Perspective on Wheat Yield and Quality with Reduced Nitrogen Supply

Wheat production has been criticized because of a Janus-faced nature for the use of nitrogen (N) fertilizer: it beneficially produces high quality and/or high yields but also has potential negative environmental impacts. Crop production and N management therefore present a sustainability dilemma. A high N application is obligatory because high wheat protein content is important for baking quality, but simultaneously, high N may cause environmental N pollution via leaching or gaseous N losses. Solutions involve both the optimization of the management of N fertilizer and breeding strategies to improve N use efficiency of the crops. A conventional but relatively easily achievable solution would be improving yield stability as a critical factor to improve N use efficiency. A novel approach may be to manipulate wheat grain N demand by selectively reducing some storage proteins to help reduce N fertilizer inputs. Wheat is an important cereal crop with a high demand for nitrogen (N) fertilizer to enable the grain protein accumulation that is necessary for baking and processing quality. Here, perspectives for the development of improved wheat genotypes with higher yield stability, better grain quality, and improved N use efficiency to lower environmental impacts are discussed. The development of improved wheat genotypes, for example, genotypes that lack storage proteins that do not contribute to baking quality (e.g., by genome editing), in combination with appropriate N fertilizer management to prevent N losses into the environment underpins a novel approach to improving N use efficiency. This approach may be particularly applicable to wheats grown for animal feed, which have lower quality and functionality requirements. Wheat is an important cereal crop with a high demand for nitrogen (N) fertilizer to enable the grain protein accumulation that is necessary for baking and processing quality. Here, perspectives for the development of improved wheat genotypes with higher yield stability, better grain quality, and improved N use efficiency to lower environmental impacts are discussed. The development of improved wheat genotypes, for example, genotypes that lack storage proteins that do not contribute to baking quality (e.g., by genome editing), in combination with appropriate N fertilizer management to prevent N losses into the environment underpins a novel approach to improving N use efficiency. This approach may be particularly applicable to wheats grown for animal feed, which have lower quality and functionality requirements. Wheat is the second most widely grown crop in the world, estimated at 200 million ha. Wheat grain consumption accounts for 19% of the calories in the global human diet [1Aksoy M. Beghin J. Global Agricultural Trade and Developing Countries. World Bank, 2005Google Scholar], while about 40% of wheat produced is fed to poultry and livestock. Wheat grain is rich in carbohydrates and has a higher protein content than other major cereals, such as rice (Oryza sativa), maize (Zea mays), rye (Secale cereale), and millet (Pennisetum glaucum) [2Pomeranz Y. Wheat. American Association of Cereal Chemists, 1988: 514-562Google Scholar]. It also contains substantial amounts of minerals (e.g., Zn, Fe), vitamins, and phytochemicals, making it a good source of nutrition [3Zörb C. et al.Metabolite profiling of wheat grains (Triticum aestivum L.) from organic and conventional agriculture.J. Agric. Food Chem. 2006; 54: 8301-8306Crossref PubMed Scopus (90) Google Scholar, 4Langenkämper G. et al.Nutritional quality of organic and conventional wheat.J. Appl. Bot. Food Qual. 2006; 80: 150-154Google Scholar, 5Shewry P.R. Wheat.J. Exp. Bot. 2009; 60: 1337-1553Crossref Scopus (838) Google Scholar, 6Hawkesford M.J. Genetic variation in traits for nitrogen use efficiency.J. Exp. Bot. 2017; 68: 2627-2632Crossref PubMed Scopus (76) Google Scholar]. Wheat is used globally for the production of bread, pasta, and other bakery products and to a small extent for industrial products. There is an absolute requirement for N for wheat growth, and crop yield and quality depend upon substantial N inputs. Initially, this drives canopy formation required for photosynthesis that, in turn, drives yield. Subsequently, the major sink is the reproductive component, namely, the wheat grain. More than half of industrially fixed N is used by agriculture, worldwide amounting to in excess of 180 Mt/year [7Hawkesford M.J. Reducing the reliance on nitrogen fertilizer for wheat production.J. Cereal Sci. 2014; 59: 276-283Crossref PubMed Scopus (245) Google Scholar]. The production and transport of N fertilizers by the Haber–Bosch process is highly energy intensive and depends on fossil fuels; however, the costs for the fertilizer for many farmers continue to be relatively low, in some cases due to state subsidies. Ready availability and cheap supply encourage overuse, and environmental problems in some agro-ecosystems remain an issue. N losses from the production system occur as nitrate (NO3−) leaching or as the gaseous products of denitrification in soil: futile di-nitrogen (N2); nitrous oxide, a greenhouse gas with 300 times the heat-trapping capacity of carbon dioxide (CO2); and finally ammonia. Except for N2, these gases contribute to pollution and climate change [8Butterbach-Bahl et al.Nitrous oxide emissions from soils: how well do we understand the processes and their controls?.Philos. Trans. R. Soc. Lond. B Biol. Sci. 2013; 386 (20130122)Google Scholar]. Furthermore, N accumulates in the soil. N losses reduce ecosystem productivity and biological diversity and contribute to eutrophication. As a consequence, maximal acceptable NO3− level in the European Union (EU) in freshwater resources is legislated to 50 μg/l. This target, however, is not achieved in some EU areas, especially those with high organic manure inputs; the disposal of N-containing manure from locally concentrated animal farming industry may be a further future threat for these regions. The environmental problems associated with N inputs continue to be an issue in many parts and countries of the world, including the USA and large regions of China. Although, substantial progress has recently been made, wheat remains the least N use efficient (NUE) major crop, whereas NUEs of maize and rice are around 25% higher [9Cui Z. et al.Pursuing sustainable productivity with millions of smallholder farmers.Nature. 2018; 555: 363-366Crossref PubMed Scopus (575) Google Scholar]. Fertilizer regulations to improve N use management with the clear goal to decrease N inputs and losses into the environment differ in each country. However, major concerns from farmers are that yield and grain quality will not be maintained at high levels if N fertilizer use is substantially reduced. This review article provides background information and some figures for what is achievable in future wheat production as well as a concept for an improved strategy, by using new wheat genotypes with higher NUE to maintain or even improve grain yield, yield stability, and quality. Globally, a growing population and per capita increasing income will translate into greater food and protein needs, using a nearly static land area that relies on intensive management of agricultural inputs. Over the past 30 years, there has been a positive correlation between cereal production and N fertilizer use (both from mineral fertilizer and organic recycling fertilizer) in developing countries. Synthetic N fertilizer use worldwide was at only 9.2 Mt N in 1960; it increased to 80.4 Mt N in 1995 and since then has steadily increased to 108 Mt N in 2015 [10Food and Agriculture Organization of the United Nations (2018) FAOSTAT, http://www.fao.org/faostat/en/Google Scholar]. Global analysis identified that in the past five decades the N use efficiency in many countries first decreased and then increased with economic growth. Despite many county-specific socio-economies and N pollution avoidance policies, the total N use efficiency, and especially that of wheat systems, remains relatively low [11Zhang X. et al.Managing nitrogen for sustainable development.Nature. 2015; 528: 51-59Crossref PubMed Scopus (1224) Google Scholar]. Overall, crop NUE on a worldwide scale is 47% [12Lassaletta L. et al.50 year trends in nitrogen use efficiency of world cropping systems: the relationship between yield and nitrogen input to cropland.Environ. Res. Lett. 2014; 9 (105011)Crossref Scopus (638) Google Scholar] [here defined as (total grain N removed – N coming from soil]/fertilizer N applied)] and for some decades was substantially lower for cereals (33%) [12Lassaletta L. et al.50 year trends in nitrogen use efficiency of world cropping systems: the relationship between yield and nitrogen input to cropland.Environ. Res. Lett. 2014; 9 (105011)Crossref Scopus (638) Google Scholar, 13Raun W.R. Johnson G.V. Improving nitrogen use efficiency for cereal production.Agron. J. 1999; 91: 357-363Crossref Scopus (1272) Google Scholar]. Wheat has three main phases of growth with considerable N demands (Figure 1). After sowing, the approximate 6 mg of total protein reserves of the kernel is sufficient to maintain the germination and growth of the seedling until the first leaf emerges. Further N has to be acquired by the root system, but at this stage the root is very small. Thus, additional fertilizer might be best applied directly as a small leaching-resistant ammonium placement below the seed row, but only if there is insufficient N from mineralization available. Despite global wheat NUE being apparently poor, root N uptake of individual wheat plants, mostly in the form of NO3−, ammonium, or even urea [14Yang H. et al.High and low affinity urea root uptake: involvement of NIP5; 1.Plant Cell Physiol. 2015; 56: 1588-1597Crossref PubMed Scopus (23) Google Scholar], is generally very efficient. Until now, there is not a single example that agronomic NUE of the best-performing crop varieties (i.e., those that are grown by farmers) was further improved by genetic targeting of uptake systems or primary N assimilation. Breeding progress over several decades in the UK, Argentina, and Italy did not increase the N uptake per unit root length and reduced root length density [15Aziz M.M. et al.Five decades of selection for yield reduced root length density and increased nitrogen uptake per unit root length in Australian wheat varieties.Plant Soil. 2017; 413: 181-192Crossref Scopus (89) Google Scholar]. However, five decades of selection for yield reduced root length density and increased N uptake per unit root length in Australian wheat varieties. For wheat to obtain high yield and quality in a humid Northern Hemisphere, a first rate of N fertilizer application (up to 60 kg N ha−1) is considered necessary at the end of winter, around growth stage 31 (ear at 1 cm; [16Zadoks J.C. et al.A decimal code for the growth stages of cereals.Weed Res. 1974; 14: 415-421Crossref Scopus (7463) Google Scholar]), before the second leaf emerges (Figure 1). A second application (up to 60 kg N ha−1) is at tillering. Within that, the N concentration in the tissue is responsible for the formation of the numbers of tillers per plant. N-accumulating genotypes that later translocate N (as a reserve for later grain filling) to the grain appear highly efficient in low N conditions, but the NUE in well fertilized conditions remains poor [17Bogard M. et al.Deviation from the grain protein concentration–grain yield negative relationship is highly correlated to post-anthesis N uptake in winter wheat.J. Exp. Bot. 2010; 61: 4303-4312Crossref PubMed Scopus (206) Google Scholar]. The third application of N fertilizer application is before growth stage 37 (flag leaf just visible), and the aim is to promote protein buildup in the ears [18Pechanek U. et al.Effect of nitrogen fertilization on quantity of flour protein components, dough properties, and breadmaking quality of wheat.Cereal Chem. J. 1997; 74: 800-805Crossref Scopus (70) Google Scholar, 19Barneix A.J. Physiology and biochemistry of source-regulated protein accumulation in the wheat grain.J. Plant Physiol. 2007; 164: 581-590Crossref PubMed Scopus (144) Google Scholar]. Currently, in modern wheat varieties, the grain protein concentration is required to be above 12% dry matter, which means that amino acids must be synthesized in high amounts in vegetative tissues and transported to the developing grain, where storage proteins are formed (Figure 1). This process is only moderately influenced by late application of high amounts of N fertilizer (up to 150 kg N ha−1), as root activity declines during maturation and may thus be responsible for the environmental problem of N losses. Excessive use of fertilizer N with total average N application rates up to >500 kg N ha−1 for winter wheat is getting rare, even in China [20Cui Z. et al.Pursuing sustainable productivity with millions of smallholder farmers.Nature. 2018; 555: 363-366Crossref PubMed Scopus (10) Google Scholar]. Such high N rates greatly will inevitably lead to large losses of N [21Liu X. et al.Nitrogen dynamics and budgets in a winter wheat–maize cropping system in the North China Plain.Field Crop Res. 2003; 83: 111-124Crossref Scopus (311) Google Scholar, 22Liu X. et al.Enhanced nitrogen deposition over China.Nature. 2013; 494: 459-462Crossref PubMed Scopus (1982) Google Scholar]. However, if N rates are massively reduced, the wheat plant cannot exploit the genetic potential to build up as much as possible protein during kernel development [23Yu X. et al.Novel insights into the effect of nitrogen on storage protein biosynthesis and protein body development in wheat caryopsis.J. Exp. Bot. 2017; 68: 2259-2274Crossref PubMed Scopus (26) Google Scholar]. However, recent farm trials in southern Germany showed that single N applications of various fertilizer forms after tillering gave the same high yield and grain protein concentrations, without increased risk of NO3− leaching, as split applications, probably because of large soil stocks due to substantial N depositions over decades [24Schulz R. et al.Is it necessary to split nitrogen fertilization for winter wheat? On-farm research on Luvisols in south-west Germany.J. Agric. Sci. 2015; 153: 575-587Crossref PubMed Scopus (34) Google Scholar]. The flag leaf is mainly important for N assimilation and serves as a main source for N metabolites such as amino acids that are subsequently transported into the developing kernels [25Barneix A. Guitman M. Leaf regulation of the nitrogen concentration in the grain of wheat plants.J. Exp. Bot. 1993; 44: 1607-1612Crossref Scopus (35) Google Scholar]. The N availability in the soil solution surrounding the root at this developmental phase is crucial for exploiting the genetic potential for protein buildup in the kernels. In organic management systems, the high availability of N at this specific period of growth (anthesis) is not easily manageable. Although it may not be sufficient to ensure the target protein concentration, soil organic N mineralization usually reaches a peak during the grain filling period because of higher temperatures compared with the vegetative growth phase (Figure 1). Therefore, the protein concentration in grains of organic farming systems, in general, is some percent lower (up to 40%) compared to that of conventional agriculture [3Zörb C. et al.Metabolite profiling of wheat grains (Triticum aestivum L.) from organic and conventional agriculture.J. Agric. Food Chem. 2006; 54: 8301-8306Crossref PubMed Scopus (90) Google Scholar, 4Langenkämper G. et al.Nutritional quality of organic and conventional wheat.J. Appl. Bot. Food Qual. 2006; 80: 150-154Google Scholar]. Thus, organic agriculture is a way to produce wheat grain with a lower N loss but that goes along with the handicap of 20–40% lower yield and the risk of lower grain protein concentration [3Zörb C. et al.Metabolite profiling of wheat grains (Triticum aestivum L.) from organic and conventional agriculture.J. Agric. Food Chem. 2006; 54: 8301-8306Crossref PubMed Scopus (90) Google Scholar]. Therefore, protein quantity in grains may differ in both management systems. Moreover, protein quality varies due to the different management systems, even when the same wheat genotypes are used [3Zörb C. et al.Metabolite profiling of wheat grains (Triticum aestivum L.) from organic and conventional agriculture.J. Agric. Food Chem. 2006; 54: 8301-8306Crossref PubMed Scopus (90) Google Scholar, 26Hellemans T. et al.Impact of crop husbandry practices and environmental conditions on wheat composition and quality: a review.J. Agric. Food Chem. 2018; 66: 2491-2509Crossref PubMed Scopus (24) Google Scholar]. Therefore, prices for these products must be 20–40% higher compared to conventional wheat grain to be profitable; however, price is determined by what people are willing to pay. Wheat is a polyploid, combining three genomes from grasses: an Agilops species that brought the BB genome, Triticum uratu that brought the AA genome, and Triticum tauschii that brought the DD genome and has been cultivated for 10 000 years. The AABBDD hexaploid genome mix of our modern wheat varieties includes bread wheat, as well as spelt [5Shewry P.R. Wheat.J. Exp. Bot. 2009; 60: 1337-1553Crossref Scopus (838) Google Scholar, 27Nesbitt M. Where was einkorn wheat domesticated?.Trends Plant Sci. 1998; 3: 1360-1385Google Scholar]. Another widely used wheat form is Triticum durum with an AABB genome, which is useful for hard wheat products such as pasta and other products around the Mediterranean basin. Modern wheat varieties may be classified as winter wheat. Winter wheat is sown in autumn and is relatively frost resistant. Spring wheat, which is sown in spring, usually has lower yields and lower protein concentrations than winter wheat. Winter wheat can be used for bread making. There are some hundreds of modern wheat varieties that might be used in a certain country, be suitable in a special climate, or be suitable for a different soil type or for specific products. Apart from yield, high resistance against fungal diseases and high grain protein concentration are major targets of modern wheat breeding. While wheat hybrids (compared to lines) do not appear to have the large yield advantage from heterosis that is found in maize, yield stability and potentially quality traits benefit from crossing lines of different heterotic groups [28Zhao Y. et al.Genome-based establishment of a high-yielding heterotic pattern for hybrid wheat breeding.Proc. Natl. Acad. Sci. U. S. A. 2015; 112: 15624-15629Crossref PubMed Scopus (128) Google Scholar, 29Mühleisen J. et al.Yield stability of hybrids versus lines in wheat, barley, and triticale.Theor. Appl. Genet. 2014; 127: 309-316Crossref PubMed Scopus (113) Google Scholar]. Varieties with higher yield stability may provide an overall benefit with respect to N losses, because farmers project and apply the seasonal fertilizer requirement before knowing whether weather conditions allow the genotype to fully retrieve its yield potential. Thus, stabile higher yielding varieties prevent excess losses in unfavorable years. In northern European countries such as Germany and the UK, wheat grains are rated according to protein concentration, and growers are paid according to the grain protein concentration. There are mills and bakeries that require a minimum of 12.8% or even higher protein concentration in wheat flour. This protein concentration requires high fertilizer rates after anthesis, for high buildup of storage protein. Despite large efforts by breeders, the negative relationship of yield with grain protein concentration is difficult to break (Figure 2A) [30Mosleth E.F. et al.A novel approach to identify genes that determine grain protein deviation in cereals.Plant Biotechnol. J. 2015; 13: 625-635Crossref PubMed Scopus (21) Google Scholar], although grain quality of modern varieties has increased by enhancing storage protein concentration (measured by near infrared spectroscopy-N content) in the grain (Figure 2B). Because of increased N fertilizer and genotypic improvement, the protein concentration in grains, on average, in German wheat genotypes rose from about 7–8% crude protein in the 1960s to 12–16% in modern genotypes (Figure 2B) [31Laidig F. et al.Evaluation of breeding progress, genetic and environmental variation and correlation of winter wheat quality traits in German official variety trials during 1983 to 2014.Theor. Appl. Genet. 2017; 130: 223-245Crossref PubMed Scopus (108) Google Scholar]. There is a limitation for increasing N in grain further because of the inverse relation of yield and protein concentration, which leads to high-yielding genotypes on the one hand and high-quality genotypes, as defined by high protein content, on the other hand (Figure 2). Although, some authors have suggested that grain protein concentration may be energy limited [32Munier-Jolain N. Salnon C. Are the carbon costs of seed production related to the quantitative and qualitative performance? An appraisal for legumes and other crops.Plant Cell Environ. 2005; 28: 1388-1392Crossref Scopus (57) Google Scholar], there is no clear indication that this is true for wheat. Therefore, farmers have to decide which market to target: the bread-making market with high quality or the animal feed market with high yield. If a farmer decides to produce bread wheat, he or she needs to use varieties with high baking potential, which in the past decades equated with high raw protein content. However, some new varieties with a high raw protein content (13–16%) performed equally (baking volume) compared to other varieties with a 1–2% lesser protein content (Figure 2B) [33Obenauf, U. (2009) Qualität neu bewerten! DLG-Mitteilungen 6, 22–24Google Scholar, 34Seling S. Bedeutung des Proteingehalts von Backweizen aus Sicht der Wissenschaft.Getreidetechnologie. 2010; 64: 103-110Google Scholar]. It may be concluded that (i) the strategy of increasing raw protein content to achieve higher baking quality is exhausted and (ii) farmers produce high raw protein by high N application, which inevitably results in high environmental losses, without any increase in grain quality. This general practice is a waste of N. The solution is to change the premium payments systems in such countries (Germany, UK) towards a system that quantifies protein fractions that contribute to grain quality (discussed in the next section), rather than just raw protein concentration. The testing method has to be fast, inexpensive, and sensitive and may involve 'lab-on-a-chip' [35Rhazi L. et al.High throughput microchip based separation and quantitation of high-molecular-weight glutenin subunits.J. Cereal Sci. 2009; 49: 272-277Crossref Scopus (23) Google Scholar] or asymmetric flow field-flow fractionation methods [36Podzimek S. Asymmetric Flow Field Flow Fractionation. John Wiley & Sons, 2012Crossref Google Scholar] to quantify the amount of gluten macro polymers, but suitable rapid and cheap spectroscopic methods should also be developed. Transcriptome studies have shown that more than 30 000 genes are expressed in the developing wheat grain [37Wan Y. et al.Transcriptome analysis of grain development in hexaploid wheat.BMC Genomics. 2008; 9: 121Crossref PubMed Scopus (172) Google Scholar], while proteomic analysis of mature grain has identified about 1125 individual components [38Skylas D.J. et al.Proteome approach to the characterization of protein composition in the developing and mature wheat grain endosperm.J. Cereal Sci. 2000; 32: 169-188Crossref Scopus (116) Google Scholar]. However, many of these components are present in small amounts and have little or no impact on the use of the grain. One protein fraction, the prolamin storage proteins, correspond to the gluten proteins and are dominant in terms of amount and impact [5Shewry P.R. Wheat.J. Exp. Bot. 2009; 60: 1337-1553Crossref Scopus (838) Google Scholar] (Figure 3). The precise number of individual gluten protein components has not been determined; together, they have been estimated to account for about 80% of the total grain protein in European wheats [39Seilmeier W. Separation and quantitative determination of high-molecular-weight subunits of glutenin from different wheat varieties and genetic variants of the variety Sicco.Eur. Food Res. Technol. 1991; 192: 124-129Google Scholar]. Gliadin proteins contribute mainly to the viscosity of the dough, whereas glutenin proteins contribute with the elasticity of the dough (Figure 3). The existence or absence of some kinds of allelic variants of high molecular weight glutenin subunits (HMW-GS) is correlated with quality of baking bread [40Wieser H. Zimmermann G. Importance of amounts and proportions of high molecular weight subunits of glutenin for wheat quality.Eur. Food Res. 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