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Abstract In this study， 50 wheat varieties promoted since the founding of the Peoples Republic of China in Shandong Province were used as materials to analyze the differences in the agronomic traits between main wheat varieties promoted in different ages. The results showed that in variety replacement process， the plant height decreased significantly， the 1 000-grain weight increased extremely significantly， and the single-stem economic yield and biological yield decreased first and then increased; the economic coefficient increased significantly， and the ear length， number of grains per ear， total number of spikelets and number of fertile spikelets had no obvious trends in the past few decades; and the changes in single-stem biological yield and economic yield were not consistent with the trend in yield per unit area， indicating that coordinating the relationship between population yield and individual yield is an important direction to improve yield.
Key words Different ages; Wheat; Agronomic traits; Evolution
Wheat （Triticum aestivum L.） is the main food crop of about 35% of the worlds population[1-5]. It ranks second in consumption in China， and bears the staple food supply of more than half of Chinas population. Its annual planting area is over 2 666.67×104 hm2 in China， accounting for about 27% of the area of food crops. Its total production is over 100 million tons， accounting for about 22% of the production of food crops[6-8]. Specifically， the winter wheat area accounts for about 85% of the total wheat area， and the yield accounts for about 89% of the total wheat yield[9-10]. Since the founding of New China， each wheat region in China has undergone seven to nine large-scale variety replacements， and each variety update increased yield by about 10%.
Shandong Province occupies a pivotal position in the selection and breeding of wheat varieties across China. In response to prominent problems such as lodging， weak disease resistance and poor quality in wheat production in specific historical periods， eight large-scale variety replacements have been conducted[11-12]. Each breeding breakthrough is significantly related to the discovery and utilization of superior germplasm resources and specific superior genes[11]. With the continuous development of wheat production and breeding， variety replacement has become an inevitable development trend[13-16]. According to statistics， in the early days of the founding of the Peoples Republic of China， 981 local varieties were used in production in Shandong Province. Before the establishment of Shandong Provincial Crop Variety Examination and Approval Committee in 1982， more than 300 wheat varieties were bred in the province， of which more than 80 had large application areas， and 26 wheat varieties were certified[17-18]. In this study， according to the division of evolutionary stages recognized by the academic community， 141 main varieties in the process of variety replacement in Shandong Province were selected as experimental materials for the fixed-point experiments in two consecutive years， and the differences in agronomic traits of the main wheat varieties promoted in different ages were discussed， so as to clarify the evolution laws of the main agronomic traits in the main varieties promoted in different ages and to provide excellent genetic germplasm resources and data support for the selection and breeding of new varieties and the innovation of cultivation technology.
Materials and Methods
Experimental materials
This study collected 141 wheat germplasm resources from the National Crop Germplasm Bank， Shandong Academy of Agricultural Sciences， and prefectural and municipal academies of agricultural sciences in Shandong Province. The collected materials meet the following conditions： ① the varieties approved since the implementation of the variety examination system and certified varieties with an annual planting area of more than 33 300 hm2， ② the varieties that had been promoted in an area over 66 700 hm2 for three consecutive years before the implementation of the variety examination system， including local varieties， introduced varieties and bred varieties， ③ the varieties that have not been approved by Shandong Province but have been approved （certified） by other provinces， and ④ the varieties that have not been approved but have special performance.
The 141 collected wheat varieties of different ages were sorted and collected， and they were propagated in the experimental bases in Jinan， Shandong and Dezhou， Shandong. Under field sparse seeding conditions （plant spacing of 5 cm， row spacing of 40 cm）， a single-factor randomized block design was used， and each variety was planted in a plot with a plot area of 3 m×3 m， with three repetitions which were consistent in basic seedlings. Before sowing， the bottom fertilizer was uniformly applied with pure N 120 kg/hm2， P2O5 105 kg/hm2， and K2O 105 kg/hm2. After the seedlings returned to green， they should be protected by a nylon net in time to prevent lodging， and pure N was applied at 120 kg/hm2 in the middle of the jointing stage. Watering was performed 3 times before overwintering， before jointing and in flowering stage， respectively. Other management was the same as the general high-yield fields. The collected 141 wheat germplasm resources were observed and evaluated by comprehensive agronomic traits， taking into account factors such as age representativeness， and 50 wheat varieties were selected as materials for further experiment and were divided into 8 representative replacement times： ① Pingyuan 50， Huangxian Dalibanmang， Youzimai， Biansuimai and Tainong 153， ② Bima 1， Bima 4， Nongda 183， Zaoyangmai， Qianjiaomai and Xuzhou 438， ③ Jinan 2， Yuejin 5， Yuejin 8 and Jinan 4， ④ Jinan 8， Hongyoubao， Luteng 1， Jinanai 6， Jinan 9 and Abo， ⑤ Taishan 1， Taishan 4， Yannong 15 ， Taishan 5， Xiaoyan 6， Baigao 38， Fengchan 3 and Aimengniu， ⑥ Jimai 13， Shannongfu 63， Lumai 1， Lumai 8， Fengkang 13 and Lumai 5， ⑦ Lumai 14， Lumai 15， Lumai 21， Lumai 22， Lu 215953， Laizhou 953 and Lumai 19， ⑧ Zimai 12， Jimai 20， Shannong 8355， Shannong 664， Jimai 22， Luyuan 502， Taishan 23 and Yannong 19.
Experimental time and location
The experiment was carried out from October 2014 to June 2016 at the Experimental Farm of Dezhou Academy of Agricultural Sciences. The experimental field had medium loam， containing organic matter 1.74%， total nitrogen 0.61 g/kg， available phosphorus 35.19 mg/kg and rapidly available potassium 87.67 mg/kg in the 0-20 cm plough layer.
Experimental design and implementation situation
Under field sparse seeding conditions， a single-factor randomized block design was used. Each variety was planted in a plot with a plot area of 3 m×3 m according to plant spacing of 5 cm and row spacing of 40 cm， with three repetitions which were consistent in basic seedlings. Before sowing， the bottom fertilizer was uniformly applied with pure N 120 kg/hm2， P2O5 105 kg/hm2， and K2O 105 kg/hm2. After the seedlings returned to green， they should be protected by a nylon net in time to prevent lodging， and pure N was applied at 120 kg/hm2 in the middle of the jointing stage. Watering was performed 3 times before overwintering， before jointing and in flowering stage， respectively. Other management was the same as the general high-yield fields.
Determination items and methods
During maturation stage， 40 single stems were randomly selected from each plot for indoor test， and indicators including plant height and internode lengths of the last three nodes， ear length， number of grains per ear， total number of spikelets and number of fertile spikelets in the ear part， and 1 000-grain weight and economic coefficient were determined. Results and Analysis
Evolution trend of plant heights of main wheat varieties promoted in different ages
Plant height has a significant impact on the efficiency of soil and light energy utilization. The reduction of plant height can reduce the distribution ratio of leaf photosynthetic products to stalks， and simultaneously improve the lodging resistance of wheat， so as to make full use of higher fertilizer and water conditions. It can be known from Table 1 that the plant heights of the replacement wheat varieties showed a very significant decrease trend with the passage of time， from 118.06 cm in the 1950s to 79.72 cm. The decrease reached 38.34 cm， and the amplitude of decrease was up to 32.48%. Among the different wheat varieties， Qianjiaomai in the second replacement had the highest plant height at 138.00 cm， and the lowest was 72.00 cm in Jimai 22 in the eighth replacement. Before the fourth replacement， the average plant heights were all above 110 cm. From the fourth replacement， the plant height decreased and tended to be stable， basically stable at about 85 cm.
This study showed that the regression equation between plant height and times of evolutions was Y=-6.289 1x+125.95， R=0.890 9， indicating that there was a significant negative correlation between plant height and times of evolutions， and the average plant height of wheat decreased by 6.29 cm each time.
Evolution trend of internode lengths of main wheat varieties promoted in different ages
As can be seen from Table 1， the internode lengths of the last three nodes of wheat varieties at different stages decreased significantly. The first internode decreased from 31.77 to 21.65 cm， showing a decrease of 10.12 cm and a decreasing amplitude of 31.85%. The second internode decreased from 24.57 to 16.65 cm， with a decrease of 7.92 cm and a decreasing amplitude of 32.23%. The third internode decreased from 17.04 to 11.25 cm， with a decrease of 5.79 cm and a decreasing amplitude of 33.98%. It showed that with the progress of evolution， the internodes all showed a downward trend. The relationships between various internode lengths and the evolution process were consistent with following equations， respectively：
Y1=-1.510 3x + 32.017 （x ranges from 1 to 8）， R=0.707 8
Y2=-0.907 6x + 23.026 （x ranges from 1 to 8）， R=0.753 7
Y3= -0.805 3x + 17.698 （x ranges from 1 to 8）， R=0.732 6
The intercept of the first internode equation was the largest， indicating that the shortening of the first internode contributed the least to the reduction of plant height. The correlation coefficient in the second internode was the largest， indicating that the length of this internode was most closely related to plant height. Evolution trends of ear traits of main wheat varieties promoted in different ages
It can be seen from Table 1 that the ear lengths of the 50 main varieties of different ages increased from 7.34 to 8.96 cm， by 22.07% with the replacement times increasing， and the ear length of the seventh replacement was the largest at 9.15 cm. The linear equation between ear length and evolution times was Y=0.165x+7.736 7， R=0.429 4， so the correlation was not significant.
The total number of spikelets changed from 19.18 for the first replacement to 20.56 for the current varieties， and the values therebetween were 18.87， 19.42， 18.27， 18.65 and 18.07， respectively. The linear equation between the total number of spikelets and the times of evolution was Y=0.107 8x+18.577， and R=0.111 4. The correlation coefficient was small， indicating that the total number of spikelets did not change much with variety evolution.
The number of fertile spikelets changed from 18.10 for the first variety replacement to 19.33 for the eighth variety replacement， and the values therebetween were 17.78， 18.33， 17.30， 17.40， 16.89 and 18.28， sequentially. The linear equation between the number of fertile spikelets and the times of evolution was Y=0.085 4x+17.553， and R=0.079 3. The correlation between the two was not significant.
Evolution trends of single-stem productivity of main wheat varieties promoted in different ages
The number of grains per ear was one of the components of yield. This study showed that the linear equation between the number of grains per ear and the times of evolutions was Y=0.157 5x+38.855， R=0.018 2， indicating that there was no correlation between the number of grains per ear and the times of evolution. 1 000-grain weight increased gradually with the evolution of the varieties （Table 1）， from 31.54 g for the first replacement to 42.31 g for the current varieties， with an increase of 10.77 g and an increasing amplitude of 34.15%. The regression equation between the 1 000-grain weight and the times of evolution was Y=1.255 8x+30.388， R=0.842 9， and the two had a very significant positive correlation. It can be seen from the intercept that each time the variety evolved， the 1 000-grain weight increased by an average of 1.255 8 g. In terms of the overall trend， within the eight times of variety evolution， the 1 000-grain weights of wheat varieties showed a very significant increase trend.
The single-stem economic yield showed a trend of first decreasing and then increasing， from 1.55 g in the first replacement to the current 1.87 g， with an increase of 0.32 g and a relative increase of 20.65%. The evolution trend of single-stem economic coefficients of main wheat varieties promoted in different ages
The single-stem economic yield and biological yield are important indicators to evaluate the individual production potential of wheat. Table 1 shows that due to the high plant height of the early varieties， the single-stem biomass yield was as high as 4.23 g. During the second， third， and fourth replacements， the plant height decreased greatly， and the single-stem biological yield also greatly decreased. In the third replacement process， the value was minimum， only 3.42 g， which decreased by 19.15% compared with the first one. Since then， the single-stem biological yield increased significantly， and that of the current varieties increased to 4.10 g. The overall trend was to fall first and then rise.
The regression equation of single-stem biomass and economic yield was Y=1.656x+1.185 6， R2=0.738 6. It showed that there was a very significant positive correlation between single-stem economic yield and biological yield. That is to say， the economic yield increases accordingly with the biological yield increasing. Therefore， increasing the single-stem biological yield of wheat is also an important way to increase economic yield. As can be seen from Table 1， the economic coefficient showed a significant and steady increase trend with the advancement of evolution， from 36.75% of the first replacement to the current 45.69%， with an increasing amplitude of 24.33%. The regression equation between the average economic coefficient and the times of evolutions of previous replacement varieties was Y=0.012 5x+0.358 3， R2=0.956 9. It showed that there was a very significant positive correlation between the economic coefficient and the times of evolution.
Conclusions and Discussion
Collection， protection and safe preservation of main wheat varieties promoted in the past 60 years
Shandong Province plays an important role in the selection and breeding of wheat varieties and the promotion of improved varieties. Since the founding of the Peoples Republic of China， there have been eight large-scale variety replacements. Each breeding breakthrough is significantly related to the discovery and utilization of superior germplasm resources and specific superior genes. In this study， we collected 141 wheat germplasm resources from the National Crop Germplasm Bank， Shandong Academy of Agricultural Sciences， and prefectural and municipal academies of Agricultural Sciences of Shandong Province， and preserved them in the germplasm bank of the Crop Germplasm Resource Center of Shandong Province after expanding propagation. The evolution trends of main agronomic traits of main wheat varieties promoted in the past 60 years
A systematic study of the main agronomic traits of main wheat cultivars promoted in different ages can provide useful inspirations from the correlation between the changes in the main traits and the changes in yield[19-20]， which will help breeders to correctly select breeding goals when improving varieties.
The results showed that plant height had a very apparent decreasing trend with the replacement of varieties， and there was a significant negative correlation between plant height and the times of evolution. The reduction in plant height was caused by shortened internodes. The shortening of the second internode contributed the most to the reduction of plant height， while the shortening of the first internode contributed the least to the reduction of plant height， but greatly improved the plant height component index， making the plant type of wheat more reasonable and facilitating wheat lodging resistance and high yield.
The correlation of ear length， total number of spikelets and number of fertile spikelets with the times of evolution was not significant during the variety replacement process， and the differences between varieties of the same age were also large. Therefore， they cannot be used as a single selection indicator for variety improvement.
The number of grains per ear differed greatly between varieties of different ages， but the change law had nothing to do with evolution， and the contribution of number of grains per ear was small in the process of variety improvement. Each evolution of wheat varieties was accompanied by an increase in 1 000-grain weight. With the replacement of varieties， the per unit yield level of wheat gradually increased in production， but the change trends of single-stem biological yield and single-stem economic yield are not consistent with the change trend of yield per unit area in production， which indicates that the level of yield depends on the coordinated development between population and individuals， and we should not only pursue the improvement of individual productivity. The economic coefficient showed a significant and steady increase trend with the advancement of evolution. The analysis showed that there was a very significant positive correlation between the average economic coefficients of all previous replacement varieties and the times of evolutions.
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Key words Different ages; Wheat; Agronomic traits; Evolution
Wheat （Triticum aestivum L.） is the main food crop of about 35% of the worlds population[1-5]. It ranks second in consumption in China， and bears the staple food supply of more than half of Chinas population. Its annual planting area is over 2 666.67×104 hm2 in China， accounting for about 27% of the area of food crops. Its total production is over 100 million tons， accounting for about 22% of the production of food crops[6-8]. Specifically， the winter wheat area accounts for about 85% of the total wheat area， and the yield accounts for about 89% of the total wheat yield[9-10]. Since the founding of New China， each wheat region in China has undergone seven to nine large-scale variety replacements， and each variety update increased yield by about 10%.
Shandong Province occupies a pivotal position in the selection and breeding of wheat varieties across China. In response to prominent problems such as lodging， weak disease resistance and poor quality in wheat production in specific historical periods， eight large-scale variety replacements have been conducted[11-12]. Each breeding breakthrough is significantly related to the discovery and utilization of superior germplasm resources and specific superior genes[11]. With the continuous development of wheat production and breeding， variety replacement has become an inevitable development trend[13-16]. According to statistics， in the early days of the founding of the Peoples Republic of China， 981 local varieties were used in production in Shandong Province. Before the establishment of Shandong Provincial Crop Variety Examination and Approval Committee in 1982， more than 300 wheat varieties were bred in the province， of which more than 80 had large application areas， and 26 wheat varieties were certified[17-18]. In this study， according to the division of evolutionary stages recognized by the academic community， 141 main varieties in the process of variety replacement in Shandong Province were selected as experimental materials for the fixed-point experiments in two consecutive years， and the differences in agronomic traits of the main wheat varieties promoted in different ages were discussed， so as to clarify the evolution laws of the main agronomic traits in the main varieties promoted in different ages and to provide excellent genetic germplasm resources and data support for the selection and breeding of new varieties and the innovation of cultivation technology.
Materials and Methods
Experimental materials
This study collected 141 wheat germplasm resources from the National Crop Germplasm Bank， Shandong Academy of Agricultural Sciences， and prefectural and municipal academies of agricultural sciences in Shandong Province. The collected materials meet the following conditions： ① the varieties approved since the implementation of the variety examination system and certified varieties with an annual planting area of more than 33 300 hm2， ② the varieties that had been promoted in an area over 66 700 hm2 for three consecutive years before the implementation of the variety examination system， including local varieties， introduced varieties and bred varieties， ③ the varieties that have not been approved by Shandong Province but have been approved （certified） by other provinces， and ④ the varieties that have not been approved but have special performance.
The 141 collected wheat varieties of different ages were sorted and collected， and they were propagated in the experimental bases in Jinan， Shandong and Dezhou， Shandong. Under field sparse seeding conditions （plant spacing of 5 cm， row spacing of 40 cm）， a single-factor randomized block design was used， and each variety was planted in a plot with a plot area of 3 m×3 m， with three repetitions which were consistent in basic seedlings. Before sowing， the bottom fertilizer was uniformly applied with pure N 120 kg/hm2， P2O5 105 kg/hm2， and K2O 105 kg/hm2. After the seedlings returned to green， they should be protected by a nylon net in time to prevent lodging， and pure N was applied at 120 kg/hm2 in the middle of the jointing stage. Watering was performed 3 times before overwintering， before jointing and in flowering stage， respectively. Other management was the same as the general high-yield fields. The collected 141 wheat germplasm resources were observed and evaluated by comprehensive agronomic traits， taking into account factors such as age representativeness， and 50 wheat varieties were selected as materials for further experiment and were divided into 8 representative replacement times： ① Pingyuan 50， Huangxian Dalibanmang， Youzimai， Biansuimai and Tainong 153， ② Bima 1， Bima 4， Nongda 183， Zaoyangmai， Qianjiaomai and Xuzhou 438， ③ Jinan 2， Yuejin 5， Yuejin 8 and Jinan 4， ④ Jinan 8， Hongyoubao， Luteng 1， Jinanai 6， Jinan 9 and Abo， ⑤ Taishan 1， Taishan 4， Yannong 15 ， Taishan 5， Xiaoyan 6， Baigao 38， Fengchan 3 and Aimengniu， ⑥ Jimai 13， Shannongfu 63， Lumai 1， Lumai 8， Fengkang 13 and Lumai 5， ⑦ Lumai 14， Lumai 15， Lumai 21， Lumai 22， Lu 215953， Laizhou 953 and Lumai 19， ⑧ Zimai 12， Jimai 20， Shannong 8355， Shannong 664， Jimai 22， Luyuan 502， Taishan 23 and Yannong 19.
Experimental time and location
The experiment was carried out from October 2014 to June 2016 at the Experimental Farm of Dezhou Academy of Agricultural Sciences. The experimental field had medium loam， containing organic matter 1.74%， total nitrogen 0.61 g/kg， available phosphorus 35.19 mg/kg and rapidly available potassium 87.67 mg/kg in the 0-20 cm plough layer.
Experimental design and implementation situation
Under field sparse seeding conditions， a single-factor randomized block design was used. Each variety was planted in a plot with a plot area of 3 m×3 m according to plant spacing of 5 cm and row spacing of 40 cm， with three repetitions which were consistent in basic seedlings. Before sowing， the bottom fertilizer was uniformly applied with pure N 120 kg/hm2， P2O5 105 kg/hm2， and K2O 105 kg/hm2. After the seedlings returned to green， they should be protected by a nylon net in time to prevent lodging， and pure N was applied at 120 kg/hm2 in the middle of the jointing stage. Watering was performed 3 times before overwintering， before jointing and in flowering stage， respectively. Other management was the same as the general high-yield fields.
Determination items and methods
During maturation stage， 40 single stems were randomly selected from each plot for indoor test， and indicators including plant height and internode lengths of the last three nodes， ear length， number of grains per ear， total number of spikelets and number of fertile spikelets in the ear part， and 1 000-grain weight and economic coefficient were determined. Results and Analysis
Evolution trend of plant heights of main wheat varieties promoted in different ages
Plant height has a significant impact on the efficiency of soil and light energy utilization. The reduction of plant height can reduce the distribution ratio of leaf photosynthetic products to stalks， and simultaneously improve the lodging resistance of wheat， so as to make full use of higher fertilizer and water conditions. It can be known from Table 1 that the plant heights of the replacement wheat varieties showed a very significant decrease trend with the passage of time， from 118.06 cm in the 1950s to 79.72 cm. The decrease reached 38.34 cm， and the amplitude of decrease was up to 32.48%. Among the different wheat varieties， Qianjiaomai in the second replacement had the highest plant height at 138.00 cm， and the lowest was 72.00 cm in Jimai 22 in the eighth replacement. Before the fourth replacement， the average plant heights were all above 110 cm. From the fourth replacement， the plant height decreased and tended to be stable， basically stable at about 85 cm.
This study showed that the regression equation between plant height and times of evolutions was Y=-6.289 1x+125.95， R=0.890 9， indicating that there was a significant negative correlation between plant height and times of evolutions， and the average plant height of wheat decreased by 6.29 cm each time.
Evolution trend of internode lengths of main wheat varieties promoted in different ages
As can be seen from Table 1， the internode lengths of the last three nodes of wheat varieties at different stages decreased significantly. The first internode decreased from 31.77 to 21.65 cm， showing a decrease of 10.12 cm and a decreasing amplitude of 31.85%. The second internode decreased from 24.57 to 16.65 cm， with a decrease of 7.92 cm and a decreasing amplitude of 32.23%. The third internode decreased from 17.04 to 11.25 cm， with a decrease of 5.79 cm and a decreasing amplitude of 33.98%. It showed that with the progress of evolution， the internodes all showed a downward trend. The relationships between various internode lengths and the evolution process were consistent with following equations， respectively：
Y1=-1.510 3x + 32.017 （x ranges from 1 to 8）， R=0.707 8
Y2=-0.907 6x + 23.026 （x ranges from 1 to 8）， R=0.753 7
Y3= -0.805 3x + 17.698 （x ranges from 1 to 8）， R=0.732 6
The intercept of the first internode equation was the largest， indicating that the shortening of the first internode contributed the least to the reduction of plant height. The correlation coefficient in the second internode was the largest， indicating that the length of this internode was most closely related to plant height. Evolution trends of ear traits of main wheat varieties promoted in different ages
It can be seen from Table 1 that the ear lengths of the 50 main varieties of different ages increased from 7.34 to 8.96 cm， by 22.07% with the replacement times increasing， and the ear length of the seventh replacement was the largest at 9.15 cm. The linear equation between ear length and evolution times was Y=0.165x+7.736 7， R=0.429 4， so the correlation was not significant.
The total number of spikelets changed from 19.18 for the first replacement to 20.56 for the current varieties， and the values therebetween were 18.87， 19.42， 18.27， 18.65 and 18.07， respectively. The linear equation between the total number of spikelets and the times of evolution was Y=0.107 8x+18.577， and R=0.111 4. The correlation coefficient was small， indicating that the total number of spikelets did not change much with variety evolution.
The number of fertile spikelets changed from 18.10 for the first variety replacement to 19.33 for the eighth variety replacement， and the values therebetween were 17.78， 18.33， 17.30， 17.40， 16.89 and 18.28， sequentially. The linear equation between the number of fertile spikelets and the times of evolution was Y=0.085 4x+17.553， and R=0.079 3. The correlation between the two was not significant.
Evolution trends of single-stem productivity of main wheat varieties promoted in different ages
The number of grains per ear was one of the components of yield. This study showed that the linear equation between the number of grains per ear and the times of evolutions was Y=0.157 5x+38.855， R=0.018 2， indicating that there was no correlation between the number of grains per ear and the times of evolution. 1 000-grain weight increased gradually with the evolution of the varieties （Table 1）， from 31.54 g for the first replacement to 42.31 g for the current varieties， with an increase of 10.77 g and an increasing amplitude of 34.15%. The regression equation between the 1 000-grain weight and the times of evolution was Y=1.255 8x+30.388， R=0.842 9， and the two had a very significant positive correlation. It can be seen from the intercept that each time the variety evolved， the 1 000-grain weight increased by an average of 1.255 8 g. In terms of the overall trend， within the eight times of variety evolution， the 1 000-grain weights of wheat varieties showed a very significant increase trend.
The single-stem economic yield showed a trend of first decreasing and then increasing， from 1.55 g in the first replacement to the current 1.87 g， with an increase of 0.32 g and a relative increase of 20.65%. The evolution trend of single-stem economic coefficients of main wheat varieties promoted in different ages
The single-stem economic yield and biological yield are important indicators to evaluate the individual production potential of wheat. Table 1 shows that due to the high plant height of the early varieties， the single-stem biomass yield was as high as 4.23 g. During the second， third， and fourth replacements， the plant height decreased greatly， and the single-stem biological yield also greatly decreased. In the third replacement process， the value was minimum， only 3.42 g， which decreased by 19.15% compared with the first one. Since then， the single-stem biological yield increased significantly， and that of the current varieties increased to 4.10 g. The overall trend was to fall first and then rise.
The regression equation of single-stem biomass and economic yield was Y=1.656x+1.185 6， R2=0.738 6. It showed that there was a very significant positive correlation between single-stem economic yield and biological yield. That is to say， the economic yield increases accordingly with the biological yield increasing. Therefore， increasing the single-stem biological yield of wheat is also an important way to increase economic yield. As can be seen from Table 1， the economic coefficient showed a significant and steady increase trend with the advancement of evolution， from 36.75% of the first replacement to the current 45.69%， with an increasing amplitude of 24.33%. The regression equation between the average economic coefficient and the times of evolutions of previous replacement varieties was Y=0.012 5x+0.358 3， R2=0.956 9. It showed that there was a very significant positive correlation between the economic coefficient and the times of evolution.
Conclusions and Discussion
Collection， protection and safe preservation of main wheat varieties promoted in the past 60 years
Shandong Province plays an important role in the selection and breeding of wheat varieties and the promotion of improved varieties. Since the founding of the Peoples Republic of China， there have been eight large-scale variety replacements. Each breeding breakthrough is significantly related to the discovery and utilization of superior germplasm resources and specific superior genes. In this study， we collected 141 wheat germplasm resources from the National Crop Germplasm Bank， Shandong Academy of Agricultural Sciences， and prefectural and municipal academies of Agricultural Sciences of Shandong Province， and preserved them in the germplasm bank of the Crop Germplasm Resource Center of Shandong Province after expanding propagation. The evolution trends of main agronomic traits of main wheat varieties promoted in the past 60 years
A systematic study of the main agronomic traits of main wheat cultivars promoted in different ages can provide useful inspirations from the correlation between the changes in the main traits and the changes in yield[19-20]， which will help breeders to correctly select breeding goals when improving varieties.
The results showed that plant height had a very apparent decreasing trend with the replacement of varieties， and there was a significant negative correlation between plant height and the times of evolution. The reduction in plant height was caused by shortened internodes. The shortening of the second internode contributed the most to the reduction of plant height， while the shortening of the first internode contributed the least to the reduction of plant height， but greatly improved the plant height component index， making the plant type of wheat more reasonable and facilitating wheat lodging resistance and high yield.
The correlation of ear length， total number of spikelets and number of fertile spikelets with the times of evolution was not significant during the variety replacement process， and the differences between varieties of the same age were also large. Therefore， they cannot be used as a single selection indicator for variety improvement.
The number of grains per ear differed greatly between varieties of different ages， but the change law had nothing to do with evolution， and the contribution of number of grains per ear was small in the process of variety improvement. Each evolution of wheat varieties was accompanied by an increase in 1 000-grain weight. With the replacement of varieties， the per unit yield level of wheat gradually increased in production， but the change trends of single-stem biological yield and single-stem economic yield are not consistent with the change trend of yield per unit area in production， which indicates that the level of yield depends on the coordinated development between population and individuals， and we should not only pursue the improvement of individual productivity. The economic coefficient showed a significant and steady increase trend with the advancement of evolution. The analysis showed that there was a very significant positive correlation between the average economic coefficients of all previous replacement varieties and the times of evolutions.
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