Drought stress and flood result in the generation and accumulation of active oxygen species, the peroxidation of membrane lipids, and reduction of nitrogen metabolism, photosynthesis, growth, and development, causing a significant decline in the qualitative and quantitative production. The water availability influences the different component of NUE and photosynthetic system and its connections. The goal of this chapter is to summarize the effect of water supply to the nitrogen and carbon metabolisms. Knowing about the value of nitrogen use efficiency and photosynthetic parameters is really a useful essential for selecting and growing the best genotypes. But what will happen with these two crucial characteristics of plants, if the environment for growing is not ideal?.
The most widely discussed projections—World Population Prospects every 2 years—are those published by the United Nations from 1951. From this year they have published knowledge about the world population that increased more than 400% over the twentieth century . Expanding populations, income growth, and urbanization have brought about quantitative and qualitative changes in the demand for food. Agriculture faces multiple challenges: it has to produce more food to feed a growing population and more feedstocks for a potentially huge bioenergy market, adopt more efficient and sustainable production methods, and adapt to climate change. Although all of the challenges are more or less hang together, thus if we will be able to find a good point to step in, it can mean a medicine for all function. Climatic change is the middle of this complex problem; it is the reason and the solution as well. According to the most recent assessment report of the Intergovernmental Panel on Climate Change (IPCC), published in 2014, levels of anthropogenic emissions of greenhouse gases are now at their highest in history . Agricultural production and its effect on land use are major sources of these emissions, by sharing methane and nitrous oxide gases. As more greenhouse gas emissions are released into the air, causing air temperatures to increase, more moisture evaporates from land and water bodies. Warmer temperatures also increase evaporation and evapotranspiration in plants and soils, which affects plant life and can reduce rainfall even more.
Plant growth and productivity are adversely affected by water deficiency and/or drought. Moreover, they are not able to solve their serious problem in a short way. Responses of plants to water stress may be assigned as either injurious change or tolerance index. Short-term acclimatization skills are not always enough mainly if any other loads are also presented. Therefore the development of plants with increased survivability and growth during water stress is a major objective in the breeding crops. All of the skills and traits of what the different plant genotypes are able to use need to be known to cope with water deficiency as single or under multiplied stress conditions.
2. Plant response to drought stress
Drought stress is one of the most common abiotic stresses for terrestrial plants [3, 4]. Drought adaptability of plants was defined as comprehensive capacity for adaptation to the drought stress and the re-watering cycle. Drought resistance and recovery determine drought adaptation of plants. Plants can increase the drought resistance through three strategies, namely, “drought escape,” “drought avoidance,” and “drought tolerance” (Figure 1). The “drought escape” strategy plants reduce life span and induce vegetative dormancy to escape severe drought stress . Drought avoidance includes increasing water uptake ability and water use efficiency: stomatal closure, root systems, high capacity for water transport from roots to leaves, and high leaf mass per leaf area . Behind the strategy of drought tolerance, the plant cells improve osmotic adjustment ability, increase cell wall elasticity to maintain tissue turgidity, elevate antioxidant metabolism, and enhance the resistance to xylem cavitation .
It is possible to categorize plant responses to drought stress in accordance with the organizational level of study: from whole plant to molecular level. At molecular level: epigenomics, which affect DNA activity without modifying the gene sequence; transcriptomics, which are changes in gene expression; proteomics, referring to changes in proteins; and finally metabolomics, which are changes in metabolites . Epigenetic mechanisms regulate chromatin structure, gene expression, transposon mobility, and DNA recombination . Several authors have reported the differential regulation of genes encoding epigenetic regulators [11, 12, 13] as well as local chromatin and DNA methylation changes in response to a variety of abiotic stresses [14, 15, 16] including drought [17, 18, 19]. Recent studies had been performed to investigate drought responses in plants using transcriptomic approaches [20, 21]. The differentially expressed transcripts help for identifying the key genes in stress response and performing functional analysis to reveal their roles in stress adaptation in plants. High-throughput proteomics has proved to be a powerful tool for the comprehensive identification of drought-responsive proteins in plants . In previous investigations, more than 2200 drought-responsive protein species have been identified in leaves ; these drought-responsive protein species are defined as 440 unique proteins on the basis of their protein sequence homology and functional domain similarity. Further progress in plant performance under stresses is expected by combining marker-assisted breeding with metabolite markers. The most dramatic effect on the metabolite composition was experienced in leaves compared with other organs . Drought stress evoked the accumulation of many amino acids (glycine, serine) including isoleucine, valine, threonine, and 4-aminobutanoate, which has been reported in both field and greenhouse experiments in many plant species .
Plants can adapt to water deficiency by a wide range of alterations in their morphology, anatomy, and physiology that have been the focus of many studies [26, 27]. Plant’s strategies on the whole plant level can lead to stomatal closure, reductions in photosynthesis and transpiration, growth inhibition, antioxidant production, and changes in hormonal composition [28, 29, 30]. Plants have evolved several strategies to cope with drought stress, including drought escape via a short life cycle or developmental plasticity, drought avoidance via enhanced water uptake and reduced water loss, as well as drought tolerance via osmotic adjustment, antioxidant capacity, and desiccation tolerance . Plant responses to drought stress also vary at different growth stages of the crop . The decrease in yield varies from 13 to 94% in the investigated crops that were under drought stress .
3. Interaction between C and N metabolism
Nitrogen is an essential macronutrient for plants, and it can affect many aspects of plant growth and metabolic pathways [34, 35, 36]. Nitrate is a primary nitrogen source for photosynthetic organisms. The assimilation of nitrate-N into amino group of amino acids contains at least three main steps: nitrate uptake, reduction of nitrate to ammonium, and incorporation of ammonium to carbon skeleton. One of the main connections is that all assimilatory power utilized by plant metabolism originates from photosynthesis. In plant leaves nitrate assimilation is a direct photosynthetic process that increases the capacity of the photosynthetic apparatus for non-cycling electron flow, overcoming the limitation imposed by CO2 fixation through the Calvin cycle . Nitrogen assimilation has an impact on the efficiency of CO2 fixation and the distribution of just fixed carbon among metabolite fractions. Under limiting light intensity condition, there is a strong competition for reducing equivalents between CO2 and nitrate assimilation . When the available nitrogen source is ammonium, there is no reduction in CO2 fixation under low light intensity; moreover ammonium has a positive effect on it . Photosynthetic reactions are involved in the synthesis, regulation, and maintenance of the enzymes of nitrate assimilation pathway. Photorespiratory nitrogen metabolism is one of the important aspects of the interactions of carbon and nitrogen.
High carbohydrate content of grain (in maize, wheat) versus low nitrogen content indicates the crucial role of photosynthesis in attaining maximum yield. With the increased use of fertilizer N being closely associated with enhanced crop yields, it seems reasonable that both carbon and nitrogen metabolisms should be considered when attempting to identify factors that limit productivity .
4. Water supply and nitrogen nutrition
Water and nitrogen are the most limiting factors in agricultural production in most parts of the world, which are able to determine and influence the carbon metabolism. Nitrogen is a vital structural component of proteins, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), nucleic acids, chlorophylls, as well as some hormones, and nitrogen fertilization is an essential agronomic management practice to enhance the crop productivity . Primary carbon metabolism is dependent on nitrogen assimilation, because much of the nitrogen pool of the plant is invested in the proteins (structural and enzymes) and chlorophyll of the photosynthetic apparatus. Leaf chlorophyll provides the platform for photosynthetic system , whereas Rubisco is the key enzyme involved in the process of photosynthesis . A major part of plant nitrogen is stored in the enzymes participating in the photosynthesis especially Rubisco, which is a key source of N recycling . The limitations in the photosynthetic process as a consequence of intensified drought stress not only impose direct drought stress-induced damages to plants but also result in light-induced oxidative stress. The lower efficiency of photosynthetic system under drought stress leads to the imbalance between absorbed light energy and its utilization in the carbon assimilation process, which in turn spares more electrons triggering the production of reactive oxygen species. Conversely, nitrogen assimilation requires a continuous supply of energy and carbon skeletons. Interconnected metabolic processes make the effect of water deficit on plant nitrogen nutrition status difficult to predict. An adequate assessment of the impacts of drought stress under different nitrogen levels on the physiological activities and yield attributes can provide the valuable insights for wheat cultivation under drought stress . Efficient nitrogen nutrition has been reported to have the potential to alleviate the drought stress damages by maintaining metabolic activities even at low tissue water potential . Abid et al.  published that higher N nutrition contributed to drought tolerance in wheat by maintaining higher photosynthetic activities and antioxidative defense system during vegetative growth periods. N-fertilized wheat plants responded more rapidly to increasing drought stress by closing stomata and reducing net photosynthesis .
The nitrogen form and the levels of nitrogen available affect root water uptake [47, 48, 49]. Synergetic transport has been found between nitrate and water uptake in roots. In plants supplied with nitrogen in both ammonium and nitrate forms, the high nitrogen supply also increased root hydraulic conductance in plants [50, 51]. The radial water transport also can be influenced by nitrogen nutrition. The water flow in apoplastic pathway is blocked by apoplastic barriers at the endodermis, and water flow continues through the symplastic pathway. The deposition of lignin and suberin in the endodermis may affect root hydraulic conductance; Ranathunge et al.  demonstrated that high ammonium supply increased this deposition and thus decrease root hydraulic conductance. Drought stress may induce the alkalinization of leaf apoplast, in tomato  and hop , and especially in plants supplied with high nitrate .
Nitrogen is an essential nutrient for plants, and it can affect dry matter production by influencing photosynthetic process in several direct and indirect ways. Drought is also a crucial abiotic factor in terms of photosynthesis and also means control for the nutrient nutrition of plants. The effect of water deficit on nitrogen nutrition has been the subject of considerable research on several plants [7, 56, 57]. The interconnected metabolic processes make it difficult to predict the effect of water deficit on plant nitrogen nutrition status and the exact regulation point in the carbon metabolism. In the future we need to know more skills and traits what the different plant genotypes are able to use to cope with water deficiency as single or under multiplied stress conditions.
The work was financed by the Higher Education Institutional Excellence Programme (20428-3/2018/FEKUTSTRAT) of the Ministry of Human Capacities in Hungary, within the framework of the 4th thematic programme of the University of Debrecen. The authors thank the support of the “Kiváló malomipari paraméterekkel rendelkező adaptív őszi búza vonalak előállítása” (AGR_PIAC_13-1-2013-0002) grant and the Arid Land Research Center of the Faculty of Agricultural and Food Sciences and Environmental Management at University of Debrecen.
Conflict of interest
We have no “conflict of interest” to declare.