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Abstract [Objectives] The study aimed to discuss the high??yield mechanism of Yugu 18. [Methods] The characteristics of sources and sinks of Yugu 18 with high quality， high yield， wide adaptability and multi??resistance to diseases were studied by using Jigu 18 as the control variety. [Results] Yugu 18 was superior in sink capability and the ability to fill sinks. In the filling period， its photosynthetic capacity was strong， and grain??leaf ratio was high. Photosynthetic products had a great contribution to grain filling after flowering， and the coordination between sources and sinks was good. [Conclusions] The research provides theoretical basis for the breeding and cultivation of high??yield foxtail millet.
Key words Yugu 18； Sources； Sinks； High yield
Yugu 18 is a variety of foxtail millet selected by the Anyang Academy of Agricultural Sciences of Henan Province from two ecological zones Anyang and Hainan. The variety has high and stable yield and wide adaptability and is very edible as millet products. In 2009， this variety was rated as first??class high??quality rice at the Eighth National Appraisal Conference of High??quality Edible Millet held by the Professional Committee of Millet Crops， the Crop Science Society of China[1]. Then Yugu 18 passed the identification of the middle?? and late??maturing groups in summer foxtail millet regions of North China， spring foxtail millet regions of Northeast China and Northwest China and the early??maturing group in spring foxtail millet regions of Northwest China， and the identification numbers are Guopin Jiangu 2012001， Guopin Jiangu 2014009 and Guopin Jiangu 2016008. This variety has been listed as the dominant variety of foxtail millet since 2014. However， the high??yield mechanism of Yugu 18 has not been reported yet. In this paper， the characteristics of sinks and sources of Yugu 18 in the process of grain filling were studied to discuss its high??yield mechanism and then provide theoretical basis for the breeding and cultivation of high??yield foxtail millet.
Materials and Methods
Tested materials
Tested materials included the variety Yugu 18 bred by Anyang Academy of Agricultural Sciences and the control variety Jigu 19 used in the regional test of new varieties of foxtail millet.
Test design
The test was conducted at the test base of Anyang Academy of Agricultural Sciences in 2016. The planting density was 600 000 plants/hm2， and the distance between plants was 40 cm， while the length of each row was 8.33 m. There were six rows in each plot， and each treatment was repeated four times. Random block arrangement was adopted. Among them， the first repetition was the sampling area， and the other three repetitions were used to measure yield. The management during the growth period was the same as conventional field management. Determination indicators and methods
In the heading and flowering period， the plants growing uniformly and heading and flowering at the same time in each plot were tagged.
Determination of photosynthetic rate Since the full heading， the net photosynthetic rate （Pn） of middle part of the flag leaf and the second last leaf was measured every 7 d. Five plants were determined in each plot， and then the values were averaged. The determination was performed from 09：00 to 11：30 on sunny days.
Leaf area index Leaf area was calculated according the formula as follows： leaf area=length of leaf vein??width of leaf vein??0.7[2]. Leaf area index （LAI） is the ratio of total area of plant leaves to land area， namely Leaf area index=The total area of leaves/Land area.
Accumulation and transport of dry matter After flowering， ten tagged plants were taken every 7 d， from which the stems， leaves and ears were taken， washed clean， and baked at 105 ?? for 30 min and at 80 ?? to constant weight. They were weighed with an analytical balance （thousandth）.
Measurement of yield After being mature， plants in the middle four rows in repeats 2-4 were harvested to calculate per unit area yield. Moreover， five plants were harvested to calculate the mass of an ear， grain mass of an ear， mass of a thousand full grains （water floating method： the grains that sink into the water bottom are full grains）， the total number of grains per ear， number of abortive grains per ear， fruiting rate， etc.
Results and Analysis
Characteristics of sources
Changes in the net photosynthetic rate of the flag leaf and the second last leaf Photosynthesis is one of the important factors affecting crop yield， especially the photosynthesis of the last three functional leaves[2-3]. Seen from Fig.1 and Fig.2， after flowering， the net photosynthetic rate of Yugu 18 and Jigu 19 tended to decrease， and the decrease of Yugu 18 tended to be gentle. At the whole filling stage， the net photosynthetic rate of the flag leaf and the second last leaf of Yugu 18 was higher than those of Jigu 19. It shows that Yugu 18 had strong photosynthetic and assimilative capacity and long photosynthesis duration and could produce and accumulate more photosynthetic products to promote its grain filling.
Changes of leaf area index As shown in Fig.3， after flowering， the leaf area index of Yugu 18 and Jigu 19 decreased. In the first 21 d after flowering， the leaf area index of Yugu 18 changes slightly and reduced gently， showing that the function of Yugu 18 leaves lasted longer. Since 28 d after flowering， the leaf area index of Yugu 18 and Jigu 19 dropped sharply， and the decrease in the leaf area index of Yugu 18 was more obvious than that of Jigu 19. It indicates that at the late filling stage， Yugu 18 could transport dry matter from the body to its grains， which laid a foundation for later high yield. Transport of dry matter
The output rate and transformation rate of dry matter in stems can be calculated as follows： the output rate of dry matter in stems=（the mass of dry matter in stems at the full heading stage??the mass of dry matter in stems at the maturation stage）/the mass of dry matter in stems at the full heading stage??100%， and the transformation rate of dry matter in stems=（the mass of dry matter in stems at the full heading stage??mass of dry matter in stems at the maturation stage）/the mass of an ear??100%[4]. According to Table 1， the amount of dry matter transported to grains of Yugu 18 from the full heading stage to the maturation stage was 45.3% higher than that of Jigu 19， and the transformation rate of dry matter in the stems of Yugu 18 was 11.7% higher than that of Jigu 19. It indicates that Yugu 18 had good coordination after heading， and can transport dry matter from its stems to grains.
Grain??leaf ratio
Grain??leaf ratio is an important indicator of coordination between sources and sinks[5]. It can be expressed as follows： grain??leaf ratio （grains/cm2）=grains per spike/the largest leaf area of a plant， and grain??leaf ratio （mg/cm2）=mass of a spike/the largest leaf area of a plant. Seen from Table 2， the grain??leaf ratio of Yugu 18 was 1.43 times higher than that of Jigu 19， and the difference was extremely significant， showing that the coordination between sources and sinks of Yugu 18 was better than that of Jigu 19.
Characteristics of sink capacity
Sink capacity and the ability to fill sinks can be calculated as follows： sink capacity=the number of effective ears per hectare??the total number of grains per ear??the weight of a thousand full seeds??10-6， and the ability to fill sinks=The number of effective ears per hectare??the total number of grains per ear??fruiting rate??the weight of a thousand seeds expect empty seeds??10-6/sink capacity[6]. Seen from Table 3， the number of effective ears， the number of grains per ear， fruiting rate， sink capacity， yield and the ability to fill sinks of Yugu 18 was superior to those of Jigu 19. The yield of Yugu 18 was up to 6 999.38 kg/hm2， 13.38% higher than that of Jigu 19. Meanwhile， the sink capacity and the ability to fill sinks of Yugu 18 were 1.3 and 1.1 times larger than those of Jigu 19. Analysis of variance shows that there were significant differences between Yugu 18 and Jigu 19 in the number of grains per ear and fruiting rate， and the differences of sink capacity， yield and the ability to fill sinks were extremely significant. It indicates that Yugu 18 was superior in sink capacity and the ability to fill sinks， which promoted the formation of its high yield. Conclusions and Discussion
Since Mason and Maskell proposed the theory of sources and sinks， it has been used to clarify the high??yield mechanism of crops. Sources refer to the organs or tissues that produce or output assimilated substances， which are usually measured by leaf area index and photosynthetic rate. Sinks refer to the organs or tissues that utilize or store assimilated substances， and spikes are the most important sinks[7].
The test results show that the leaf area index and grain??leaf ratio of Yugu 18 were relatively large and had long photosynthesis duration and high net photosynthetic rate， which was conducive to the accumulation of dry matter and filling of grains. Yugu 18 was superior to Jigu 19 in sink capacity and the ability to fill sinks， and the transformation rate of dry matter in the stems of Yugu 18 was obviously higher than that of Jigu 19. Studies have shown that larger storage capacity and better coordination between sources and sinks are the basis of high yield. This is the important reason for the high yield of Yugu 18. Many foxtail millet ears are large， but the filling of poor grains is poor， so that its potential for high yield has not been realized[8].
References
[1] YAN HS， LIU JR， WANG SY， et al. Breeding of a new variety of foxtial millet Yugu 18[J]. Crops， 2012（3）： 147-148. （in Chinese）.
[2] WANG YQ，YANG XH，LI B， et al. Study on the relation between leaf area， photosynthetic rate and yield of wheat[J]. Acta Agriculturae Boreali??Sinica， 2008， 23（B10）： 10-15. （in Chinese）.
[3] TAN CL， ZHANG HX， DAI ZY， et al. Characteristics of sink， source and flow in good quality indica rice Yangdao 6[J]. Scientia Agricultura Sinica， 2003， 36（1）： 26-30. （in Chinese）.
[4] ZHAO BH， WANG P， ZHANG HX， et al. Analysis on source??sink and grain??filling characteristics of two??line hybrid rice Yangliangyou 6[J]. Chinese Journal of Rice Science， 2006， 20（1）： 65-72. （in Chinese）.
[5] LI ZX， ZHAO SJ， MENG QW， et al. Photosynthetic characteristics in non??leaf organs of winter wheat cultivars differing in grain??leaf ratio[J]. Acta Agronomica Sinica， 2004， 30（5）： 419-426. （in Chinese）.
[6] LIAO YP， CHEN ZM， HE XY， et al. Sink， source and flow characteristics of rice variety Yuexiangzhan with high harvest index[J]. Chinese Journal of Rice Science， 2001， 15（1）： 73-76. （in Chinese）.
[7] ZHANG ZX， LI Z， CHEN J， et al. Characteristics of sink??source??flow in "II Youhang No. 1" and "II Youhang No. 2" super hybrid rice[J]. Chinese Journal of Eco??Agriculture， 2011， 19（2）： 326-330. （in Chinese）.
[8] YANG JC， ZHANG JH. Grain??filling problem in ??super?? rice[J]. Journal of Experimental Botany，2010，61（1）：1-5.
Key words Yugu 18； Sources； Sinks； High yield
Yugu 18 is a variety of foxtail millet selected by the Anyang Academy of Agricultural Sciences of Henan Province from two ecological zones Anyang and Hainan. The variety has high and stable yield and wide adaptability and is very edible as millet products. In 2009， this variety was rated as first??class high??quality rice at the Eighth National Appraisal Conference of High??quality Edible Millet held by the Professional Committee of Millet Crops， the Crop Science Society of China[1]. Then Yugu 18 passed the identification of the middle?? and late??maturing groups in summer foxtail millet regions of North China， spring foxtail millet regions of Northeast China and Northwest China and the early??maturing group in spring foxtail millet regions of Northwest China， and the identification numbers are Guopin Jiangu 2012001， Guopin Jiangu 2014009 and Guopin Jiangu 2016008. This variety has been listed as the dominant variety of foxtail millet since 2014. However， the high??yield mechanism of Yugu 18 has not been reported yet. In this paper， the characteristics of sinks and sources of Yugu 18 in the process of grain filling were studied to discuss its high??yield mechanism and then provide theoretical basis for the breeding and cultivation of high??yield foxtail millet.
Materials and Methods
Tested materials
Tested materials included the variety Yugu 18 bred by Anyang Academy of Agricultural Sciences and the control variety Jigu 19 used in the regional test of new varieties of foxtail millet.
Test design
The test was conducted at the test base of Anyang Academy of Agricultural Sciences in 2016. The planting density was 600 000 plants/hm2， and the distance between plants was 40 cm， while the length of each row was 8.33 m. There were six rows in each plot， and each treatment was repeated four times. Random block arrangement was adopted. Among them， the first repetition was the sampling area， and the other three repetitions were used to measure yield. The management during the growth period was the same as conventional field management. Determination indicators and methods
In the heading and flowering period， the plants growing uniformly and heading and flowering at the same time in each plot were tagged.
Determination of photosynthetic rate Since the full heading， the net photosynthetic rate （Pn） of middle part of the flag leaf and the second last leaf was measured every 7 d. Five plants were determined in each plot， and then the values were averaged. The determination was performed from 09：00 to 11：30 on sunny days.
Leaf area index Leaf area was calculated according the formula as follows： leaf area=length of leaf vein??width of leaf vein??0.7[2]. Leaf area index （LAI） is the ratio of total area of plant leaves to land area， namely Leaf area index=The total area of leaves/Land area.
Accumulation and transport of dry matter After flowering， ten tagged plants were taken every 7 d， from which the stems， leaves and ears were taken， washed clean， and baked at 105 ?? for 30 min and at 80 ?? to constant weight. They were weighed with an analytical balance （thousandth）.
Measurement of yield After being mature， plants in the middle four rows in repeats 2-4 were harvested to calculate per unit area yield. Moreover， five plants were harvested to calculate the mass of an ear， grain mass of an ear， mass of a thousand full grains （water floating method： the grains that sink into the water bottom are full grains）， the total number of grains per ear， number of abortive grains per ear， fruiting rate， etc.
Results and Analysis
Characteristics of sources
Changes in the net photosynthetic rate of the flag leaf and the second last leaf Photosynthesis is one of the important factors affecting crop yield， especially the photosynthesis of the last three functional leaves[2-3]. Seen from Fig.1 and Fig.2， after flowering， the net photosynthetic rate of Yugu 18 and Jigu 19 tended to decrease， and the decrease of Yugu 18 tended to be gentle. At the whole filling stage， the net photosynthetic rate of the flag leaf and the second last leaf of Yugu 18 was higher than those of Jigu 19. It shows that Yugu 18 had strong photosynthetic and assimilative capacity and long photosynthesis duration and could produce and accumulate more photosynthetic products to promote its grain filling.
Changes of leaf area index As shown in Fig.3， after flowering， the leaf area index of Yugu 18 and Jigu 19 decreased. In the first 21 d after flowering， the leaf area index of Yugu 18 changes slightly and reduced gently， showing that the function of Yugu 18 leaves lasted longer. Since 28 d after flowering， the leaf area index of Yugu 18 and Jigu 19 dropped sharply， and the decrease in the leaf area index of Yugu 18 was more obvious than that of Jigu 19. It indicates that at the late filling stage， Yugu 18 could transport dry matter from the body to its grains， which laid a foundation for later high yield. Transport of dry matter
The output rate and transformation rate of dry matter in stems can be calculated as follows： the output rate of dry matter in stems=（the mass of dry matter in stems at the full heading stage??the mass of dry matter in stems at the maturation stage）/the mass of dry matter in stems at the full heading stage??100%， and the transformation rate of dry matter in stems=（the mass of dry matter in stems at the full heading stage??mass of dry matter in stems at the maturation stage）/the mass of an ear??100%[4]. According to Table 1， the amount of dry matter transported to grains of Yugu 18 from the full heading stage to the maturation stage was 45.3% higher than that of Jigu 19， and the transformation rate of dry matter in the stems of Yugu 18 was 11.7% higher than that of Jigu 19. It indicates that Yugu 18 had good coordination after heading， and can transport dry matter from its stems to grains.
Grain??leaf ratio
Grain??leaf ratio is an important indicator of coordination between sources and sinks[5]. It can be expressed as follows： grain??leaf ratio （grains/cm2）=grains per spike/the largest leaf area of a plant， and grain??leaf ratio （mg/cm2）=mass of a spike/the largest leaf area of a plant. Seen from Table 2， the grain??leaf ratio of Yugu 18 was 1.43 times higher than that of Jigu 19， and the difference was extremely significant， showing that the coordination between sources and sinks of Yugu 18 was better than that of Jigu 19.
Characteristics of sink capacity
Sink capacity and the ability to fill sinks can be calculated as follows： sink capacity=the number of effective ears per hectare??the total number of grains per ear??the weight of a thousand full seeds??10-6， and the ability to fill sinks=The number of effective ears per hectare??the total number of grains per ear??fruiting rate??the weight of a thousand seeds expect empty seeds??10-6/sink capacity[6]. Seen from Table 3， the number of effective ears， the number of grains per ear， fruiting rate， sink capacity， yield and the ability to fill sinks of Yugu 18 was superior to those of Jigu 19. The yield of Yugu 18 was up to 6 999.38 kg/hm2， 13.38% higher than that of Jigu 19. Meanwhile， the sink capacity and the ability to fill sinks of Yugu 18 were 1.3 and 1.1 times larger than those of Jigu 19. Analysis of variance shows that there were significant differences between Yugu 18 and Jigu 19 in the number of grains per ear and fruiting rate， and the differences of sink capacity， yield and the ability to fill sinks were extremely significant. It indicates that Yugu 18 was superior in sink capacity and the ability to fill sinks， which promoted the formation of its high yield. Conclusions and Discussion
Since Mason and Maskell proposed the theory of sources and sinks， it has been used to clarify the high??yield mechanism of crops. Sources refer to the organs or tissues that produce or output assimilated substances， which are usually measured by leaf area index and photosynthetic rate. Sinks refer to the organs or tissues that utilize or store assimilated substances， and spikes are the most important sinks[7].
The test results show that the leaf area index and grain??leaf ratio of Yugu 18 were relatively large and had long photosynthesis duration and high net photosynthetic rate， which was conducive to the accumulation of dry matter and filling of grains. Yugu 18 was superior to Jigu 19 in sink capacity and the ability to fill sinks， and the transformation rate of dry matter in the stems of Yugu 18 was obviously higher than that of Jigu 19. Studies have shown that larger storage capacity and better coordination between sources and sinks are the basis of high yield. This is the important reason for the high yield of Yugu 18. Many foxtail millet ears are large， but the filling of poor grains is poor， so that its potential for high yield has not been realized[8].
References
[1] YAN HS， LIU JR， WANG SY， et al. Breeding of a new variety of foxtial millet Yugu 18[J]. Crops， 2012（3）： 147-148. （in Chinese）.
[2] WANG YQ，YANG XH，LI B， et al. Study on the relation between leaf area， photosynthetic rate and yield of wheat[J]. Acta Agriculturae Boreali??Sinica， 2008， 23（B10）： 10-15. （in Chinese）.
[3] TAN CL， ZHANG HX， DAI ZY， et al. Characteristics of sink， source and flow in good quality indica rice Yangdao 6[J]. Scientia Agricultura Sinica， 2003， 36（1）： 26-30. （in Chinese）.
[4] ZHAO BH， WANG P， ZHANG HX， et al. Analysis on source??sink and grain??filling characteristics of two??line hybrid rice Yangliangyou 6[J]. Chinese Journal of Rice Science， 2006， 20（1）： 65-72. （in Chinese）.
[5] LI ZX， ZHAO SJ， MENG QW， et al. Photosynthetic characteristics in non??leaf organs of winter wheat cultivars differing in grain??leaf ratio[J]. Acta Agronomica Sinica， 2004， 30（5）： 419-426. （in Chinese）.
[6] LIAO YP， CHEN ZM， HE XY， et al. Sink， source and flow characteristics of rice variety Yuexiangzhan with high harvest index[J]. Chinese Journal of Rice Science， 2001， 15（1）： 73-76. （in Chinese）.
[7] ZHANG ZX， LI Z， CHEN J， et al. Characteristics of sink??source??flow in "II Youhang No. 1" and "II Youhang No. 2" super hybrid rice[J]. Chinese Journal of Eco??Agriculture， 2011， 19（2）： 326-330. （in Chinese）.
[8] YANG JC， ZHANG JH. Grain??filling problem in ??super?? rice[J]. Journal of Experimental Botany，2010，61（1）：1-5.