Elsevier

Field Crops Research

Volume 102, Issue 1, 30 April 2007, Pages 22-32
Field Crops Research

In winter wheat (Triticum aestivum L.), post-anthesis nitrogen uptake and remobilisation to the grain correlates with agronomic traits and nitrogen physiological markers

https://doi.org/10.1016/j.fcr.2007.01.002Get rights and content

Abstract

In wheat, nitrogen (N) uptake and remobilisation after flowering contributes largely, in Northern countries, to grain yield and grain protein content. The aim of our study was first to estimate the proportion of N taken up and remobilised to the grain as well as their relative efficiency using 15NO3-labelling at flowering. The validity of the technique was assessed in comparison to the N budget calculation method on five winter wheat cultivars grown for 2 years at low and high fertilization input. We estimated that on average 71.2% of grain N originates from remobilisation with significant genotypic differences. Among the five genotypes, significant differences were also found for both N remobilisation efficiency (from 69.8 to 88.8%) and N translocation efficiency (from 89.7 to 93.4%). In parallel, during 1 year, we monitored physiological markers representative of N assimilation and recycling at two sampling dates during the grain filling period. We then examined if there was any relationship between these physiological markers, N absorption and remobilisation estimates and agronomic traits related to yield and grain N content. Nitrate reductase (NR) activity was highly correlated to N absorbed post-flowering and to grain protein content. Glutamine synthetase (GS) activity was even more highly correlated than NR activity to the amount of N remobilised and grain yield. The use of physiological traits such as NR and GS activities as markers of the wheat N status is discussed.

Introduction

To attain maximum yields, modern cereals cultivars require large amounts of fertilizers since the genotypes currently cultivated in developed countries were mostly selected under non-limiting fertilization conditions (Bänziger et al., 1997, Presterl et al., 2003). Among all the fertilizers applied in the field, nitrogen (N) is the most important for plant growth, plant productivity and grain quality (Frink et al., 1999). However, in developed economies, N use efficiency (NUE; defined as grain dry matter per unit of N available from the soil, fertilizer included), is very low and estimated to be approximately 33% (Raun and Johnson, 1999). Thus, in Asia, Europe and Northern America, intensive agricultural practices (Singh, 2005) have led to both higher production costs and a greater risk from environmental hazards such as ground and surface water pollution by nitrate leaching (Mariotti, 1996). Reducing the amount of N fertilizers applied to the field without producing a N deficiency, will be the main challenge faced by breeders in selecting for cereal cultivars that absorb and/or metabolise N more efficiently.

In wheat, grain N content rather than yield components is largely influenced by the amount of N taken up after anthesis and by the amount of remobilised N originating from pre-anthesis uptake since these two sources of N are used for storage protein synthesis (Dupont and Altenbach, 2003). After anthesis, leaves become a source of N. N in leaves is recycled following protein hydrolysis and exported in the form of amino acids to grains (Feller and Fischer, 1994, Masclaux et al., 2000). Sixty to 95% of grain N comes from the remobilisation of N stored in roots and shoots before anthesis (Van Sanford and MacKown, 1986, Palta and Fillery, 1995a, Palta and Fillery, 1995b). A less important fraction of seed N comes from post-flowering N uptake and N translocation to the grain. After flowering, both the size and the N content of the grain can be significantly reduced under N deficient conditions (Dupont and Altenbach, 2003). However, it is still not clear whether it is plant N availability (including the N taken up after anthesis and the remobilised N originating from uptake before anthesis), or storage protein synthesis that limit the determination of grain yield in general and grain protein deposition in particular.

In wheat grown in the field, remobilisation efficiency of the N taken up before anthesis (NRE) was shown to be variable depending on the genotypes examined (Cox et al., 1985, Van Sanford and MacKown, 1987). However, Barbottin et al. (2005) showed that when there are no environmental factors limiting grain filling, the differences in the amount of remobilised N originating from uptake before anthesis were mainly due the capacity of the plant to store N in sink organs until this period. Remobilisation of the N stored before anthesis and N uptake after anthesis are generally estimated by calculating the difference between the amount of total N present at anthesis and the amount of total N present at harvest in the different parts of the plant. However, the results obtained by this method (called apparent remobilisation method) can be subject to large experimental errors due to the necessity to combine data obtained at two different sampling dates. The use of 15N stable isotope labelling is a good alternative that generally allows the estimation of N uptake after anthesis and N remobilisation of pre-anthesis stored N from source organs in a less biased and more precise manner. This was achieved by applying 15N nitrate to maize plants grown in hydroponics (Cliquet et al., 1990) or by infiltrating 15N-urea to the leaves of wheat plants grown the field (Palta and Fillery, 1995a, Palta and Fillery, 1995b). In order to characterize genotypic differences for N remobilisation of the N stored before anthesis and post-anthesis N uptake in wheat, field experiments were conducted on 2 years using post-flowering 15N labelling at high and low levels of N fertilization on five cultivars of winter wheat selected for their different sensitivity to a N stress (Le Gouis et al., 2000). When 15N is applied just after flowering, we showed that both N uptake after flowering and N remobilisation of pre-anthesis stored N can be estimated with a greater accuracy and reproducibility compared to the method measuring the difference between the amount of N present at flowering and the amount of N amount present at maturity. In addition, a critical and statistical analysis of the 15N-labelling technique was performed in comparison with the more traditional N balance method.

To identify physiological traits that may be involved in the control of NUE, we have monitored in parallel the changes in biochemical markers representative of the transition between primary N assimilation during vegetative growth (including nitrate and ammonia assimilation in shoots and roots) and N remobilisation (amino acids released following protein hydrolysis in source organs) during the grain filling period. These biochemical markers including N metabolites (total N, nitrate, ammonium and amino acids) and nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activity have been successfully used both in wheat (Kichey et al., 2006) and in maize (Hirel et al., 2005b) to depict the transition sink organs to source organs in the two crop species.

Correlations between the relative values of the physiological markers for N assimilation and recycling, yield and its components, and the capacity of the plant to absorb N after flowering and remobilise the N accumulated before anthesis were then evaluated. We have then determined whether there was any significant genetic correlation between these three classes of traits under low and high N fertilization conditions. In addition, we have examined the influence of the level of N fertilization on these genetic correlations.

Section snippets

Agronomic studies and 15N-labelling experiments

The winter wheat cultivars Arche, Récital, Renan, Shango and Soissons commonly cultivated in France and differing for their response to N deficiency (Le Gouis et al., 2000) were sown on 16 October 2001 and 15 October 2002 at Estrées-Mons INRA experimental station (Somme, northern France) at a density of 250 seeds m−2.

The soil classified as a deep loam soil (Orthic Luvisol, FAO, 2006) contained an average of 190 g clay kg−1, 730 g silt kg−1, 52 g sand kg−1 and 19 g organic matter kg−1 with a pH of 8.1 and its

Estimation of post-anthesis N uptake and remobilisation of N accumulated before anthesis

The repeatability of the estimations of post-anthesis uptake of N was highly improved in 2002 using 15N-labelling (Table 1). In 2003, the apparent remobilisation method and the 15N-labelling method gave approximately the same results. Both in 2002 and 2003, the estimation of the amount of N accumulated before anthesis and further remobilised to the grain was more repeatable using 15N-labelling (Table 1). On average, both methods gave almost the same mean quantities of N taken up after anthesis

Discussion

The amount of N taken up after anthesis and of N remobilised coming from pre-anthesis uptake during grain filling by the five cultivars was different. Both N absorption and N translocation efficiencies were also variable. The changes in metabolite content and enzyme activities representative of N assimilation and N remobilisation during the grain filling period demonstrated that they were subjected to genotypic variability. By the mean of correlation studies we have examined the relationship

Conclusion

In this study we showed that 15N-labeling performed in the field at flowering is a valuable tool to estimate the genetic variability for N uptake, N assimilation and N recycling in different wheat cultivars. This technique combined with the measurement of physiological traits such as GS and NR activities revealed that both enzymes are potential markers to estimate the proportion of N absorbed or N remobilised invested in grain yield elaboration or grain N content. The best way to help breeders

Acknowledgments

Financial support by the Conseil Régional de Picardie (IBFBio project no. 2001.3) is greatly acknowledged. We thank Olivier Delfosse (INRA Laon, France) for realizing the 15N and total N analyses.

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