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Nitrogen Management

Accumulation and Translocation of Nitrogen in Spring Cereal Cultivars Differing in Nitrogen Use Efficiency

^a MTT Agrifood Research Finland, Plant Production Research, FIN-31600 Jokioinen, Finland
^b Kemira GrowHow, Research Center, PO Box 2, FIN-02271 Espoo, Finland

^* Corresponding author (susanna.muurinen{at}mtt.fi)

Received for publication April 6, 2006.

	ABSTRACT

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Since northern European agricultural practices are likely to go toward systems with lower inputs of N fertilizers, it is desirable to develop cultivars with increased yield potential associated with higher nitrogen use efficiency (NUE). This study determined the extent of variation in NUE-related parameters, specifying the primary traits contributing to the difference in nitrogen remobilization efficiency (NRE) on four spring cereal crops, two- and six-row barley (Hordeum vulgare L.), oat (Avena sativa L.), and wheat (Triticum aestivum L.). Field experiments were conducted in Finland during 2001, 2002, and 2003 under two N regimes (0 and 90 kg N ha^–1). Wheat had relatively high N content at maturity even though NUE was low, whereas oat and barley had higher NUE. A comparison of nitrogen harvest index (NHI) and NRE revealed that both were low for wheat, linking this with high N uptake after anthesis, suggesting that in wheat the proportion of the assimilated N used immediately in the developing grain is greater than in barley and oat. There was no strong N translocation from vegetative parts of the main shoots in wheat, which exhibited higher competition for N between vegetative and reproductive organs. Plant breeders could use these findings to their advantage in breeding spring cereal crops that not only produce high yield but also efficiently use available N in northern growing conditions.

Abbreviations: BPE, biomass production efficiency • BPE_vege, modified biomass production efficiency • HI, harvest index • NA, nitrogen uptake after anthesis • NHI, nitrogen harvest index • NRE, nitrogen remobilization efficiency • NUE, nitrogen use efficiency • UPE, nitrogen uptake efficiency • UTE, nitrogen utilization efficiency

	INTRODUCTION

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AGRICULTURAL EMISSIONS to the environment have escalated as fertilizer use has steadily increased (Tilman et al., 2001). Based on this phenomenon the European Community has launched several directives to reduce such agricultural emissions to the environment (Van Alphen and Stoorvogel, 2000). Under northern growing conditions, N, as one of the main inputs for cereal production systems, represents a particular threat to the environment (Granstedt, 2000).

Reduced use of N fertilizer is likely to decrease both production costs and pollution, but could also result in reduced yields and quality if crops experience temporary N deficiency (Cassman et al., 2003). Hatfield and Prueger (2004) demonstrated that N fertilizer consumption had not changed significantly since 1978 in Western Europe or in the USA, whereas at the same time the NUE had decreased for cereals. This observation combined with available information demonstrating wheat yields leveling off in the 1990s (Evans, 1998; Slafer and Peltonen-Sainio, 2000), sets high demands on plant breeders to develop cultivars with increased yield potential associated with higher NUE-improved ability to absorb N more efficiently from the soil and partition the greater part of the absorbed N into the grain. However, NUE is a complex trait. Moll et al. (1982) and Ortiz-Monasterio et al. (1997) showed that NUE comprises N uptake efficiency (UPE) and N utilization efficiency (UTE), the latter of which can be further split into harvest index (HI) and biomass production efficiency (BPE).

Results from experiments have shown genetic variation in cereals for UPE (Kelly et al., 1995; Singh and Arora, 2001) and for UTE (Woodend et al., 1986; Papakosta, 1994; Singh and Arora, 2001). For wheat, UPE accounts for most of the variation in NUE at low N availability (Ortiz-Monasterio et al., 1997; Le Gouis et al., 2000). Recent results obtained under northern growing conditions showed that UPE has a stronger relationship with NUE than UTE (Muurinen et al., 2006). However, there were indications of differences between cereal species in their NUE to UTE relationship. The association was especially strong for oat (Muurinen et al., 2006), which occurs, as also noted by Isfan (1993), regardless of N supply. Furthermore, Delogu et al. (1998) showed that in low N input environments winter barley had higher UTE than winter wheat and UTE was also associated with higher NHI. This indicates retranslocation efficiency of N from vegetative plant parts to the grain. Many studies have indicated that 70% or more of the N harvested in seeds is derived from N remobilized from senescing vegetative plant parts (Austin et al., 1977; Cox et al., 1985; Papakosta and Gagianas, 1991). Therefore, improved understanding of plant N requirements and dynamics, particularly BPE and NRE from vegetative parts among species and cultivars, is needed to determine better NUE.

The aims of the present study were to: (i) determine the extent of variation in NUE, UPE, UTE, NHI, NRE and BPE among different cultivars of spring cereals adapted to northern growing conditions; (ii) assess their relative importance with respect to N availability; and (iii) specify the primary traits contributing to the differences among the cultivars in NRE.

	MATERIALS AND METHODS

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Experimental Design
The experiments were conducted during the 2001 and 2002 growing seasons on a heavy clay soil, tentatively classified as a Vertic Cryaquept (Yli-Halla and Mokma, 2001) and in 2003 on a sandy clay soil, tentatively classified as a fine Typic Cryaquept (Yli-Halla and Mokma, 2001) in fields of MTT Agrifood Research Finland, Jokioinen, Finland (60°49' N, 23°30' E). The experimental field was characterized as sufficient in P content in all the 3 yr (8.1–25.4 mg L^–1, Vuorinen and Mäkitie, 1955) to the soil depth of 20 cm. Nitrogen content of the soil before the experiment was 4.78 to 16.75 mg L^–1 in the 0- to 20-cm layer. Other nutrient contents were adequate in topsoil as well, according to the common classification used in Finland. Soil pH varied from 6.2 to 6.7 in 0- to 20-cm layer (Vuorinen and Mäkitie, 1955). Spring wheat, barley (both two- and six-row cultivars), and oat were evaluated. Crops were sown on 10 May 2001, 29 Apr. 2002, and 13 May 2003, at seeding rates of 500 seeds m^–2 for oat and barley and 600 seeds m^–2 for wheat. Weeds were controlled with MCPA [(4-chloro-2-methylphenoxy) acetic acid] at 1000 mL ha^–1 at early growth stages (Zadoks growth stage ZGS12 in 2002 and ZGS13 in 2003, Zadoks et al., 1974).

In 2001 (July) and 2002 (June) the plots were irrigated twice with a total of 30 mm during the dry periods. No irrigation was needed in 2003. The experiments were arranged as split-plots with four replicate blocks. Two N fertilizer application rates, 0 kg N ha^–1 (control) and 90 kg N ha^–1 as ammonium nitrate were applied to main plots as banded at seeding. The 90 kg N ha^–1 fertilizer application rate is the basic recommendation level used in Finland (Ministry of Agriculture and Forestry, 2001). The species were randomly assigned to subplots containing three randomly assigned cultivars within crops as split plots of 12.5 m² (10 by 1.25 m, 10 rows orientated east–west). Each crop used in the experiment included one local landrace or a long-strawed old cultivar and two modern cultivars (Table 1). Old cultivars were Tammi (wheat), Uurainen and Olli (barley), and Jalostettu Maatiainen (Jama oat). Modern cultivars were Vinjett and Manu (wheat), Scarlett, Inari, Rolf, and Kunnari (barley), and Suomi and Aslak (oat).

The following parameters were calculated (terminology according to Moll et al., 1982; Ortiz-Monasterio et al., 1997; Cregan and van Berkum, 1984; Cox et al., 1985): (i) NUE (kg kg^–1 N) multiplying N uptake efficiency by the N utilization efficiency, (ii) UPE as the ratio of [plant total aboveground N at fertilization_(N90) – plant total aboveground N of control_(N0)] and applied fertilizer amount, (iii) UTE (kg kg^–1 N) by multiplying HI by the biomass production efficiency, (iv) BPE_vege (kg kg^–1 N) as the ratio of plant total aboveground vegetative biomass to plant total aboveground N, (v) HI as the ratio of total grain yield to total aboveground biomass at maturity, (vi) NHI as the ratio of total N in grain to total aboveground plant N at maturity, (vii) N uptake after anthesis (NA) (mg) as total aboveground N at maturity minus total aboveground N at anthesis, and (viii) NRE as the ratio of amount of remobilized N (grain N content at maturity – NA) to total aboveground N at anthesis.

Statistical Analyses
The data for 3 yr were analyzed with SAS PROC MIXED (Littell et al., 1996). To compare cultivars, the lsmeans statement of the MIXED procedure was used to produce the Tukey-Kramer method based tests. Before the analysis, accordance of the data with the assumption of equality of group variances and the normality assumption of errors were checked using Box Cox diagnostic plots (Neter et al., 1996). The year, N treatment, and cultivar were analyzed as fixed effects, while replicate nested years was analyzed as a random effect. Correlations between NRE and other main shoot and tiller traits from matured plants were evaluated by Spearman's correlation.

	RESULTS

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Weather Conditions
Average air temperatures were similar in 2001, 2002, and 2003 (Table 2). The amount and distribution of rainfall differed between years. In 2001, precipitation was close to the long-term average at the beginning of the growing season, but decreased later and rains were uneven during June. Consequently, the crops were irrigated during the beginning of July (twice with 15 mm each time). During July 2001 the crops did not suffer from drought. Crops were harvested at the end of August. In 2002 rains were uneven and much of the rain fell during a very short period. Suboptimal rainfall distribution in combination with high maximum daily average temperatures during July (23°C) and the beginning of August (25°C) caused water stress despite the irrigation. The overall result was exceptionally early maturity and the crops were harvested at the beginning of August. In 2003, precipitation was relatively evenly distributed during the growing season. Also in 2003, the average maximum temperatures were high during July (25°C) and August (20°C), and harvest was completed by mid-August. Such seasonal differences in growing conditions likely accounted for year and year x N application rate interaction effects for all the measured traits.

Total aboveground N content at anthesis averaged across species was lowest in 2002 (12.1 mg plant^–1) compared with 20.5 and 28.1 mg plant^–1 in 2001 and 2003, respectively (P < 0.0001). Mean values for N content differed only between the two- and six-rowed barley (P < 0.04) (Table 5). Interaction between year and crops was significant (P < 0.0001) even though response of the species to yearly growing conditions were quite similar. Within the years there were differences among crops only in 2001 since oat clearly had higher N content than two-row barley and wheat. Mean N content at anthesis did not vary by cultivar within a species, except on wheat. Since, the long-strawed, old cultivar Tammi had lower N content than Manu and Vinjett. There was no consistent pattern of N content for any of the cultivars over years.

Total aboveground N content at maturity differed markedly (P < 0.0001) over years. No interaction occurred between crops and N application rates or cultivars and N application rates. Two-row barley, oat, and wheat differed significantly (P < 0.001). Oat had the highest and two-row barley the lowest mean values. This was also the case in 2002, while in 2001 wheat had the lowest value though it did not differ from two-row barley. In 2003 the crops did not differ from each other (Table 5). Only for wheat were there significant differences between cultivars in all years: Tammi differed from other cultivars in 2001 and Vinjett in 2002 and 2003.

Nitrogen taken up after anthesis differed over years (P < 0.0001), whereas N treatment did not have a significant effect on NA. Of the four crops, there were no differences between two- and six-row barley, and wheat and oat both had similarly higher levels of NA (Table 5). Within crops only oat had comparable NA amounts over the years, whereas for the others NA was clearly lowest in 2002. Within crops the mean NA of the cultivars differed only for two-row barley and oat. The old, long-strawed, two-row cultivar Uurainen and oat cultivar Aslak had clearly lower NA compared with the other cultivars. There was a year x cultivar interaction only for wheat. Accordingly the new high-yielding cultivar Vinjett had the highest NA on 2001 and 2003 when the growing conditions were more favorable than in year 2002 when the cultivars Manu and Tammi had higher NA values. In addition NA was especially high in 2003.

Wheat had the lowest UTE in all 3 yr (Table 6). There were differences among barley cultivars; values ranged from 28.0 to 59.7 kg kg^–1 N for two-row barley and from 37.1 to 53.5 kg kg^–1 N for six-row barley depending on year. Low-yielding two-row barley cultivar Uurainen had the lowest UTE and differed from other two-row barleys. The low-yielding six-row barley cultivar Olli performed similarly among six-row barleys. For oat and wheat, the high-yielding cultivars Suomi and Vinjett differed in mean UTE values from other cultivars in 2002 and 2003.

Nitrogen remobilization efficiency differed (P < 0.03) among years for barley and wheat. In 2002, NRE values were highest for all crops (Table 6). Nitrogen remobilization efficiency was highest at 90 kg N ha^–1 for all species; since average NRE ratios at 0 kg N ha^–1 and 90 kg N ha^–1 were respectively 0.60 and 0.72 in 2001, 0.74 and 0.77 in 2002, 0.63 and 0.64 in 2003. However, no interactions between years, cultivars, and N application rates were recorded within the crops. Mean NRE differed among crops. Wheat had the lowest NRE, while oat and six-row barley had the highest values. Low-yielding oat cultivar Jama had the lowest value in all years, but only in 2001 and 2003 significantly. For six-row barley and wheat there were no differences among the cultivars within years. However, there were differences among the wheat cultivars over years, and Manu appeared to be the most stable.

There were strong negative correlations between NRE and main shoot leaf N content at 0 kg N ha^–1 in 2002 and 2003 for two-row barley (Table 8). Other traits strongly correlated with NRE in 2002 were those related to leaf characteristics, whereas in 2003 N content of different plant parts had negative correlation with NRE. At 90 kg N ha^–1 NRE correlated negatively with tiller leaf N content and tiller spike number in 2002 and 2003 (Table 8). Strong negative relationships were established between NRE and tiller traits in 2003. For oat, the main shoot leaf and straw N content were clearly negatively correlated with NRE at both N application rates. However, for the control N treatment a strong negative correlation was also established between NRE and tiller leaf N content and the proportion of N in the tillers as a fraction of total N content. For wheat, NRE was strongly related to tiller traits at both N application rates. Tiller biomass and tiller N content were the traits correlating negatively with NRE, whereas the proportion of dead tillers was positively correlated with NRE. For six-row barley only a negative correlation between NRE and tiller leaf N content was recorded at both N application rates in 2001 and 2003, whereas the relationship was positive in 2002 at the control rate. Other traits strongly and negatively correlated with NRE were main shoot straw N content and proportion of tiller N from total N at 90 kg N ha^–1 in 2002 and 2003.

	DISCUSSION

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Of the main traits contributing to differences in NUE, N uptake contributed to NUE in wheat when grown at different N application rates, but especially at low N availability (May et al., 1991; Le Gouis et al., 2000). We did not find significant differences in mean UPE among species. However, cultivars differed within species indicating that low-yielding cultivars also had lower UPE. In this study, contribution of N application rate to N uptake was studied by analyzing the difference in total N content of the plant at heading and at maturity. Lack of species x N application rate and cultivar x N application rate interactions for total N content at each growth stage studied indicated that species and cultivars responded similarly to N application rates. Of the four spring cereals, oat had significantly higher mean total plant N content at maturity, even though there was large variation over years.

Wheat had relatively high N content at maturity even though NUE was lowest. Therefore, our results do not fully agree with earlier studies suggesting higher N uptake contributed to improved NUE in wheat (Van Sanford and MacKown, 1986; May et al., 1991; Le Gouis et al., 2000). However, this could be partly explained by low total N content of the wheat plant at heading. At heading, barley (both two- and six-row) N uptake was 70 to 73% of the total N uptake, whereas that for wheat and oat averaged 64%. Our results for barley are in line with the earlier studies (Bulman and Smith, 1994; Delogu et al., 1998), while wheat had much lower N uptake until heading than the 90 to 100% reported by Clarke et al. (1990) and Heitholt et al. (1990). According to our results, oat and wheat had higher postheading N uptake than barley. The most likely reason for this is that oat and wheat have a much longer growing period than barley under northern growing conditions (Juuti, 1988; Mukula and Rantanen, 1989; Peltonen-Sainio et al., 2006). This possibly indicates that oat and wheat would exhibit higher competition for N between vegetative and reproductive organs.

Alternatively BPE, as a measure of total plant biomass produced per unit N absorbed, also demonstrates N concentration in the entire plant and describes the internal N requirement in many agronomically important species (Gourley et al., 1994). Our modified BPE_vege values for spring cereals showed that wheat had high vegetative biomass production even during the end of the growing season in each year, which was in line with the average total biomass results from 90 kg N ha^–1 treatment (wheat, 1180 g m^–2; oat, 1470 g m^–2; barley, 1065 g m^–2) reported by Muurinen and Peltonen-Sainio (2006). These results, and those for total N content of the plant at maturity, suggest that wheat would show competition for N between vegetative and reproductive organs. Therefore, part of the accumulated N possibly would not be translocated to filling grains.

As UTE demonstrates the ability of the plant to translocate N taken-up into the grains, it is considered to be an essential physiological parameter contributing to improved NUE (Isfan, 1993). This finding was also supported by results from our study, as wheat had significantly lower UTE at both N application rates. The higher UTE found for oat and barley is also in line with an earlier study of Delogu et al. (1998). They showed that barley outperformed wheat regarding UTE. Furthermore, Muurinen et al. (2006) showed a clear relationship between UTE and NUE in oat and barley. However, there are two different N sources for grain filling: absorbed N from the soil after heading and remobilized N from the vegetative plant parts. Hence, we investigated the source of improved UTE.

The proportion of the total plant N located in the grain at maturity, NHI, was significantly lower in wheat than in oat and barley. Average NHI ratios for barley and wheat, 0.77 and 0.74 respectively, were similar to those cited previously (Spiertz and De Vos, 1983; Ugalde, 1993; Bulman and Smith, 1994; Noulas et al., 2004). However, the ratios for oat were higher in all study years when compared with values of 69 to 75% reported by Rattunde and Frey (1986), McMullan et al. (1988), and Welch and Leggett (1997).

Nitrogen remobilization efficiency, which represents a measure of the ability of the crop to remove N from vegetative tissue, was inconsistent for some crops over the 3 yr. Dry weather conditions during grain filling in 2002 suppressed N movement in the soil as indicated by lower postanthesis N uptake compared with 2001 and 2003. This decreased N availability during grain filling might also have increased NRE values for all cultivars of two-row barley and wheat during 2002. Wheat NRE varied markedly during the 3 yr and its mean value was significantly lower in comparison with that for the other species. This result was consistent with the findings of Papakosta and Gagianas (1991), Cox et al. (1985), and Palta et al. (1994). However, the results from different N treatments in this study did not support the idea of lower N supply (0 kg N ha^–1) having a higher NRE than the 90 kg N ha^–1 treatment. Thus 90 kg N ha^–1 treatments had higher NRE in all 3 yr for all crops. However, Barbottin et al. (2005) showed that in some cases in wheat the NRE could be stable across environments and genotypes. Our results for oat and six-row barley indicated stable mean NRE values within the years, suggesting that NRE for these crops might not be so controlled by environment. Our NRE ratios for barley were 0.65 to 0.78 and they varied less than the reported 47 to 66% for spring barley by Przulj and Momcilovic (2001). The mean values reported by Cregan and van Berkum (1984) and Barbottin et al. (2005) for wheat varied from 0.66 to 0.92, whereas our results for wheat ranged from 0.47 to 0.72. Our results for oat were in line with those reported for barley and wheat (Przulj and Momcilovic, 2001).

A comparison of NHI and NRE in this study revealed that both were low for wheat, average for barley (both two- and six-row), and high for oat. Linking this with differences in N taken up after anthesis, suggests that in wheat the proportion of assimilated N used immediately in the developing grain is greater than in barley and oat. Also the comparison between barley and oat indicated that barley had more efficient preheading assimilation processes and above all better translocation ability. Similar differences between species were reported by Cregan and van Berkum (1984), who compared wheat, soybean [Glycine max (L.) Merr.], and maize (Zea mays L.). Within a species there were indications that similar assimilation efficiency differences were apparent among cultivars. Wheat cultivar Vinjett and oat cultivar Jama had lower NRE and higher NHI than other cultivars, pointing out that a large proportion of assimilated N is immediately used by the developing grain rather than first being incorporated into leaf or stem proteins, as is likely to be the case in other wheat and oat cultivars.

Correlation between NRE and leaf and tiller traits measured from mature plants indicated that NRE had a strong negative relationship between main shoot leaf and straw N content and tiller traits for barley and oat. This could indicate that barley removed more N from leaves and tillers at late grain filling, therefore affirming that there would not have been strong competition between vegetative parts and grain filling. Contrary to this, vegetative parts and grains of oat likely competed more for N at higher N treatments as there were no clear strong, negative correlations between NRE and tiller traits, suggesting that some of the tillers could have been competing for N with filling grains. However, wheat NRE correlated strongly and negatively mainly with tiller traits. This suggests that N translocation would have been from tillers at a very late grain-filling phase, whereas there was no strong N translocation from vegetative parts of the main shoots, which could still have been competing with filling grains for N.

In conclusion, several partly interrelated reasons could explain the exceptionally low NUE for wheat compared with that of barley and oat under northern growing conditions. The low N translocation ability in wheat is, however, the most likely explanation. Wheat and oat plants accumulate N slowly when compared with barley, in line with the slower growth rate of those species under long days. However, N translocation efficiency of wheat appeared lower than that for oat and especially for barley. Plant breeders in northern growing conditions should take advantage of the variation between wheat cultivars in NRE and NHI, so that in the future selected genotypes are both high yielders and efficient utilizers of N.

	ACKNOWLEDGMENTS

 
Päivi Lehkonen, Petra Manninen, Riikka Puntila, Jaana Nissi, Tarja Hannula, Leila Salo, Irma Noppa, and Aino Lahti are thanked for their valuable assistance. Christian Eriksson is thanked for his help with the statistics. Kesko/K-group and Boreal Plant Breeding are thanked for providing the seeds of old and new cultivars for the experiments. The study was financed by the Academy of Finland (project 53592), Kemira GrowHow Oyj, MTT Agrifood Research Finland and the foundations of Kemira Oy, Tiura, Emil Aaltonen and the Scientific Agricultural Society of Finland.

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