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Published online 1 February 2006
Published in Crop Sci 46:561-568 (2006)
© 2006 Crop Science Society of America
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CROP PHYSIOLOGY & METABOLISM

Breeding Effects on Nitrogen Use Efficiency of Spring Cereals under Northern Conditions

S. Muurinen*,a, G. A. Slaferb and P. Peltonen-Sainioa

a MTT Agrifood Research Finland, Plant Production Research, FIN-31600 Jokioinen, Finland
b Research Professor of ICREA (Institució Catalana de Recerca i Estudis Avancats) at the Dep. of Crop Production and Forestry, Centre UdL-IRTA, Univ. of Lleida, Alcalde Rovira Roure 191, 25198 Lleida, Spain

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


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Spring cultivars of barley (Hordeum vulgare L.), oat (Avena sativa L.), and wheat (Triticum aestivum L.) are the most important crops in Finnish agricultural systems. The increasing need to reduce pollution from N fertilizer is concomitantly strengthening the importance of improving the understanding of nitrogen use efficiency (NUE) of these crops. The aim of this work was to study the differences in NUE, defined as the crop's ability to produce yield with one available N unit, among spring cereal cultivars, and to determine the achievements of plant breeding in NUE under northern European growing conditions. Field experiments were conducted in Finland during 2003 and 2004. Samples from matured plants of 17 to 18 cultivars of each of the three cereal species released between 1909 and 2002 were studied. There were no clear differences in NUE among modern spring cultivars. However, there were cultivar differences within species and significant NUE improvements on wheat and particularly for oat across time. There was no clear trend of NUE and year of release of cultivars in two-row spring barley, probably because breeding for malting barley involves consistent selection for low-protein cultivars. The study revealed that most breeding effects on NUE were associated with changes in nitrogen uptake efficiency (UPE).

Abbreviations: BPE, biomass production efficiency • HI, harvest index • NUE, nitrogen use efficiency • UPE, nitrogen uptake efficiency • UTE, nitrogen utilization efficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SPRING BARLEY, OAT, AND WHEAT are important crops in the Nordic region (Denmark, Finland, Norway, and Sweden). Finland has larger areas sown to oat (424 000 ha) and barley (550 000 ha) than to wheat (174 000 ha), in contrast to the other Nordic countries (Food and Agriculture Organization of the United Nations, 2005). As N is one of the main inputs in cereal cultivation and more than 50% of the arable land in Finland is used for cereal production, N has become an important contributor to agricultural pollution through fertilizer leaching and runoff. More than 42% of N losses to the Baltic Sea resulting from human activities are from agriculture (Granstedt, 2000), and according to Bouraoui et al. (2004), climate change is likely to result in slightly increased total runoffs in Finland. Because of such ecological as well as economical factors, northern European agricultural practices are expected to reduce N fertilizer application rates. Therefore, more efficient N use in cereal production represents a current challenge for agronomical research, and especially for plant breeding programs in the northern growing areas, to minimize pollution risks, and maximize farmer income.

Moll et al. (1982) defined NUE as the grain yield produced per unit of available N, and is used here also to describe how much yield was produced by 1 g of available N in soil. Nitrogen use efficiency (g g–1 N) is determined by the ability of the plant to extract soil N (UPE, %), and the ability of the plant to convert the absorbed N into harvested grain yield (nitrogen utilization efficiency, UTE, g g–1 N). Moll et al. (1982) speculated that the role of UPE is emphasized in determining NUE as soil N supply increases. However, Ortiz-Monasterio et al. (1997) reported that UPE is an important component of NUE under low N conditions.

Several studies showed improved NUE in modern wheat cultivars (Austin et al., 1980; Slafer et al., 1990; Calderini et al., 1995; Ortiz-Monasterio et al.,1997; Foulkes et al., 1998; Reynolds et al., 1999) and improved agronomic efficiency of modern barley cultivars (Isfan, 1990; Delogu et al., 1998; Abeledo et al., 2003) compared with their predecessors. However, there were hardly any correlations between total N uptake and year of cultivar release (Slafer et al., 1990; Calderini et al., 1995; Foulkes et al., 1998), although a positive trend was reported for six-row spring barley (Bulman et al., 1993) and oat (Wych and Stuthman, 1983; Welch and Leggett, 1997). On the other hand, UTE appears to be the trait mainly affected by plant breeding (Slafer et al., 1990; Ortiz-Monasterio et al., 1997). Nitrogen utilization efficiency comprises harvest index (HI) and biomass production efficiency (BPE, g g–1 N). Harvest index has increased substantially through plant breeding (Austin et al., 1980; Bulman et al., 1993), and these improvements have largely contributed to improved UTE (Calderini et al., 1999).

Barley, oat, and wheat have been grown in Finland since the 14th century, though extensive cultivation did not begin until the 20th century. Structured plant breeding also started at the beginning of the 20th century and depended on selection from landraces and crossing with foreign varieties (Aikasalo, 1988; Juuti, 1988; Rekunen, 1988). The main strategy for plant breeding programs aimed at better yield stability, shorter stature, and early maturity (Peltonen-Sainio and Karjalainen, 1991). Oat and barley cultivars are generally better adapted to Finnish growing conditions than wheat. Oat grows well in a cool climate and in acidic soils, whereas barley is well adapted to a short growing period typical of Finland (Mukula and Rantanen, 1989a, 1989b). Wheat is generally less well adapted to such conditions and performs less well on acidic soils than oat, and requires a longer growing period than barley and oat.

Different breeding strategies and differing adaptation of cereal species to the harsh Nordic growing conditions could indicate differences among the species in NUE. Knowing whether the species differ in NUE could help to design more sustainable production systems. As there is little information about NUE of spring wheat and barley under Nordic growing conditions and practically no information on oat NUE, this study aimed to analyze differences in NUE among spring cereals grown in Finland. Nitrogen use efficiency levels of modern cultivars are a consequence of past management practices and breeding efforts. Understanding how breeding has modified physiological traits could help to identify alternative production options, such as possible different roles that species can play in crop rotations designed to meet the demands of modern agriculture. We therefore also aimed to determine the achievements of plant breeding for NUE and its components (UPE, UTE, HI, and BPE). The experiments included numerous wheat, barley, and oat cultivars released between 1901 and 2002.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The study was conducted during two consecutive growing seasons in 2003 and 2004 on a sandy clay soil, tentatively classified as a fine Typic Cryaquept (Yli-Halla and Mokma, 2001) in a field located at the MTT Agrifood Research Finland, Jokioinen, Finland (60°49' N, 23°30' E). The experimental field was characterized as low in organic C (2.6–3.6%) at 0–20 cm with LECO CN-2000 (St. Joseph, MI) and sufficient in P content (9.6–12.3 mg L–1, Vuorinen and Mäkitie, 1955) to the soil depth of 20 cm. Other nutrient contents were adequate in topsoil as well, according to the common classification used in Finland. Soil pH varied from 6.1 to 6.5 between blocks in 20 cm (Vuorinen and Mäkitie, 1955). Seventeen to eighteen cultivars of spring wheat, two-row barley, and oat released between 1901 and 2002 were evaluated (Table 1). Cultivars were sown on 2 June 2003 and on 12 May 2004 in a split-plot arrangement in a randomized complete block design with three replicates. Each block contained 18 main plots (net size 1 by 1 m) with six rows (row width = 17 cm). Main plot was period of release of cultivars. Three species were randomly assigned to the inner rows within the plots as subplots. The border rows were sown with a modern wheat cultivar under test to obviated edge effects. Weeds were controlled with MCPA [(4-chloro-2-methylphenoxyl) acetic acid] at a rate of 1000 mL ha–1 at Growth Stage 13 (Zadoks et al., 1974). At sowing, 70 kg N ha–1 was applied as ammonium nitrate. In 2004, a growing season characterized by heavy rainfall, plant stands were supported with nets to prevent lodging.


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  		Table 1. Name, country of origin, year of release (or from when known), pedigree or geographical origin, and duration of cultivation in Finland for spring wheat, oat, and barley included in an experiment conducted at Jokioinen during 2003 and 2004 to determine breeding effects on nitrogen use efficiency.

 
Soil samples were collected from the plots immediately before sowing in both years to record the mineral N content. Because of the large number of plots, N was sampled only from the upper 20 cm. Two subsamples per plot were taken, samples from 10 plots close to each other within block were bulked, and the soil was homogenized. Two subsamples per block were taken and stored frozen (–30°C) in plastic bags until analysis. The extractable ammonium and nitrate N content was analyzed from thawed soil samples (100 g) by extracting in 250-mL 2 M KCl overnight and analyzing the extract by Skalar Auto-Analyser (Scalar Analytical B.V., De Breda, the Netherlands) for NH4+–N and NO3–N (Krom, 1980; Greenberg et al., 1980). Earlier measurements taken in the same field for the complete profile to the 100-cm depth (Salo, 1999) were used to estimate the proportion of N in different soil profiles (Table 2). Using both measurements, mineral N available to the crop at sowing was estimated for each plot as follows:

Formula
where MN is mineral N (kg ha–1) on 20 cm, EN is estimated proportion of N in soil profile, and c is coefficient for soil level (Table 2). The sum of these values, plus the N contributed by the fertilizer, was considered to represent the pool of total available N (Le Gouis et al., 2000). Estimated proportions of N in the soil profile apply only for this particular experimental field, since the method used somewhat underestimated the total N available, as mineralization takes place during the growing season. This was, however, expected to be similar for all plots in a year, and hence the relative comparison between species and breeding trends was considered to be valid.


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  		Table 2. The calculations of soil inorganic N (NH4–N + NO3–N) contents in experimental fields in 2003 and 2004.

 
Plant samples were collected by hand at maturity by uprooting each cultivar row. Twenty plants of each cultivar in each replication were selected for further analysis. The samples were divided into main shoots and tillers, and further separated into vegetative mass and heads. Samples were dried at 60°C for 2 d and weighed. Samples were ground and N content of the plant parts was measured using a modified version of the Kjeldahl procedure with a Kjeltec Auto 1030 Analyzer (Tecator Ab, Höganäs, Sweden). Both NUE and its components were calculated as shown in Table 3.


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  		Table 3. Traits calculated from mature plants and procedure for calculation for an experiment conducted at Jokioinen during 2003 and 2004.

 
The data for 2 yr were analyzed together using the SAS MIXED procedure (Littell et al., 1996). Before analysis, equality of group variances and normality of error assumptions were checked using Box.Cox diagnostic plots (Neter et al., 1996). The year and cultivar were analyzed as fixed effects, while replicate nested years was analyzed as a random effect. Robust REGRESSION was used to analyze the mutual trait changes and trends in each year.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Rainfall exceeded the long-term average during most of the growing season in both years (Table 4). The only exception was during the grain-filling period in 2003. In 2003, the temperatures were above average, while in 2004 they were close to average. In 2003 the plants matured faster and were harvested during August, whereas in 2004 the growing period was longer and harvest was during September. On both experimental years, the estimated total N in soil before fertilization was high. In 2003 the mean content of total N at 0 to 20 cm had higher variation than in 2004, changing from 40 to 85 kg ha–1 (Table 2).


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  		Table 4. Monthly meteorological data for 2003–2004 growing seasons and long term (1971–2000) averages at Jokioinen, Finland.

 
Nitrogen Use Efficiency of Modern Wheat, Barley, and Oat Cultivars
To compare the spring cereals most widely grown in Finland (wheat, oat, and barley) and to establish whether they differed in NUE, an average of the three most recently released cultivars (between 1993 and 2002) within each species was calculated. Despite the 2 yr being quite different, differences in NUE of modern cultivars were not statistically significant among species for either of the two experiments (Fig. 1 ). However, there were statistically significant differences between years (P = 0.02 for UPE) and among species (P = 0.002 for UTE and P = 0.03 for UPE); also, species x year interaction was significant for UTE (P = 0.002) and UPE (P = 0.02) (Fig. 1). Wheat had higher UPE and lower UTE than barley and oat in 2004, while no difference was evident in 2003.


Figure 1
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  		Fig. 1. Comparison of modern spring cereal species on (A) nitrogen use efficiency (NUE), (B) nitrogen uptake efficiency (UPE), and (C) nitrogen utilization efficiency (UTE) during 2 yr (2003 and 2004). Columns with different letters indicate a significant difference at P = 0.05.

 
Genetic Improvement of Nitrogen Use Efficiency
Relationships between NUE measurements for each year and year of cultivar release were evaluated by linear regression. As there were no significant year effects on NUE and trends in both years were similar, though slopes differed in magnitude, responses were calculated from means across 2 yr (Fig. 2 ). There were significant linear relationships between NUE and year of release of wheat and oat cultivars, but not for barley (Fig. 2). The genetic gain estimated for NUE in oat was highest (0.13 g g–1 N yr–1) and the improvement achieved in wheat was intermediate between that value and the lack of improvement in barley.


Figure 2
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  		Fig. 2. Changes in nitrogen use efficiency with year of release for cultivars of wheat, oat, and barley. The values are the means from 2 yr.

 
It is noteworthy that the intercepts of the regressions for cultivars released at the early 1900s differed markedly among species, being –233.7 ± 67.5 for oat, 59.1 ± 73.9 for barley, and –85.2 ± 59.98 for wheat (Fig. 2). When the cultivars released at the early 1900s were studied more closely there were marked differences in NUE among the species. The NUE of old oat cultivars was much lower than the NUE of wheat and barley cultivars (Fig. 3 ), even though there are no differences in NUE of modern cultivars of the species (Fig. 1). Thus, there seems to be a negative trend between achieved genetic gains of NUE and the mean NUE values of old cultivars released before 1950, that is, in the slope of NUE improvement and the actual values of this variable of each species as indicated in Fig. 3.


Figure 3
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  		Fig. 3. Improvement of oat, wheat, and barley nitrogen efficiency described by slope plotted against the mean nitrogen use efficiency (NUE) values of old cultivars (released before 1950). Bars indicate the standard errors of the means.

 
There was a clear and consistent association between NUE and UPE for all three species (Fig. 4 ). In contrast, the association between NUE and UTE was not clear for any of the species. Oat was the only species showing a significant relationship between NUE and UTE, though this relationship was far less clear than that between NUE and UPE (Fig. 4).


Figure 4
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  		Fig. 4. Relationships between nitrogen use efficiency, nitrogen uptake efficiency, and N utilization efficiency from 2 yr (closed symbols for 2003 and open symbols for 2004) for wheat, oat, and barley.

 
Correlations between UPE, UTE, NUE, and other traits measured from mature plants were calculated using average values across 2 yr. The UPE was highly correlated with plant total N content (g m–2) for all the species (Table 5). Total plant biomass (g m–2) and grain yield (g m–2) were also positively correlated with UPE. Nitrogen utilization efficiency was positively correlated with the N harvest index for all species (Table 5). In addition, UPE and NUE were positively correlated with grain yield for all species. Similarly, UPE the NUE correlated positively with total plant N content for the three species (Table 5).


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  		Table 5. Regression equations y = a + bx + c x year (1 or 2) and correlations between nitrogen use efficiency (NUE), nitrogen uptake efficiency (UPE), nitrogen utilization efficiency (UTE), and other traits measured from samples of mature plants grown in an experiment conducted at Jokioinen during 2003 (Year 1) and 2004 (Year 2). Correlations were calculated from average values across 2 yr.{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
According to our hypothesis, spring cereals in Finland would differ in their NUE due to differences in adaptation to the short growing season and soil characteristics. However, our results indicate that NUE values for modern wheat, barley, and oat cultivars were similar (Fig. 1). The findings therefore do not support the hypothesis that oat or barley would have improved NUE over wheat due to the better adaptation of oat to acidic soils or barley's adaptation to a short growing season. The lack of clear differences in NUE in modern cultivars of the three cereals may imply that breeding has been equally efficient for spring cereals.

There is little information on NUE values for spring cereals under Nordic growing conditions. The NUE values for wheat (average of all wheat cultivars, 29.4 g g–1 N) in our study were similar to mean values (26–44 g g–1 N) reported under different growing conditions (Ortiz-Monasterio et al., 1997). However, the soil mineral N estimation used in this study did not include the mineralization of N during the growing season and therefore calculated NUE values are not unambiguously comparable with values obtained with other methods. The reported values for barley and oat largely represented agronomic efficiency of N use rather than NUE and varied widely depending on growing conditions (Isfan, 1993; Delogu et al., 1998; Sinebo et al., 2003). Despite this, the values for barley (average of all barley cultivars, 32.8 g g–1 N) and oat (27.1 g g–1 N) fitted in the range of reported values for wheat.

There were differences in NUE among cultivars of the species, but they represented consistent differences between old and new cultivars of oat and wheat, not barley. Similar differences for wheat were reported from areas differing in growing conditions from those in the Nordic areas (Calderini et al., 1995). The genetic gains in NUE for wheat and oat attributable to plant breeding reflect genetic gains in grain yields for the species (Ortiz et al., 1998; Slafer and Peltonen-Sainio, 2001; Öfversten et al., 2004). However, Górny (2001) for barley indicated that as the N stress increased, the landraces and old cultivars became superior to modern ones in UTE. Thus, even though the fertilization rate of our experiments was set to be close to the minimal recommendation levels in Finland (70 kg N ha–1), the total mineral N levels in the experiments turned out to be very high. This could have given better advantages for the modern cultivars with the higher response to high N levels, and therefore promote the higher NUE of modern oat and wheat cultivars compared with old cultivars. Nevertheless, similar to our results, Ortiz-Monasterio et al. (1997) demonstrated that under moderate fertilization rate, the NUE values of the modern wheat cultivars were higher than on old cultivars. However, the postulate of Ortiz et al. (2002) of improvement in two-row barley grain yields in the Nordic areas is not supported by our results.

It appears that NUE values for landraces and old cultivars at the beginning of the 20th century have influenced the extent of NUE improvement during the subsequent breeding process under northern growing conditions. It seems that barley landraces and old cultivars already had higher NUE values, whereas wheat and oat landraces and old cultivars had very low NUE values (Figs. 2 and 3). The differences in initial NUE values could be explained by the fact that barley was the more common crop in Finland after the 14th century, whereas oat and wheat were minor crops at that time (Aikasalo, 1988).

Genotypic differences in NUE seem to have been mostly associated with UPE for the three species (Fig. 4). Even though wheat and barley showed no increase in N uptake during the last century as also suggested by Løes (2003), oat was significantly improved in N uptake in this study. This is in agreement with N uptake of oat grown in the northern USA (Wych and Stuthman, 1983).

The lack of differences in N uptake of wheat and barley could indicate that breeding has not directly changed UPE. However, it could be that UPE of old wheat and barley cultivars was already high or that there has been no variation among cultivars and lines used in breeding for higher yields and improved yield stability. Probably breeding has always taken place in high-yielding environments so that the plant's ability to take up N is a trait that is not selected for under such growing conditions. Moreover, as two-row barley breeding in the Nordic region is concentrated on malting barley, it involves consistent selection for low-protein cultivars (Bertholdsson, 1998), which could similarly limit UPE. Thus, it seems that there has not been changed demand for nutrients in wheat and barley, but increased grain yields result from increased efficiency to convert soil N to grain yield, that is, UTE, as suggested by Calderini et al. (1995).

Wheat and oat had improved UTE, in line with results for Nordic spring wheat (Ortiz et al., 1998). However, there was no genetic gain in UTE for barley (data not shown). Although wheat and oat showed improved UTE during the 2 yr, only for oat was there a clear positive relationship between NUE and UTE (Fig. 4). However, Ortiz-Monasterio et al. (1997) defined UTE as a combination of HI and BPE and showed that new wheat cultivars had improved HI rather than BPE. Earlier studies on spring wheat and barley grown in the Nordic region showed that improvement in UTE was achieved through reduced plant height and lodging and improving yields via improved HI (Ortiz et al., 1998, 2002). Also in our study, UTE and NUE were positively correlated with HI for wheat and oat. Therefore, the NUE would be improved through UTE. However, there was no evidence for improvement of UTE for barley.

The cultivars currently utilized have been selected because of their greater ability to respond to applied N. However, because use of high rates of N fertilizer associates with a range of environmental problems, the need to reduce N fertilizer rates has further emphasized the need for genotypes with improved NUE. Improvement of NUE of spring cereals under northern growing conditions in the future would require introduction of new wheat and barley germplasm, since breeding efforts to date appear to have exhausted variation for the trait in current material. This is indicated by the reduced rate of NUE improvement for barley and wheat. Wild Hordeum species or exotic germplasm might provide genes needed to improve UPE and UTE in Nordic spring barley (Górny, 2001 and Ortiz et al., 2002). According to our results, there might also be marked variation among local barley landraces in NUE, which could prove useful in future breeding efforts. Breeding for improved NUE in oat has not yet leveled off, and therefore future improvements could be achieved from selection for improved N uptake, thereby simultaneously increasing UTE. As HI and BPE determine UTE, to increase UTE in the future would need more efficient improvement of BPE, while simultaneously maintaining a high HI. Oat breeding has already been shown to have increased biomass production in earlier studies (Peltonen-Sainio, 1991). However, BPE was negatively correlated with NUE in this study. Therefore, increased N uptake would be the avenue to address NUE in oat.


   ACKNOWLEDGMENTS
 
Jaana Nissi, Kirsi Raiskio, Tarja Hannula, and Aino Lahti are thanked for valuable assistance. We thank Boreal Plant breeding employees for providing help and advice for establishment of experiments. Christian Eriksson is thanked for his help with the statistics. The Nordic Gene Bank is thanked for providing seeds of landraces and old varieties for the experiment. The study was financed by the Academy of Finland (project 53592), Agrifood Research Finland (MTT), and the foundations of Kemira Oy, Tiura, Emil Aaltonen, and the Scientific Agricultural Society of Finland.

Received for publication May 4, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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