype, age of the plant, tissue under investigation, type and extent of the stress and other agro-climatic conditions. Biological nitrogen fixation and nitrate assimilation appear to be more sensitive than the ammonium assimilation possibly because re-assimilation of ammonia released in the tissue due to catabolism of N molecules is essential for the survival of plants. In addition, synthesis of stress metabolites (proline, glycine betaine and poly amines) is directly dependent on the availability of primary amino acid, glutamic acid. There are few well coordinated and detailed studies, determining the key factors involved in regulating NUE under abiotic stresses.

Abstract Intensification of agriculture during the last few decades has resulted in considerable depletion of the productivity of Indian soils and the balance sheet of potassium (K) is highly negative due to very low level of K fertilization. Although the need for balanced fertilization is known for many years, it is not always observed in practice by farmers. Since the effects of nitrogen are easily seen and crops look more vigorous and yield better, excessive use of nitrogenous fertilizers, accompanied by an inadequate supply of other nutrients, is considered by farmers as a reasonable insurance against yield losses. Often, the lack of K restricts nitrogen uptake and utilization, and hence plant growth. When K is limiting, nitrogen fertilizers are used inefficiently by the plants. This nutrient imbalance is thus closely associated with low nitrogen-use-efficiency (NUE) of crop plants, which is detrimental to crop production and environment – because of the consequences of N unused by the crop.

Abstract During the past five decades, the role of nitrogen (N) fertilizer in augmenting foodgrain production has widely been recognized the world over. In Indian soils, N deficiency is recognized as a universal problem and its consumption has increased by more than 300 times during the last 60 years.

Abstract Increases in atmospheric concentrations of CO2 are likely to have major physiological consequences for plants, due to the major effects of atmospheric CO2 on photosynthetic rates and the centrality of photosynthesis to plant metabolism. We review what is known about the effects of growth at elevated levels of CO2 on the protein and N concentrations of plants in general, and food crops in particular. Growth at elevated CO2 typically reduces plant tissue N concentrations by 10-15%. The physiological mechanisms behind this decrease in tissue protein and N are likely complex, and include both dilution from increased tissue concentrations of non-structural carbohydrates and decreased root uptake of N.

Abstract Nitrogen is the most limiting nutrient controlling production of all agricultural systems. Application of N fertilizer has increased food production tremendously all over the world. However, it also contributes towards global warming and ozone layer depletion through emission of nitrous oxide, a greenhouse gas (GHG). Therefore, there is a need to assess the possibilities of N management for climate change abatement and food security enhancement, while minimizing environmental and human health impacts. In this chapter, role of gaseous-N emission on N use and various mitigation options for Indian agriculture have been discussed. For the base year 2007, the direct and indirect N2O emissions from Indian agricultural soils was estimated to be 136.29 Gg (35.3 Tg CO2 equivalent) and 30.61 Gg (5.8 Tg CO2 equivalent), respectively.