Wheat (Triticum aestivum L.) is the second largest food crop in the world and the third largest food crop in China. 40% of the world's population mainly feeds on Wheat (Liu et al., 2018). Wheat production has great significance to ensure national food security. The development of wheat in China can be divided into four stages, they are the recovery growth from 1949 to 1957, the stable growth from 1958 to 1978, the rapid growth of per unit area yield from 1979 to 1999 and the synchronous improvement of field and quality from 2000 to 2016 (He et al., 2018). Wheat development has experienced a period from slow to fast, and finally reach to a steady and medium-speed growth stage of increasing yield and quality. Worldwide, China's wheat yield has been at a high level. With the continuous improvement of people's living standards, higher requirements have been put forward for the improvement of wheat quality. Under the background of structural adjustment of wheat development and agricultural supply side reform, the future consumer market has higher requirements for wheat breeding, and the development of wheat needs the promotion of high yield, high quality and stress resistance (Li et al., 2019). Especially under the promotion of supply side structural reform, wheat breeding should pay more attention to the coordinated development of high yield, high efficiency, green and health. Shandong Province is the main wheat producing province. In recent years, the wheat yield per unit area in Tai'an City is higher than that in Shandong Province. According to the latest statistics in 2018, the wheat yield per unit area is 6 090 kg/hm², while the wheat yield per unit area in Tai'an City reached 6 555 kg / hm². As the main breeding unit in Tai'an City, the new varieties bred by Wheat Research Institute of Tai'an Academy of Agricultural Sciences in recent years have made a certain contribution to wheat production in Tai'an City. At the same time, with the continuous adjustment of wheat breeding objectives, high-quality breeding is also making continuous development. Since 1990, 16 varieties have been bred, analysing the evolution of yield and quality indicators has important reference significance. Therefore, this study summarized China and Shandong province wheat varieties selected in recent 30 years, and analyzed the evolution of yield, agronomic traits and grain quality, in order to provide data support for the future wheat breeding work of high yield and quality and the promotion of new varieties.

1 Results and Analysis

1.1 Yield and agronomic traits of bred varieties

The introduction of Taikemai (Taishan) series new wheat cultivars released by China and Shandong province in recent 30 years is shown in Table 1.

Table 1 Introduction of Taikemai (Taishan) series winter wheat cultivars released in recent thirty years

According to the published data of released varieties, the yield and related agronomic characters of varieties in recent 30 years were counted (Table 2). In general, the variation range of yield, yield increase and spike No. were larger than 10%, and the variation coefficient of kernels per spike, 1000-grain weight, growth period and plant height were smaller, less than 10%. From the perspective of yield, the maximum yield reached 9192.75 kg/hm², the minimum value was 5971.50 kg/hm², the average value was 8016.34 kg/hm², and the coefficient of variation reached 11.67%. According to the latest statistical data of the National Bureau of Statistics (2018), the average yield of winter wheat in Shandong Province is 6090.00 kg/hm², and the average yield of winter wheat in China is 5497.08 kg/hm². The average, maximum and minimum values of Taikemai (Taishan) were higher than the national level, and the average and maximum values were also higher than the level of Shandong province. The coefficient of variation of yield increase was 44.93%, which ranked the first in yield and agronomic traits. The main reason was that the yield increase range of early bred varieties, later high-quality varieties and dry land varieties was relatively small. Among the three yield components, the coefficient of variation of spike No. was the largest, which indicated that the selection and improvement of spike No. changed greatly in the breeding process. The coefficient of variation of growth period was the smallest, concentrated in 228 ~ 242 days. The plant height ranged from 73.3 cm to 85.3 cm, with an average of 79.0 cm, which was in the middle level, and the coefficient of variation was 4.81, with a small gap.

Table 2 Statistics analysis of yield and agronomic traits of wheat cultivars released in recent thirty years

The evolution trend of yield and agronomic characters of the bred varieties were shown in Figure 1. The yield of the varieties selected before TKM31 (2018) showed an increasing trend, later, the change of yield was not obvious. In terms of yield increase rate, before TS23 (2004), the yield increase rate continued to increase, after TS24 (2005), the yield increase rate showed a trend of decreasing and then increasing steadily, remained relatively stable. The main reason is that we began to pay attention to high quality, drought resistance and water saving, as well as the breeding of characteristic wheat. The spike No. and kernels per spike showed the opposite trend of decreasing first and then increasing, and increasing first and then decreasing, respectively. The change trend of kernels per spike and yield was consistent, which showed that the increase of yield was due to the gradual increase of kernels per spike. The 1000 grain weight increased gradually and then leveled off. The growth period increased slowly and then decreased, which was related to the gradual increase of temperature in recent years, the delay of wheat sowing time and the acceleration of growth process. Plant height showed a downward trend, but the decline rate tended to be gentle, and the decline potential was not large.

Figure 1 Evolution in yield and agronomic traits of wheat cultivars released in recent thirty years

1.2 Grain quality traits of bred varieties

Table 3 counted the grain quality traits of bred varieties in the past 30 years. Among them, the protein content varied from 12.7% to 15.6%, the average value was 13.9%, the coefficient of variation was 5.35%, and the coefficient of variation was the smallest. The wet gluten content ranged from 28.2% to 38.2%, with an average value of 32.9%, and a coefficient of variation was 9.15%, ranking second. The water absorption ranged from 51.7 to 66.3 mL/100 g, the average value was 60.1 mL/100 g, and the coefficient of variation was 6.83%, ranking third. The stabilization time ranged from 1.5 to 11.0 min, the average value was 4.5, and the coefficient of variation was 60.90%, which reached the maximum. It shows that in recent years, the stability time of bred varieties has a large difference, the variation types were rich, the wet gluten content was the second, the water absorption was the third, and the protein content was the smallest.

Table 3 Statistics analysis of grain quality traits of wheat cultivars released in recent thirty years

The evolution in grain quality traits of wheat cultivars released in recent thirty years is shown in Figure 2. It can be seen that the protein content showed an increasing trend, the wet gluten content was relatively stable, and the water absorption changed are not obvious. The stabilization time showed an increasing trend, especially in the past ten years, the number of bred high-quality wheat varieties has gradually increased, and the stabilization time was a key reference indicator, and the increasing trend was more obvious.

Figure 2 Evolution in grain quality traits of wheat cultivars released in recent thirty years

According to the national standard special wheat variety quality (GB/T 17320-2013), the certified varieties were classified (Table 4). Among the bred varieties, 1 (Taishan 27) reached the standard for strong gluten in all four indicators, and 2 (Taikemai 33, Taikemai 36) met the four indexes of medium-strong gluten. There were 5 varieties (Taishan 21, Taikemai 31, Taikemai 30, Taikezimai 1 and Taikemai 44) have reached the mid-gluten standard, and 0 variety reached the soft gluten standard in all four indicators. According to the "Main Crop Variety Approval Standards (National Level)", the wheat that does not meet the requirements of soft gluten is also medium gluten wheat. Therefore, in addition to meeting the standards of strong gluten, medium strong gluten and medium gluten, 8 varieties were classified as medium gluten wheat.

Table 4 The number of the key quality traits of wheat cultivars released in recent thirty years

2 Discussion

From 1990 to 2018, the sown area of winter wheat in China decreased from 25.93 million hectares to 22.74 million hectares, the total output increased from 85 million tons to 125 million tons, and the yield per unit area increased from 3287 kg/hm² to 5497 kg/hm² (National Bureau of Statistics). Under the circumstance of declining planting area, yield per unit and total yield increased, mainly due to the selection and breeding of high-yielding new varieties and efficient cultivation techniques (Li, 2010). The study found that the annual genetic progress of the yield potential of wheat varieties bred in Shandong Province from 1969 to 2006 was 0.82% or 59 kg/hm². The increase in the number of grains, especially the increase in the number of grains per unit area, played a significant role (Xiao et al., 2012). By analyzing the regional test varieties of Huanghuai wheat from 2001 to 2009, it was found that among the three elements of yield composition, kernels per spike had the greatest influence (Wang et al., 2013). In this study, the yield of bred varieties before 2018 showed a gradual upward trend, which was basically consistent with the change trend of kernels per spike and 1000-grain weight. This was consistent with the results of previous studies. After 2018 (including 2018), the changes in the field of bred varieties were not obvious, showing a new situation in which the output of high-yield varieties is more prominent, the number of high-quality varieties has increased, the drought-resistant and water-saving wheat has improved, and the special wheat has made breakthroughs. It can be seen that during this period, the number of kernels per spike of the variety decreased slightly, and the yield increased steadily.

In the early 1980s, the research on wheat quality improvement began. At present, the main problem with the quality of wheat varieties in China is that the stability time is not high, or the three indicators of stability time, protein content, and wet gluten content are not coordinated (Wei et al., 2013). For the improvement of wheat quality in Huanghuai wheat area, the current focus of improvement is to strengthen the improvement of protein quality and balance various quality traits (Hu et al., 2010). Studies have found that there are close genetic relationships among breeds in some regions, and the phenomenon of reduced genetic diversity (Liu et al., 2000; Zhan et al., 2006). In general, the rate of the stabilization time meets the standard of medium strong gluten is 26.7% among the certified Taikemai (Taishan) wheat varieties, which is a relatively low proportion. At the same time, it is only 20.0% that the four indicators meet the medium-strength gluten. The main problems are short stabilization time and uncoordinated quality indicators. It shows that the genetic diversity of bred varieties needs to be enriched in quality traits. Studies have found that the quality indicators of most varieties vary greatly from year to year and from place to place (Wang et al., 2013). For example, in the two-year regional test of the bred nationally approved variety Taikemai 30, the stabilization time was 6.9 min and 15 min. However, in recent years, varieties such as Taikemai 33 and Taikemai 36 have relatively balanced quality indicators, meeting the level of medium-strong gluten. It can be seen that the wheat breeding of Tai'an Academy of Agricultural Sciences has made breakthroughs in quality improvement.

In summary, the breeding goals of the Taikemai series of wheat bred in the past 30 years have been gradually adjusted, early maturity and high yield gradually adjusted to high yield and early maturity, further high-yield and stress resistance, so far, high-yield, high-quality, high-efficiency, multi-resistance, cost-saving and safety. At the same time, bred new wheat varieties such as Taikemai 33 and Taikemai 36, have excellent yield and grain quality traits, and various indicators are relatively balanced. They can be used as high-yield, high-quality wheat parents for high-yield and high-quality wheat breeding.

3 Materials and Methods

3.1 Basic situation of selected materials

Taking the wheat varieties approved by the Wheat Research Institute of Tai'an Academy of Agricultural Sciences in the past 30 years as the research object (Table 1), the yield and quality data of the varieties are derived from the China Seed Industry Big Data Platform, China Knowledge Network, and the Shandong Provincial Wheat Variety Approval Announcement (sorted by year of approval). Among the 16 varieties bred, there are 13 water-land varieties (except for dryland and purple wheat varieties), 2 dry-land varieties (Taikemai 30, Taikemai 32), and 1 purple wheat variety (Taikezimai 1). As the Lumai 18, which was bred in the early years, did not perform relevant quality analysis, only the data of yield and agronomic traits were collated and analyzed. The nationally approved varieties bred in the later period lacked relevant data on sedimentation value. Therefore, except for Lumai 18 which is not used for quality analysis, the other 15 varieties are analyzed for protein content, wet gluten content, water absorption and stabilization time.

3.2 Data analysis

The data were processed and analyzed by Microsoft Office Excel 2007 and SPSS18.0, and origin 9.0 software was used to make a diagram.

Authors’ contributions

Sun Yingying is the experimental designer and executor of this study, completing the data analysis and the writing of the first draft of the paper; Wang Ruixia, LV Guangde, Wang Chao, Sun Xianyin, Mi Yong, Qi Xiaolei, Mu qiuhuan, Chen Yongjun and Wu Ke participate in the experimental design and analysis of the experimental results; Qian Zhaoguo is the designer and director of the project, guiding the experimental design, data analysis, paper writing and research modification. All authors read and approved the final manuscript.

Acknowledgements

This study was jointly funded by Shandong Province Fine Seed Engineering Project "Cultivation of new varieties with high quality" (2019LZGC001-3), the guidance plan of technology plan of Tai'an City (2019NS094), the national wheat modern agricultural industry technology system(CARS-3-2-21)and the genetic breeding post of the wheat innovation team of modern agricultural production system in Shandong Province(SDAIT-01-05).

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