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Article

Influence of Nitrogen Application Rate on Wheat Grain Protein Content and Composition in China: A Meta-Analysis

by
Hao-Yuan An
1,
Jing-Jing Han
2,
Qian-Nan He
1,
Yi-Lin Zhu
1,
Peng Wu
1,
Yue-Chao Wang
1,
Zhi-Qiang Gao
1,
Tian-Qing Du
1 and
Jian-Fu Xue
1,*
1
College of Agriculture, Shanxi Agricultural University, Jinzhong 030800, China
2
College of Agricultural Economics and Management, Shanxi Agricultural University, Jinzhong 030801, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(6), 1164; https://doi.org/10.3390/agronomy14061164
Submission received: 22 April 2024 / Revised: 26 May 2024 / Accepted: 27 May 2024 / Published: 29 May 2024
(This article belongs to the Special Issue Sustainable Management and Tillage Practice in Agriculture)
Browse Figures
Versions Notes

Abstract

:
The nitrogen application rate (NAR) has a significant effect on the contents of wheat grain protein and its composition. There is still no consensus regarding the appropriate NAR, given the differences in studied conditions and influence of factors such as geographical location, climate, and soil nutrient contents. In this study, 66 papers related to wheat grain protein and its composition published from 1984 to 2021 were selected for meta-analysis in comprehensively evaluating the response of wheat grain protein content and composition to NAR in China. The results reveal that NAR significantly increased total protein content by 9.49–28.6%, gliadin by 9.13–30.5%, glutenin by 12.9–45.4%, albumin by 5.06–15.8%, and globulin by 8.52–24.0% of wheat grain in China, respectively, compared to no nitrogen application. The optimal NAR is 240–300 kg ha−1 when specific planting conditions are not being considered. Under different growing conditions, the NAR that provided the greatest increase in wheat grain protein and its composition varied as follows: 180–240 kg ha−1 in Northwest China and at >100 m altitudes; >300 kg ha−1 in North China and at <100 m altitudes and lower soil base nutrient levels; 240–300 kg ha−1 in Southeast China, with higher soil nutrients levels and for all average annual temperatures and precipitation ranges. In conclusion, the results of the present study reveal that it is feasible to systematically enhance the contents of wheat grain protein and its related fractions by appropriate NAR under different cropping conditions.

1. Introduction

Wheat is one of the world’s major food crops, supplying approximately one-fifth of the human daily protein requirement (FAOSTAT database, https://www.fao.org/faostat/zh/#data/FBS, accessed on 13 October 2023). Improving wheat quality is thus also very important for ensuring food security and a healthy diet for the people in China. Grain protein is an important index for evaluating the nutritional quality of wheat and can be divided into two categories: soluble proteins that are easily dissolved in water, ethanol, and other solvents (i.e., albumin and globulin) and stored proteins (i.e., gliadin and glutelin) [1]. In pursuit of higher yields, large amounts of nitrogen fertilizers are applied in wheat production. Can the contents of wheat grain protein and its composition (i.e., albumin, globulin, gliadin and glutelin) be improved by optimizing the nitrogen application rate (NAR)? The relevant information to answer this question is presently lacking.
Systematic nitrogen fertilizer management strategies are important not only for enhancing wheat yield [2,3] but also for optimizing major agronomic practices that affect the content of grain protein and its composition in wheat [4]. In recent years, there have been numerous studies on the impact of nitrogen application on wheat grain protein content, yet universal conclusions are still lacking. For example, some studies have shown that nitrogen fertilizer application can increase the contents of grain protein and its composition in wheat to a certain extent [5,6]; however, the opposite conclusions have been reached in other studies. Wang et al. [7] reported that with increasing NAR, the content of stored protein increases while the content of soluble protein significantly decreases. There is still no consensus on the effect of NAR on the content of grain protein and its composition in wheat under different experimental conditions. There are many factors that influencing the regulatory effect of NAR on the contents of grain protein and its composition in wheat. For example, temperature and precipitation are the most important climatic factors affecting wheat growth and development [8]. The growth and reproduction of wheat vary with temperature, precipitation, and light. Altitude conditions can affect the physiological and biochemical processes in wheat, such as photosynthesis and the growing period, as a result of variations in solar radiation, temperature, and precipitation [9]. The initial soil nutrient contents in different wheat planting regions are quite varied [10], resulting in differing responses of the wheat grain protein content and its composition to NAR.
Temperature and precipitation regulate the accumulation of protein and its composition in wheat grain under different NARs by affecting photosynthesis, growth rate, nutrient use efficiency, and the transfer of nutrients from sources to sinks [11,12]. Appropriate increases in temperature may enhance the protein content of wheat grain via plant uptake of nitrogen from soil [13]. Conversely, higher precipitation generally results in lower protein content in wheat, which may be due to reduced nitrogen availability for protein synthesis caused by nitrogen leaching into the subsoil [14]. However, other studies have shown that precipitation does not have a significant effect on the wheat total grain protein content [15]. In addition, there are few studies on the direct effect of altitude on the contents of total grain protein and its composition. The initial soil nutrient contents can affect the absorption and utilization of nitrogen fertilizer by wheat and thus subsequently affect the accumulation of total grain protein and its composition [16,17]. Zhang et al. [18] reported that the application of nitrogen fertilizer to low-fertility soil can effectively increase the grain protein content and its composition in wheat. However, quantitative evaluations are lacking regarding the effects of NAR on total grain protein and its composition over large geographical ranges.
At present, most studies on the effects of NAR on total grain protein and its composition are based on independent field experiments. These studies have advanced our understanding of the effects of NAR on total grain protein and its composition and the related mechanisms; however, it is still difficult to quantitatively evaluate the effects of geographical factors, climatic conditions, the initial soil nutrients, and other factors on the contents of total grain protein and its composition. Meta-analysis can be used to comprehensively and quantitatively analyze existing experimental data, systematically analyze the sources of differences between results, and effectively resolve problems related to the fact that independent experiments are limited by their experimental conditions [19,20]. It is helpful to further clarify the effect of NAR on the contents of total grain protein and its composition in wheat. Meta-analyses are now being widely used in agriculture, such as to analyze the effects of farming measures on crop yield [21], soil carbon sequestration [22], and greenhouse gas emission [23]. In recent years, Solomon et al. [24] analyzed the effect of NAR on crop yield and nitrogen utilization in Ethiopia based on meta-analysis. However, there are still few studies on the effect of NAR on wheat protein content and its composition based on meta-analysis [25].
To this end, we selected 66 peer-reviewed studies from different wheat planting areas in China. We hypothesized that nitrogen content regulates the contents of total grain protein and its composition in wheat through factors such as geographical conditions, climatic conditions, and soil basic nutrient contents. Therefore, the objectives of this study are (1) to clarify the effect of NAR on the content of total grain protein and its composition in wheat under different geographical conditions, climatic conditions, soil basic nutrient contents, and other regulatory factors, and (2) to provide a theoretical basis for enhancing protein and its composition in wheat in China by optimizing nitrogen application.

2. Materials and Methods

2.1. Data Preparation

In this study, we comprehensively evaluated the effect of NAR on the contents of total grain protein and its composition (i.e., gliadin, glutenin, albumin, and globulin) in wheat under different planting conditions in China. We pre-defined the following search terms to identify relevant articles for the meta-analysis: “nitrogen”, “wheat”, “protein” and “China” in China National Knowledge Infrastructure (http://www.cnki.net/, accessed on 1 November 2021) and Web of Science (http://apps.webofknowledge.com/, accessed on 1 November 2021). The following criteria were considered when selecting the appropriate paired experiments to avoid any publication bias: (1) the test site is located in mainland China, and the test material is wheat; (2) the experiment is an outdoor field experiment; (3) the experiment includes a control (i.e., no nitrogen application) and nitrogen application treatment under the same conditions, and the rate of nitrogen application is clear; (4) the research on content includes at least total grain protein or any protein component.

2.2. Data Description

According to the above description, 66 studies were ultimately selected, with a dataset comprising 2332 paired sets on the contents of total grain protein and its composition in wheat. These paired sets include data for 973 pairs on total protein, 371 pairs on gliadin, 416 pairs on glutenin, 287 pairs on albumin, and 285 pairs on globulin. The total grain protein content and composition in wheat from the literature were the response variables. Treatments with no nitrogen fertilizer (0 kg ha−1, N0) application served as the controls, and the nitrogen fertilizer treatments were divided into five groups according to application rate: 1–120 kg ha−1 (N≤120), 121–180 kg ha−1 (N121–180), 181–240 kg ha−1 (N181–240), 241–300 kg ha−1 (N240–300), and >300 kg ha−1 (N>300). In addition to these five groups, meta-data related to the driving factors in each study were collected and added to the database as explanatory variables, as follows: (1) geographic conditions, including region and altitude, where regions were selected based on the main wheat cropping regions in China; (2) climatic conditions, including mean annual temperature (MAT) and mean annual precipitation (MAP); (3) the initial soil nutrients, including soil organic matter (SOM), total nitrogen (TN), alkali hydrolyzable nitrogen, available phosphorus, and available potassium. The specific subgroups are shown in Table 1.

2.3. Data Analysis

In this study, the logarithm of the response ratio (R) was used as the effect value (E) to describe the effect of NAR on the contents of total grain protein and its composition in wheat [26]. The formula is as follows:
E = ln R = ln ( X 1 / X 0 )
where X1 and X0 are the average values of contents of grain protein or its composition in wheat in the experimental and control groups, respectively.
To more intuitively reflect the effect of NAR on the increase in contents of total grain protein and its composition in wheat, the above reaction ratio was converted into a percentage change rate [27], and the formula is as follows:
Z Y = exp ln R − 1 × 100 %
In meta-analyses, the standard deviation reflects the importance of the results of each study. In our meta-analysis, the standard deviation directly reflects the values in the literature. If the standard deviation is missing from the literature, the value is taken as 10% of the average value of each variable [4,28].

2.4. Statistical Analysis

Microsoft Office Excel 2007 (Microsoft Corp., Redmond, WA, USA) was used for preliminary data processing. Metawin 2.1 (Sinauer Associates Inc., Sunderland, MA, USA) was used to calculate the comprehensive effect of wheat quality indicators and conduct subgroup analysis, and OriginPro 2023 Learning Edition (OriginLab. Inc. Northampton, MA, USA) software was used for graphing.

3. Results

3.1. NAR Increases the Contents of Total Protein and Its Composition in Wheat Grain

Compared to N0, the application of nitrogen fertilizer significantly increased the content of total grain protein and its composition in wheat. With the increase in NAR, the total protein content in wheat grain gradually increased (Figure 1a). Compared to N0, the total protein content in wheat grain increased by more than 25% when the NAR exceeded 240 kg ha−1. The highest increase in gliadin, gluten, albumin, and globulin contents in wheat grain was under N241–300, increasing by 30.5%, 45.4%, 15.8%, and 24.0%, respectively, compared to N0 (Figure 1b–e).

3.2. Effects of Geographical Conditions on Total Grain Protein and Its Composition in Wheat under Different NARs

NAR significantly increased the total protein content in wheat grain compared to N0 in all regions (Figure 2a), increasing by 8.69–40.1% in Southeast China, 15.5–28.1% in Northwest China, and 7.89–29.5% in North China. In addition, the contents of gliadin, glutenin, albumin and globulin in wheat grain significantly increased in North China when the NAR ranged from 0 kg ha−1 to 300 kg ha−1. NAR of 181–300 kg ha−1 significantly increased the gliadin content by 19.2–27.2% and the glutenin content by 26.4–27.8% in wheat grain compared to N0 in Northwest China. In Southeast China, the glutenin, albumin, and globulin contents at N241–300 increased by 37.6%, 15.6%, and 27.4% compared to N0, respectively (Figure 2).
Compared to N0, nitrogen application significantly increased total grain protein content by 8.19–31.4% at an altitude of 100 m or lower and 11.3–21.3% at altitudes higher than 100 m (Figure 3a). The highest total protein content was observed at N181–240 when the altitude exceeded 100 m, as well as at N241–300 when the altitude was 100 m or lower. When the altitude was ≤100 m, nitrogen application significantly increased the gliadin content by 7.54–30.5% (Figure 3b), the glutenin content by 10.9–48.0% (Figure 3c), the albumin content by 5.17–16.1% (Figure 3d), and the globulin content by 8.93–25.1% (Figure 3e) compared to N0. Meanwhile, the greatest increase in the content of these protein composition in wheat grain at N241–300 was observed at an altitude of ≤100 m. Compared to N0, NAR of N1–300 significantly increased the gliadin and glutenin content at an altitude of >100 m.

3.3. Effects of Climatic Conditions on Total Grain Protein and Its Composition in Wheat under Different NARs

Overall, the total protein content in wheat grain increased significantly under nitrogen application compared to N0, increasing by 7.20–19.5% at MATs of 12 °C or lower and 9.79–28.7% at MATs exceeding 12 °C (Figure 4a). At MATs of 12 °C or lower, the contents of all protein composition were significantly higher at N181–300 compared to N0. The NAR of N>300 significantly increased the contents of all protein composition by 4.61–44.7% compared to N0 MATs exceeding 12 °C (Figure 4b–e). The highest contents of total grain protein and its composition were observed at N241–300. Additionally, NAR had a greater effect on the increase in total protein and gluten content at MATs higher than 12 °C, and on the increase in albumin and globulin at MATs of 12 °C or lower.
Compared to N0, nitrogen application significantly increased total grain protein content by 13.4–19.4% for precipitation below 400 mm, 12.4–27.1% for 400–600 mm precipitation, and 7.89–28.7% for precipitation exceeding 600 mm, (Figure 5a). The NAR exceeded 120 kg ha−1 significantly increased all protein composition contents by 6.75–32.3% for 400–600 mm precipitation, and all protein composition contents by 7.49–30.5% for precipitation exceeding 600 mm, compared to N0 (Figure 5b–e). The highest contents of grain protein composition were observed at N241–300 when precipitation exceeded 600 mm.

3.4. Effects of the Initial Soil Nutrients on Wheat Grain Protein and Composition under Different NARs

Compared to N0, nitrogen application led to a significant increase in total grain protein content (Figure 6a). The increase was 14.0–41.1% at SOM levels below 10 g kg−1, 9.38–31.9% at 10–14 g kg−1 SOM, and 9.43–30.8% at SOM levels exceeding 14 g kg−1. The NAR exceeded 180 kg ha−1 significantly increased the contents of all protein composition by 15.4–37.9% at 10–14 g kg−1 SOM compared to N0. The NAR of N121–300 significantly increased the gliadin content by 7.56–28.7%, the glutenin content by 9.46–42.4%, and the globulin content by 5.61–30.4%. Additionally, the gliadin and globulin contents were significantly reduced by 7.40% and 8.01%, respectively, compared to N0.
NAR significantly increased total protein content in wheat grain compared to N0 at different TN levels, increasing by 14.01–43.4% at TN below 0.75 g kg−1, 7.35–37.5% at 0.75–1 g kg−1 TN, and 8.73–30.7% at TN higher than 1 g kg−1 (Figure 7a). The NAR of 120–300 kg ha−1 significantly increased the gliadin content by 15.3–20.7%, the glutenin content by 16.4–32.1%, the albumin content by 8.64–19.5%, and the globulin content by 18.1–36.2% at 0.75–1 g kg−1 TN (Figure 7b–e).
Nitrogen application significantly increased total grain protein content compared to N0 by 7.41–32.7% at alkali hydrolyzable nitrogen levels of 60–100 mg kg−1 and 6.49–18.5% when alkali hydrolyzable nitrogen exceeded 100 mg kg−1 (Figure 8a). Nitrogen application significantly increased the gliadin content by 10.7–27.3%, the glutenin content by 14.7–31.4%, the albumin content by 8.27–25.3%, and the globulin content by 9.50–31.5% at alkali hydrolyzable nitrogen levels of 60–100 mg kg−1 (Figure 8b–e). In addition, the gliadin and globulin contents were significantly reduced by 7.40% and 8.01%, respectively, at N>300 compared to N0 when alkali hydrolyzable nitrogen exceeded 100 mg kg−1.
Compared to N0, nitrogen application significantly increased total grain protein content by 12.5–51.1% at available phosphorus levels of 5–10 mg kg−1, 8.62–26.3% at available phosphorus levels of 10–20 mg kg−1, and 8.7–30.1% at available phosphorus levels exceeding 20 mg kg−1 (Figure 9a). The NAR of 120–300 kg ha−1 significantly increased the gliadin content by 16.2–30.7%, the glutenin content by 29.5–58.3%, the albumin content by 6.83–18.5%, and the globulin content by 17.6–29.4% compared to N0 when available phosphorus exceeded 20 mg kg−1 (Figure 9b–e).
Compared to N0, nitrogen application significantly increased total grain protein content by 8.10–45.5% at an available potassium level of less than 80 mg kg−1, 5.56–23.6% at available potassium levels of 80–100 mg kg−1, and 9.71–29.7% at available potassium levels exceeding 100 mg kg−1. The highest total protein content was observed under N>240–300 for available potassium of less than 80 mg kg−1 and for 80–100 mg kg−1, as well as under N>300 when the available potassium exceeded 100 mg kg−1 (Figure 10). Nitrogen application significantly increased the gliadin content by 10.8–15.8% and increased the glutenin content by 13.8–37.4% at available potassium levels of 80–100 mg kg−1.

4. Discussion

4.1. The Positive Response of Total Grain Protein Content and Composition in Wheat to NAR

The NAR is one of the main factors affecting the content of total grain protein and its composition in wheat [29]. In the current study, nitrogen application significantly increased the contents of protein and its composition in wheat grain. In particular, the NAR of N>240–300 resulted in the highest contents of total grain protein and its composition compared to N0. This may be because the application of nitrogen fertilizer promotes the accumulation of nitrogen in wheat grain. The formation of total grain protein is closely related to the absorption of nitrogen by crops and the remobilization of nitrogen after flowering [30]. The nitrogen absorbed by plants before flowering can be used to maintain and prolong photosynthesis. In the late stage of reproductive growth, the nitrogen absorbed by plants can be used to meet demands related to the growth and development of grains [31], which is conducive to increasing the accumulation of grain protein. Excessive nitrogen supply would result in the vigorous vegetative growth of wheat, which may reduce the ratio for transfer and distribution of photosynthetic assimilation products to the grain after anthesis and thereby affect grain protein formation. In addition, vigorous photosynthesis could promote the synthesis of wheat starch, which may result in competition for carbon and nitrogen in the grain. However, starch could dilute protein to some extent [32,33], thus reducing the protein content of the grain. Appropriate nitrogen application can increase the net photosynthetic rate of wheat in the early stage to a certain extent, increase the storage volume of wheat grain, and promote the transfer of nitrogen to the grain. This can help in reducing the aforementioned “dilution effect” [34] and, ultimately, synergistically enhance the yield and quality of wheat.

4.2. Response of Grain Protein and Its Composition to Climatic and Geographical Conditions under Different NARs

Temperature and precipitation are the main climatic factors that limit the contents of wheat grain protein composition [35]. In general, the soluble protein (i.e., albumin and globulin) of wheat grain is mainly concentrated in the aleurone layer, while the stored protein (i.e., gliadin and glutelin) is mainly concentrated in the endosperm. Shi et al. [36] reported that the protein content in the aleurone layer of wheat grain is the highest under low temperature conditions, possibly because the amino acids required for grain protein synthesis are transported from the aleurone layer to the endosperm in the early stage [37]. This may explain why higher albumin and globulin contents were observed at temperatures of ≤12 °C compared to >12 °C for the same NAR in the current study. Qin et al. [38] reported that for each 1 °C increase in temperature, the yield of wheat increases by 0.43%, and this increase in yield can somewhat reduce the protein content of wheat grain [33], which is aligned with the findings of the present study.
In most cases, the contents of total grain protein and its composition decreased with increasing MAP in this study, which may be due to the fact that higher precipitation increases nitrogen leaching and runoff in soil [39], resulting in a decrease in the nitrogen available to wheat.
Additionally, we found that NAR below 240 kg ha−1 increases the wheat grain protein content to a greater extent in Northwest China. On the one hand, there is less precipitation in this region, which favors the accumulation of grain protein. On the other hand, nitrogen application may effectively alleviate the negative impact of water shortage on crops [40]. However, a NAR exceeding 240 kg ha−1 results in a relatively small increase in wheat grain protein content in Northwest China compared to North China and Southeast China. This may be because excessive application of nitrogen under conditions of water shortage aggravates the degree of stress, which is not conducive to the growth and development of wheat. It was also found that NAR of N>240–300 increases the contents of protein and its composition in wheat grain to a greater extent at altitudes below 100 m than at those exceeding 100 m. This may be because the low-altitude areas are mainly located in the Huang-Huai-Hai region of China. There is high soil fertility in addition to abundant precipitation in the early stage of wheat growth [13] in this region, making it suitable for the growth of wheat. Moreover, the temperature in this region is higher during the late stage of growth. In the late stage, these high temperatures will lead to a decrease in wheat grain weight but increase in the protein content of wheat grain [41].

4.3. Response of Grain Protein and Its Composition to Initial Soil Nutrient Contents under Different NARs

The results of this meta-analysis demonstrate that nitrogen application in high-fertility soils can still significantly increase the protein content of wheat grain but to a lesser extent. Some studies have found that under high-fertility soil conditions, appropriate nitrogen application can significantly increase the protein content of wheat grain [42], while excessive nitrogen application in high-fertility soil will attenuate the increase in wheat grain protein content [43], similar to our results. This may be due to the high nitrogen supply capacity of high-fertility soil; wheat is highly dependent on soil nutrients, consumes more soil nutrients, consumes less chemical fertilizer, and thus the effect of nitrogen application on grain protein is poor. Moreover, some scholars believe that soil organic matter and total nitrogen content are the main limiting factors affecting crop yield [44]. We found, however, that the content of wheat grain protein and its composition did not increase with increases in soil organic matter and total nitrogen content. This may be due to the fact that organic matter and total nitrogen are not adequate representatives for characterizing the soil nitrogen supply capacity [45].
In this study, we found that under N>240–300, the content of each protein component was high when the soil available potassium content was also high. Li et al. [16] found that soil available potassium has a significant positive effect on wheat grain protein content, which is consistent with the results of this study. Zhang et al. [46] found that with increasing phosphorus application, the protein content of wheat grain decreases, which may be due to the increase in wheat yield caused by phosphorus fertilizer in addition to the dilution of protein in grains. Our meta-analysis results also revealed that when the soil available phosphorus content was low, nitrogen application led to a greater increase in wheat grain protein content. This may be due to the “compensatory and symmetrical response” between N and P [47]. When the supply of phosphorus in the soil is insufficient, the plant can compensate for this deficiency by increasing nitrogen uptake and utilization. When the nitrogen supply increases, the absorption and utilization of phosphorus by plants will also increase. Meanwhile, phosphorus plays an important regulatory role in the growth and development of wheat [48], including through energy transfer, nucleic acid formation, and coenzyme function [49]. Phosphorus deficiency can lead to poor root development in crops, thereby reducing plant growth [48], and the co-application of P and K increases the nitrogen use efficiency of wheat [50]. Therefore, the contents of nitrogen, phosphorus, and potassium in soil should be simultaneously regulated to ensure the efficient increase of wheat grain protein and its composition.

4.4. Limitations and Prospects

In this study, we conducted a meta-analysis based on the specified criteria. The goal was to better understand the effect of nitrogen application rate on the content of total grain protein and its composition in wheat that were under N treatment, which is shown for different categorical factors in Figure S1. However, this study has several limitations. Firstly, farm management practices, such as fertilizer application methods and number of nitrogen follow-ups, varied across studies, which may have introduced uncertainty into the study results. Secondly, the increase in protein content may also be due to a reduction in grain weight and thus grain yield. However, the yield data have not been collected in our study. In future studies, we will consider the effect of nitrogen application rate on the total protein and its composition in the context of wheat yield. In addition, we focused on optimizing nitrogen application to enhance the contents of wheat grain protein and its composition in this study. Excessive nitrogen fertilizer application can cause a cascade of environmental problems, such as water eutrophication, smog, acid rain, stratospheric ozone depletion, and biodiversity loss [51]. However, we have ignored the environmental impacts of excessive application of nitrogen fertilizer. This may result in our recommended nitrogen rate being overestimated. In future studies, it is important to take environmental effects into account when assessing the effect of nitrogen application on protein content.

5. Conclusions

From our meta-analysis, we found that the NAR of 240–300 kg ha−1 resulted in the highest increases in total protein, gliadin, gluten, albumin, and globulin contents, compared to no nitrogen application, based on studies conducted in China. Our results can serve as a reference for improving the grain protein content by optimizing the nitrogen application rate in China. However, the effect of greater nitrogen fertilizer application on environmental problems should be considered in the future.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14061164/s1, Figure S1: the content of total grain protein and its composition in wheat under N0 treatment for different categorical factors.

Author Contributions

Conceptualization, J.-F.X. and H.-Y.A.; methodology, J.-F.X.; writing—original draft preparation, H.-Y.A., J.-J.H. and Q.-N.H.; resources, J.-F.X. and Z.-Q.G.; formal analysis, J.-F.X., J.-J.H. and H.-Y.A.; writing—review and editing, J.-F.X., P.W., Y.-C.W. and T.-Q.D.; data curation, H.-Y.A., J.-J.H., Q.-N.H. and Y.-L.Z.; funding acquisition, J.-F.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key R&D Program of China (2021YFD1901102), Graduate Education Reform and Quality Improvement Program of the College of Agriculture, Shanxi Agricultural University (2023YDT01), Graduate Education Reform and Quality Improvement Program of College of Agriculture, Shanxi Agricultural University (2023YCX14, 2023YCX19, 2023YCX35).

Data Availability Statement

All data related to this study are open access, and the databases, websites, and software information used have been detailed in the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 14 01164 g001
Figure 1. Effects of NAR on contents of total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain. NAR indicates nitrogen application rate. The 95% confidence interval is represented by the error line, and the invalid line is represented by the dotted line. When there is no intersection between the error line and the invalid line, the difference is significant (p < 0.05). These numbers in brackets represent the number of corresponding grouped data pairs. The same definitions are used in subsequent figures.
Figure 1. Effects of NAR on contents of total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain. NAR indicates nitrogen application rate. The 95% confidence interval is represented by the error line, and the invalid line is represented by the dotted line. When there is no intersection between the error line and the invalid line, the difference is significant (p < 0.05). These numbers in brackets represent the number of corresponding grouped data pairs. The same definitions are used in subsequent figures.
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Figure 2. Changes in contents of total grain protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat in different regions of China. SEC, NWC, and NC indicate Southeast China, Northwest China, and North China, respectively. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 2. Changes in contents of total grain protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat in different regions of China. SEC, NWC, and NC indicate Southeast China, Northwest China, and North China, respectively. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 3. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain at different altitudes. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 3. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain at different altitudes. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 4. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different mean annual temperatures (MATs). Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 4. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different mean annual temperatures (MATs). Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 5. Changes of total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain at different levels of mean annual precipitation (MAP). Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 5. Changes of total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain at different levels of mean annual precipitation (MAP). Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 6. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different soil organic matter (SOM) levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 6. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different soil organic matter (SOM) levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 7. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different total nitrogen (TN) levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 7. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different total nitrogen (TN) levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 8. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different alkali hydrolyzable nitrogen levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 8. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different alkali hydrolyzable nitrogen levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 9. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different available phosphorus levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 9. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different available phosphorus levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Figure 10. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different available potassium levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
Figure 10. Changes in total protein (a), gliadin (b), glutenin (c), albumin (d), and globulin (e) in wheat grain for different available potassium levels. Thereinto, i, ii, iii, iv, and v indicate changes total protein and its composition under N>300, N241–300, N181–240, N121–180, and N≤120, respectively.
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Table 1. Classification factors and subgroups based on these classification factors that affect the contents of total grain protein and its composition in wheat for meta-analysis.
Table 1. Classification factors and subgroups based on these classification factors that affect the contents of total grain protein and its composition in wheat for meta-analysis.
Categorical FactorSubgroups
123
Geographical conditionsregionNorthNorthwestSoutheast
altitude (m)≤100>100
Climatic conditionsMAP (mm)<400400–600>600
MAT (°C)<12>12
Initial soil nutrientsSOM (g kg−1)<1010–14>14
TN (g kg−1)<0.750.75–1>1
alkali hydrolyzable nitrogen (mg kg−1)<6060–100>100
available phosphorus (mg kg−1)5–1010–20>20
available potassium (mg kg−1)<8080–100>100
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An, H.-Y.; Han, J.-J.; He, Q.-N.; Zhu, Y.-L.; Wu, P.; Wang, Y.-C.; Gao, Z.-Q.; Du, T.-Q.; Xue, J.-F. Influence of Nitrogen Application Rate on Wheat Grain Protein Content and Composition in China: A Meta-Analysis. Agronomy 2024, 14, 1164. https://doi.org/10.3390/agronomy14061164

AMA Style

An H-Y, Han J-J, He Q-N, Zhu Y-L, Wu P, Wang Y-C, Gao Z-Q, Du T-Q, Xue J-F. Influence of Nitrogen Application Rate on Wheat Grain Protein Content and Composition in China: A Meta-Analysis. Agronomy. 2024; 14(6):1164. https://doi.org/10.3390/agronomy14061164

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An, Hao-Yuan, Jing-Jing Han, Qian-Nan He, Yi-Lin Zhu, Peng Wu, Yue-Chao Wang, Zhi-Qiang Gao, Tian-Qing Du, and Jian-Fu Xue. 2024. "Influence of Nitrogen Application Rate on Wheat Grain Protein Content and Composition in China: A Meta-Analysis" Agronomy 14, no. 6: 1164. https://doi.org/10.3390/agronomy14061164

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