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Abstract [Objectives]The aim was to investigate the effects of corona field and dielectric separation on the seed vigor of hybrid rice.
[Methods]The seeds of 6 hybrid rice varieties were processed with corona field， dielectric separation and corona field + dielectric separation.
[Results]When the field intensity was 200 km/m and the processing time was 4 min， the seed vigor was significantly increased. The seed vigor of Y Liangyou 9918 （2014） increased the most， and the germination potential and the germination rate increased by 17 and 12 percentage points respectively compared to the control. Corona field reduced seed conductivity， and for seedlings， it decreased their MDA content， increased proline content， improved peroxidase activity， and enhanced root activity. When the rotating speed of the drum was 30 r/min， the dielectric separation voltage of 4 500 V could achieve good separation effect， and the vitality of seeds at different grades was significantly different. Under the optimized corona field combined with optimized voltage processing of dielectric separation， the seed vigor at all grades from the separation was significantly higher than that of the dielectric separation processing alone. The combined separation processing separated no seeds with germination rate of higher than 80% from the hybrid rice seeds with germination rate of lower than 60%， yet it could effectively separate the seeds with germination rate of higher than 80% from the hybrid rice seeds with germination rate of 65%-70%. After the combined processing， the germination rates of separated Grade I seeds of Wangliangyou 6 （2015）， N Liangyouhuazhan （2015）， Wangliangyou 6 （2016） increased by 20， 15， 20 percentage points from the germination rates of the control， while the germination rates of N Liangyouhuazhan （2016） and Y Liangyou 9918 （2016）， which had the germination rates of higher than 80%， increased by 3 and 9 percentage points from the control， suggesting insignificant separation effect.
[Conclusions]This study provided a new technical approach for the processing of hybrid rice seeds.
Key words Corona treatment； Dielectric separation； Hybrid rice； Seed vigor
Nowadays， the large amount of seed used for mechanized sowing of hybrid rice has led to high production costs， which is not conducive to the development of hybrid rice. Rice producers urgently need hybrid rice seeds with high germination rate and good vigor. However， the germination rate and vigor of hybrid rice seeds sold on the market often cannot meet the requirements of mechanized production. Therefore， to further develop hybrid rice， hybrid rice seed vigor must be improved. There have been a large number of studies on electric field treatment of seeds and dielectric separation. Huang et al.[1]found that the pretreatment to wheat seeds with electrostatic field significantly increased the proline content of seedling leaves， increased the activity of catalase， and reduced the relative conductivity and MDA content of the seedling leaves. Dew et al.[2]investigated the effect of presowing electric field treatment on Phaseolus mungo， finding out that with the increase of field intensity， the seed germination rate and the activity of αamylase also increased correspondingly. Dielectric separation is to separated the seeds of different vitalities by using the electromagnetic field to amplify the differences of the electrical properties of seeds （conductivity， polarization properties， permittivity， membrane capacitance）[3]. Lu et al.[4]found that dielectric separation could increase the germination rate and vigor of stale cotton seeds， which was conducive to the cultivation of strong seedlings. Li et al.[5]suggested that the germination rate and seedling mass of the Class A cotton seeds significantly increased when the rotating speed of the separating roller was of 20 r/min and the selecting voltage was -6 kV （DC）， and both the germination rate and seedling mass of the seeds first processed with electric field and then separated with the dielectric separator under the optimum conditions were significantly higher than those of the seeds processed solely with the electric field or separated with the dielectric separator. Conventional seed processing generally uses seed physical properties to classify seeds into different grades， thereby improving seed quality and seed use value. However， seeds classified according to physical properties cannot fully meet the requirements of mechanized production for vitality， and corona field treatment and dielectric separation are to first increase the vitality of the seed itself and then separate the seeds according to the size of the vitality， so that more seeds that meet the requirements of mechanized production can be effectively processed. The purpose of this study was to improve seed vigor and separate seeds according to the vigor through screening the optimal conditions for corona field and dielectric separation to treat hybrid rice seeds， so as to provide a new technical pathway for the processing of hybrid rice seeds.
Materials and Methods
Test materials
Test rice varieties were N Liangyouhuazhan （produced in 2015， 2016）， Wangliangyou 6 （produced in 2015， 2016）， Y Liangyou 9918 （produced in 2014， 2016）. The initial moisture content of the seeds was 11.2%-12.2%. The seeds of N Liangyouhuazhan and Wangliangyou 6 were provided by Hunan Hoping Seeds Co.， Ltd.， and seeds of Y Liangyou 9918 were provided by Hunan Hengde Seed Technology Co.， Ltd. The test devices included a corona field processing unit and a dielectric separation test set[5].
Processing methods
Corona field processing
A total of 5 levels were set to the processing intensities of corona field， namely 80 KV/m （E1）， 120 KV/m （E2）， 160 KV/m （E3）， 200 KV/m （E4）， 240 KV/m （E5）； and there were also 5 levels of processing time， namely 1 min （T1）， 2 min （T2）， 4 min （T3）， 6 min （T4）， and 8 min （T5）. The seeds without any processing were served as the control.
Dielectric separation processing
The drum rotating speed for dielectric separation was 30 r/min， and 4 separation voltage levels were set， which were 1 500， 2 500， 3 500， and 4 500 V， respectively. The seeds were classified into I， II， and III grades under each separating voltage conditions. The seeds without any processing were served as the control.
Corona field + dielectric separation processing The seeds under the optimal processing conditions of corona filed treatment （field intensity of 200 KV/m for 4 min） in 1.2.1 were selected to conduct the dielectric separation under the optimal conditions in 1.2.1 （drum rotating speed of 30 r/min， separating voltage of 4 500 V）. Examining items and methods
Determination of vitality
Referring to standard germination test[6]， the determination of vitality had 4 replicates. The germination potential was recorded on the 3rd d. On day 7， the germination rate was calculated， and the seedlings with normal growth were cut and then dried in an oven at 108℃ for 0.5 h， followed by drying at 80℃ for 48 h before weighing the dry weights of the seedlings.
Germination Index （GI） = ∑ （Gt/Dt）
Vitality Index （VI） = GI×S
Where， Gt is the number of germinated shoots on day t； Dt is the corresponding number of germination days； S is the dry weight of seedlings （g）.
Methods for the determination of physiological and biochemical indexes
A total of 100 normal seeds were selected randomly and soaked in a conical flask for 24 h. Conductivity was measured using DDS11A conductivity meter referring to the method of Duan et al.[7]. The germination culture was performed according to the standard germination test on each treated seed. The normal germinating seedlings were cut on the 4th d of germination to measure the activities of malondialdehyde， proline， and peroxidase. On the 7th d of germination， normal germinating roots were cut to carry out the determination of root activity. The determination of physiological indicators was based on the method of Xiao et al.[8]. Each measurement was repeated 3 times.
Corona field treatment and dielectric separation were conducted at Shandong Agricultural University， and other tests were carried out in the Key Laboratory for Rapeseed Breeding of Hunan Province.
Data analysis
Excel2007 and DPS7.05 were used for data sorting and statistical analysis.
Results and Analysis
Effect of Corona field on seed vigor of N Liangyouhuazhan （2015）
As shown in Table 1， the twofactor treatment of the processing intensity and time of corona field had a certain interaction effect. Both the low field intensity with long processing time and high field intensity with short processing time could significantly increase seed vigor， while the improvement of seed vigor was not significant for the treatments of small field intensity with short processing time and high field intensity with long processing time. After treated with corona fields of E1T5， E2T4， E3T1， E3T2， E3T5， E4T1， and E4T3， the seed vigor indicators of N Liangyouhuazhan （2015） got more significant increases than the seeds of the control， and the increases in the 7 treatments all reached the significant levels. Therefore， they could be used as the optimization parameters for the corona field treatment of other hybrid rice seeds. Agricultural Biotechnology2018
Effect of corona field on the seed vigor of different hybrid rice varieties
As shown in Table 2， after the treatment of E4T3， the seed vigor indicators of different hybrid rice varieties got significantly increases， but the increases of seed vigor of different varieties were different. The increase of seed vigor was the highest for the seeds of Y Liangyou 9918 （2014）， while the lowest for Wangliangyou 6 （2016）. As for the same variety， the seeds with low vigor had the largest increases and best effects.
Effect of corona field treatment of seeds on physiological and biochemical indexes of seedlings
As shown in Table 3， corona field processing reduced seed conductivity， and for seedlings， it decreased their MDA content， increased proline content， improved peroxidase activity， and enhanced root activity. The conductivity of seeds of N Liangyouhuazhan （2015） decreased first and then increased with the increase of field intensity. Seed conductivity was the lowest at 51.74 us/cm when treated with E4T3. The conductivities of seeds of Wangliangyou 6 （2015） and Y Liangyou 9918 （2014） decreased with the increase of field intensity， and the conductivities of both varieties were the lowest in treatment E5T3， 18.60 and 20.60 us/cm·g， respectively， which were significantly lower than that of the control. The MDA contents of the rice seedlings of the 3 varieties all reduced first and then increased with the increase of filed intensity. The MAD contents were low for the seeds of N Liangyouhuazhan （2015） and Y Liangyou 9916 （2014） treated with E4T3， which were 0.93 and 0.42 nmol/g， respectively. Seedlings of Wangliangyou 6 （2015） had low MDA content of 3.96 nmol/g after treatment with E2T3， which was significantly lower than that of the control. For the seedlings of the 3 rice varieties， the proline contents first increased and then reduced with the increase of field intensity. The proline contents were high in the seedlings from the seeds of N Liangyouhuazhan （2015） and y Liangyou 9916 （2014） treated with E2T3， which were 4.01 and 0.56 ug/g， respectively. The proline content of seedlings from seeds of Wangliangyou 6 （2015） treated with E4T3 was high of 1.11 ug/g， which was significantly higher than that of the control. The POD activities of the seedlings of the 3 rice varieties all increased with the increase of field intensity. The seedlings from the seeds of N Liangyou hauzhan （2015）， Wangliangyou 6 （2015） and Y Liangyou 996 （2014） under treatments E5T3， E5T3 and E3T3 had relatively higher POD contents of 17.32， 19.39 and 17.39 U/（g·min）， respectively， which were significantly higher than that of the control. After the seeds treated with different field intensities， the activities of root systems of the seedlings got significantly improvement. The root activity of the seedlings from the seeds of N Liangyouhuazhan （2015） and Wangliangyou （2015） first increased and then decreased with the increase of field intensity， and both reached the highest after treatment E3T3， which were 1.81 and 2.11 ug/（g·h）， respectively. After the seeds of Y Liangyou 9918 （2014） processed with different field intensities， the root activity of seedlings increased with the increase of field intensity， and it was the highest of 3.00 ug/（g·h） at treatment E5T3， which was significantly higher than that of the control. Effect of dielectric separation on the seed vigor of N Liangyouhuazhan （2015）
As shown in Table 4， with the increase of separation voltage， the proportion of Grade I seeds of N Liangyouhuazhan （2015） gradually decreased， the proportion of Grade III seeds gradually increased， and the proportion of Grade II seeds showed no big change. Moreover， the seed vigor indicators of seeds of Grade I and II all increased. According to Table 4， the separation effect of the seeds of N Liangyouhuazhan was the best under the separation voltage of 4 500 V， so it could be used as the optimal voltage for dielectric separation to treat the seeds of other hybrid rice varieties.
Effect of dielectric separation on seed vigor of different hybrid rice varieties
As shown in Table 5， after the seeds of all varieties were processed with dielectric separation under the voltage of 4 500 V， the proportions of seeds of Grade I and II were similar and small. There were no big differences in the 1 000grain weight of the seeds of Grade I， Grade II， Grade II and CK. The 1 000grain weight of the seeds of Grade I was slightly greater than that of Grade II seeds， and the 1 000grain weight of the seeds of Grade II was slightly greater than that of Grade II seeds. The various seed vigor indicators of seeds of Grade I were significantly higher than those of seeds of Grade II， Grade II and CK， and the various seed vigor indictors of seeds of Grade II were significantly higher than those of seeds of Grade II and CK， while the various seed vigor indictors of seeds of Ck were significantly higher than those of seeds of Grade III. The greatest increases in seed vigor of Grade I and II were found in the seeds of Wangliangyou 6， which had the germination potentials increased by 22 and 14 percentage points from the control， germination rates increased by 17 and 9 percentage points from the control， germination indexes increased by 23.27% and 11.32% from the control， and vigor indexes increased by 109.61% and 41.69% from the control， respectively.
Effect of corona field + dielectric separation on seed vigor of different hybrid rice varieties
As shown in Table 6， there were significant differences in the seed vigor of the seeds of all grades after separating the seeds processed with corona field treatment， and the seed vigor increased significantly. Compared with the data in Table 5， the various seed vigor indicators of the seeds of all grades processed with corona field + dielectric separation were significantly higher than those for the seeds processed with dielectric separation alone. The seed vigor of Wangliangyou 6 （2016） increased the highest. Compared with the seeds of Grade I， Grade II and Grade III separated from solely dielectric separation， those separated from the combined treatments had the germination potentials increased by 12， 14 and 9 percentage points， germination rates increased by 7， 8 and 10 percentage points， germination indexes increased by 18.97%， 19.25% and 16.97%， vigor indexes increased by 16.94%， 31.61% and 55.51%， respectively. Therefore， the combination of optimal corona field process with dielectric separation could well improve the vigor of seeds of all grades. Conclusion and Discussion
The tests results show that the twofactor treatment of the processing intensity and time of corona field has a certain interaction effect. Both the low field intensity with long processing time and high field intensity with short processing time could significantly increase seed vigor. Germination potential， germination rate， germination index， and vigor index are the most sensitive macroscopic indicators in the germination period after electric field treatment. The results of this study indicate that appropriate corona field treatment can significantly improve the various indicators of seed vigor， which is consistent with the findings of Wang et al.[9]. The seed vigor increases significantly in the corona field treatment under the field intensity of 200 KV/m for 4 min， and the increases of various seed vigor indicators are greater for the seeds with low vigor of the same variety， which is consistent with the results of Xu et al.[10]. Dielectric separation can better classify hybrid rice seeds according to the vigor size. After separation， the seed vigor indicators of seeds of different grades show significant differences， and the separation effect is the best at the drum rotating speed of 30 r/min with separating voltage of 4 500 V. After processing with the combination of optimized corona field and dielectric separation， the seeds of Grade I， II and III not only show significant differences in seed vigor， but also show significant increases in the vigor from the seeds separated from dielectric separation alone.
The test results show that corona field processing and dielectric separation cannot separate seeds with germination rates of higher than 80% from the hybrid rice seeds with the germination rates of lower than 60%， while the corona field processing and dielectric separation can separate seeds with germination of higher than 80% from the seeds with the germination rates of 65%-79%. For the seeds of Wangliangyou 6 （2015）， N Liangyouhuazhan （2015） and Wangliangyou 6 （2016）， the germination rates of seeds of Grade I separated from the combined treatment are increased by 20， 15 and 20 percentage points from the control. For the seeds of N Liangyouhuazhan （2016） and Y Liangyou 9918（2016） which have the germination rates of higher than 80%， the germination rates of seeds of Grade I separated from the combined treatment are increased by 3 and 9 percentage points respectively compared to the control， and the separation effect is relatively insignificant. The conductivity of the seed leachate can reflect the degree of damage of the seed membrane system[11]. The test results show that appropriate corona field treatment can significantly reduce the seed leachate conductivity， and the membrane system can be repaired to a certain extent after the seed is treated by the electric field. MDA content is an important index reflecting the strength of membrane peroxidation and the degree of plasma membrane destruction[12]. This study shows that after the seeds are processed with appropriate corona field treatment， the MDA content of seedlings is significantly reduced， which is consistent with the results of Gao et al.[13]. Proline is an effective osmotic regulator in plants. Adversity causes massive accumulation of free proline in the plant[14]. This study shows that after the seeds are processed with appropriate corona field treatment， the proline content of seedlings increase significantly， indicating that after the electric field treatment to seeds， the seedlings increase the stress resistance through the accumulation of proline. POD can remove the toxic H2O2， and it is an important antioxidant enzyme in the enzymatic defense system of membrane lipid peroxidation in plants[15]. The results of this study indicate that the appropriate corona field treatment to seeds can significantly increase the POD content of seedlings， so that the seedlings can maintain normal metabolic levels. The activity of the root system is an important physiological indicator of plant growth. It directly affects the growth and nutritional status of the aboveground parts and the yield level[16]. This study shows that suitable corona field treatment to seeds can significantly improve the root activity of seedlings， which is consistent with the results of Gao et al.[17].
The corona field treatment of the seed can repair the seed membrane system， enhance the stress resistance， increase the internal enzyme activities of the seed， and thus the improve the seed vigor. It is this kind of repair effect that makes the corona field treatment better improve the vitality of the seed with low vigor. Dielectric separation can separate the seeds of hybrid rice according to the level of their vitality. Processing with corona field treatment before separation to improve seed vigor can significantly improve the vigor of seeds of all grades after separation， which provides a new way to meet the requirements of current mechanized production for seeds with high vitality， and also provides a reference for the seed companies to process aged seeds. References
[1]HUANG HY， DU N， HAN Z. Effects of wheat seedling cold resistance through treatment of seeds with high voltage electrostatic field[J]. Seed， 2016， 35（8）： 24-26， 31.
[2]BISWAS D， SADHU TK， BHATTACHARYYA R. Effect of presowing electric field treatment of the seeds on some physiological parameters of Phaseolus mungo L.[J]. Indian Journal of Plant Physiology，2016，21（3）： 1-4.
[3]ZHAO JP. The new advances of study on seed dielectric separation technology[J]. Scientia Silvae Sinicae， 2000（S1）： 97-103.
[4]LU CM. Preliminary analysis on reuses of stale cotton seeds[J]. Seed， 2012，31（5）： 98-100.
[5]LI FD， ZHANG XG， LIU XZ， et al. Effects of electric field processing and dielectric separation on cotton seed germination rate and seedling mass[J]. Transactions of the Chinese Society of Agricultural Engineering， 2010， 26（9）： 128-132.
[6]ZHANG CQ， WANG JH. Seed inspection[M]. Beijing： China Education Press， 2006： 51-58.
[7]DUAN YH， LI XX， LI WH. Determination of seed activity of hybrid rice using conductivity method[J]. Hunan Agricultural Sciences， 2010（23）： 17-19.
[8]XIAO LT， WANG SG. Experimental technique of plant physiology[M]. Beijing： China Agriculture Press， 2005： 65-221.
[9]WANG QY， LU GZ， ZHAO YQ. Experimental of bourgeoning paddy rice seeds with high voltage electrostatic field[J]. Journal of Yunnan Agricultural University， 2005（01）： 147-150.
[10]XU J， TAN M， ZHANG CQ， et al. Improving paddy seed vigor by corona discharge field processing and dielectric separation[J]. Transactions of the Chinese Society of Agricultural Engineering， 2013，29（23）： 233-240.
[11]WANG ZB， WANG SC， XU GL. Method for determination of seed vitalityconductivity method[J]. Seed， 1998（03）： 70.
[12]YANG CX， LI XL， GAO DS， et al. Effects of chilling stress on membrane lipid peroxidation and activities of protective enzymes in nectarine flower organs[J]. Journal of Fruit Science， 2005， 22（1）： 69-71.
[13]GAO WN， GU XQ， QIAO XL， et al. Effect of highvoltage electrostatic field on the seed vigor of aged rice seeds[J]. Acta Agriculturae BorealiOccidentalis Sinica， 2007（2）： 37-40.
[14]ZHAO HW， WANG XP， YU MF， et al. Effects of drought stress and rewatering on antioxidant system and proline content in rice during tillering stage[J]. Journal of Northeast Agricultural University， 2016，47（2）： 1-7.
[15]HUANG SM， ZHANG HB， YU HZ. Relation of the activities of SOD， POD， CAT and the content of MDA in leaf to grainleaf ratio of rice[J]. Hybrid Rice， 2017，32（4）： 76-80.
[16]XU GW， SUN HZ， LU DK， et al. Differenced in root ultrastructure and activity of rice roots under different irrigation and nitrogen supply levels[J]. Plant Nutrition and Fertilizer Science， 2017， 23（3）： 811-820.
[17]GAO WN， WANG GX， ZHANG H， et al. Effect of high voltage electrostatic field on the sprout roots of aging rice seeds[J]. Journal of Northwest SciTech University of Agriculture and Forestry （Natural Science Edition）， 2006（11）： 83-86.
Editor： Na LI Proofreader： Xinxiu ZHU
[Methods]The seeds of 6 hybrid rice varieties were processed with corona field， dielectric separation and corona field + dielectric separation.
[Results]When the field intensity was 200 km/m and the processing time was 4 min， the seed vigor was significantly increased. The seed vigor of Y Liangyou 9918 （2014） increased the most， and the germination potential and the germination rate increased by 17 and 12 percentage points respectively compared to the control. Corona field reduced seed conductivity， and for seedlings， it decreased their MDA content， increased proline content， improved peroxidase activity， and enhanced root activity. When the rotating speed of the drum was 30 r/min， the dielectric separation voltage of 4 500 V could achieve good separation effect， and the vitality of seeds at different grades was significantly different. Under the optimized corona field combined with optimized voltage processing of dielectric separation， the seed vigor at all grades from the separation was significantly higher than that of the dielectric separation processing alone. The combined separation processing separated no seeds with germination rate of higher than 80% from the hybrid rice seeds with germination rate of lower than 60%， yet it could effectively separate the seeds with germination rate of higher than 80% from the hybrid rice seeds with germination rate of 65%-70%. After the combined processing， the germination rates of separated Grade I seeds of Wangliangyou 6 （2015）， N Liangyouhuazhan （2015）， Wangliangyou 6 （2016） increased by 20， 15， 20 percentage points from the germination rates of the control， while the germination rates of N Liangyouhuazhan （2016） and Y Liangyou 9918 （2016）， which had the germination rates of higher than 80%， increased by 3 and 9 percentage points from the control， suggesting insignificant separation effect.
[Conclusions]This study provided a new technical approach for the processing of hybrid rice seeds.
Key words Corona treatment； Dielectric separation； Hybrid rice； Seed vigor
Nowadays， the large amount of seed used for mechanized sowing of hybrid rice has led to high production costs， which is not conducive to the development of hybrid rice. Rice producers urgently need hybrid rice seeds with high germination rate and good vigor. However， the germination rate and vigor of hybrid rice seeds sold on the market often cannot meet the requirements of mechanized production. Therefore， to further develop hybrid rice， hybrid rice seed vigor must be improved. There have been a large number of studies on electric field treatment of seeds and dielectric separation. Huang et al.[1]found that the pretreatment to wheat seeds with electrostatic field significantly increased the proline content of seedling leaves， increased the activity of catalase， and reduced the relative conductivity and MDA content of the seedling leaves. Dew et al.[2]investigated the effect of presowing electric field treatment on Phaseolus mungo， finding out that with the increase of field intensity， the seed germination rate and the activity of αamylase also increased correspondingly. Dielectric separation is to separated the seeds of different vitalities by using the electromagnetic field to amplify the differences of the electrical properties of seeds （conductivity， polarization properties， permittivity， membrane capacitance）[3]. Lu et al.[4]found that dielectric separation could increase the germination rate and vigor of stale cotton seeds， which was conducive to the cultivation of strong seedlings. Li et al.[5]suggested that the germination rate and seedling mass of the Class A cotton seeds significantly increased when the rotating speed of the separating roller was of 20 r/min and the selecting voltage was -6 kV （DC）， and both the germination rate and seedling mass of the seeds first processed with electric field and then separated with the dielectric separator under the optimum conditions were significantly higher than those of the seeds processed solely with the electric field or separated with the dielectric separator. Conventional seed processing generally uses seed physical properties to classify seeds into different grades， thereby improving seed quality and seed use value. However， seeds classified according to physical properties cannot fully meet the requirements of mechanized production for vitality， and corona field treatment and dielectric separation are to first increase the vitality of the seed itself and then separate the seeds according to the size of the vitality， so that more seeds that meet the requirements of mechanized production can be effectively processed. The purpose of this study was to improve seed vigor and separate seeds according to the vigor through screening the optimal conditions for corona field and dielectric separation to treat hybrid rice seeds， so as to provide a new technical pathway for the processing of hybrid rice seeds.
Materials and Methods
Test materials
Test rice varieties were N Liangyouhuazhan （produced in 2015， 2016）， Wangliangyou 6 （produced in 2015， 2016）， Y Liangyou 9918 （produced in 2014， 2016）. The initial moisture content of the seeds was 11.2%-12.2%. The seeds of N Liangyouhuazhan and Wangliangyou 6 were provided by Hunan Hoping Seeds Co.， Ltd.， and seeds of Y Liangyou 9918 were provided by Hunan Hengde Seed Technology Co.， Ltd. The test devices included a corona field processing unit and a dielectric separation test set[5].
Processing methods
Corona field processing
A total of 5 levels were set to the processing intensities of corona field， namely 80 KV/m （E1）， 120 KV/m （E2）， 160 KV/m （E3）， 200 KV/m （E4）， 240 KV/m （E5）； and there were also 5 levels of processing time， namely 1 min （T1）， 2 min （T2）， 4 min （T3）， 6 min （T4）， and 8 min （T5）. The seeds without any processing were served as the control.
Dielectric separation processing
The drum rotating speed for dielectric separation was 30 r/min， and 4 separation voltage levels were set， which were 1 500， 2 500， 3 500， and 4 500 V， respectively. The seeds were classified into I， II， and III grades under each separating voltage conditions. The seeds without any processing were served as the control.
Corona field + dielectric separation processing The seeds under the optimal processing conditions of corona filed treatment （field intensity of 200 KV/m for 4 min） in 1.2.1 were selected to conduct the dielectric separation under the optimal conditions in 1.2.1 （drum rotating speed of 30 r/min， separating voltage of 4 500 V）. Examining items and methods
Determination of vitality
Referring to standard germination test[6]， the determination of vitality had 4 replicates. The germination potential was recorded on the 3rd d. On day 7， the germination rate was calculated， and the seedlings with normal growth were cut and then dried in an oven at 108℃ for 0.5 h， followed by drying at 80℃ for 48 h before weighing the dry weights of the seedlings.
Germination Index （GI） = ∑ （Gt/Dt）
Vitality Index （VI） = GI×S
Where， Gt is the number of germinated shoots on day t； Dt is the corresponding number of germination days； S is the dry weight of seedlings （g）.
Methods for the determination of physiological and biochemical indexes
A total of 100 normal seeds were selected randomly and soaked in a conical flask for 24 h. Conductivity was measured using DDS11A conductivity meter referring to the method of Duan et al.[7]. The germination culture was performed according to the standard germination test on each treated seed. The normal germinating seedlings were cut on the 4th d of germination to measure the activities of malondialdehyde， proline， and peroxidase. On the 7th d of germination， normal germinating roots were cut to carry out the determination of root activity. The determination of physiological indicators was based on the method of Xiao et al.[8]. Each measurement was repeated 3 times.
Corona field treatment and dielectric separation were conducted at Shandong Agricultural University， and other tests were carried out in the Key Laboratory for Rapeseed Breeding of Hunan Province.
Data analysis
Excel2007 and DPS7.05 were used for data sorting and statistical analysis.
Results and Analysis
Effect of Corona field on seed vigor of N Liangyouhuazhan （2015）
As shown in Table 1， the twofactor treatment of the processing intensity and time of corona field had a certain interaction effect. Both the low field intensity with long processing time and high field intensity with short processing time could significantly increase seed vigor， while the improvement of seed vigor was not significant for the treatments of small field intensity with short processing time and high field intensity with long processing time. After treated with corona fields of E1T5， E2T4， E3T1， E3T2， E3T5， E4T1， and E4T3， the seed vigor indicators of N Liangyouhuazhan （2015） got more significant increases than the seeds of the control， and the increases in the 7 treatments all reached the significant levels. Therefore， they could be used as the optimization parameters for the corona field treatment of other hybrid rice seeds. Agricultural Biotechnology2018
Effect of corona field on the seed vigor of different hybrid rice varieties
As shown in Table 2， after the treatment of E4T3， the seed vigor indicators of different hybrid rice varieties got significantly increases， but the increases of seed vigor of different varieties were different. The increase of seed vigor was the highest for the seeds of Y Liangyou 9918 （2014）， while the lowest for Wangliangyou 6 （2016）. As for the same variety， the seeds with low vigor had the largest increases and best effects.
Effect of corona field treatment of seeds on physiological and biochemical indexes of seedlings
As shown in Table 3， corona field processing reduced seed conductivity， and for seedlings， it decreased their MDA content， increased proline content， improved peroxidase activity， and enhanced root activity. The conductivity of seeds of N Liangyouhuazhan （2015） decreased first and then increased with the increase of field intensity. Seed conductivity was the lowest at 51.74 us/cm when treated with E4T3. The conductivities of seeds of Wangliangyou 6 （2015） and Y Liangyou 9918 （2014） decreased with the increase of field intensity， and the conductivities of both varieties were the lowest in treatment E5T3， 18.60 and 20.60 us/cm·g， respectively， which were significantly lower than that of the control. The MDA contents of the rice seedlings of the 3 varieties all reduced first and then increased with the increase of filed intensity. The MAD contents were low for the seeds of N Liangyouhuazhan （2015） and Y Liangyou 9916 （2014） treated with E4T3， which were 0.93 and 0.42 nmol/g， respectively. Seedlings of Wangliangyou 6 （2015） had low MDA content of 3.96 nmol/g after treatment with E2T3， which was significantly lower than that of the control. For the seedlings of the 3 rice varieties， the proline contents first increased and then reduced with the increase of field intensity. The proline contents were high in the seedlings from the seeds of N Liangyouhuazhan （2015） and y Liangyou 9916 （2014） treated with E2T3， which were 4.01 and 0.56 ug/g， respectively. The proline content of seedlings from seeds of Wangliangyou 6 （2015） treated with E4T3 was high of 1.11 ug/g， which was significantly higher than that of the control. The POD activities of the seedlings of the 3 rice varieties all increased with the increase of field intensity. The seedlings from the seeds of N Liangyou hauzhan （2015）， Wangliangyou 6 （2015） and Y Liangyou 996 （2014） under treatments E5T3， E5T3 and E3T3 had relatively higher POD contents of 17.32， 19.39 and 17.39 U/（g·min）， respectively， which were significantly higher than that of the control. After the seeds treated with different field intensities， the activities of root systems of the seedlings got significantly improvement. The root activity of the seedlings from the seeds of N Liangyouhuazhan （2015） and Wangliangyou （2015） first increased and then decreased with the increase of field intensity， and both reached the highest after treatment E3T3， which were 1.81 and 2.11 ug/（g·h）， respectively. After the seeds of Y Liangyou 9918 （2014） processed with different field intensities， the root activity of seedlings increased with the increase of field intensity， and it was the highest of 3.00 ug/（g·h） at treatment E5T3， which was significantly higher than that of the control. Effect of dielectric separation on the seed vigor of N Liangyouhuazhan （2015）
As shown in Table 4， with the increase of separation voltage， the proportion of Grade I seeds of N Liangyouhuazhan （2015） gradually decreased， the proportion of Grade III seeds gradually increased， and the proportion of Grade II seeds showed no big change. Moreover， the seed vigor indicators of seeds of Grade I and II all increased. According to Table 4， the separation effect of the seeds of N Liangyouhuazhan was the best under the separation voltage of 4 500 V， so it could be used as the optimal voltage for dielectric separation to treat the seeds of other hybrid rice varieties.
Effect of dielectric separation on seed vigor of different hybrid rice varieties
As shown in Table 5， after the seeds of all varieties were processed with dielectric separation under the voltage of 4 500 V， the proportions of seeds of Grade I and II were similar and small. There were no big differences in the 1 000grain weight of the seeds of Grade I， Grade II， Grade II and CK. The 1 000grain weight of the seeds of Grade I was slightly greater than that of Grade II seeds， and the 1 000grain weight of the seeds of Grade II was slightly greater than that of Grade II seeds. The various seed vigor indicators of seeds of Grade I were significantly higher than those of seeds of Grade II， Grade II and CK， and the various seed vigor indictors of seeds of Grade II were significantly higher than those of seeds of Grade II and CK， while the various seed vigor indictors of seeds of Ck were significantly higher than those of seeds of Grade III. The greatest increases in seed vigor of Grade I and II were found in the seeds of Wangliangyou 6， which had the germination potentials increased by 22 and 14 percentage points from the control， germination rates increased by 17 and 9 percentage points from the control， germination indexes increased by 23.27% and 11.32% from the control， and vigor indexes increased by 109.61% and 41.69% from the control， respectively.
Effect of corona field + dielectric separation on seed vigor of different hybrid rice varieties
As shown in Table 6， there were significant differences in the seed vigor of the seeds of all grades after separating the seeds processed with corona field treatment， and the seed vigor increased significantly. Compared with the data in Table 5， the various seed vigor indicators of the seeds of all grades processed with corona field + dielectric separation were significantly higher than those for the seeds processed with dielectric separation alone. The seed vigor of Wangliangyou 6 （2016） increased the highest. Compared with the seeds of Grade I， Grade II and Grade III separated from solely dielectric separation， those separated from the combined treatments had the germination potentials increased by 12， 14 and 9 percentage points， germination rates increased by 7， 8 and 10 percentage points， germination indexes increased by 18.97%， 19.25% and 16.97%， vigor indexes increased by 16.94%， 31.61% and 55.51%， respectively. Therefore， the combination of optimal corona field process with dielectric separation could well improve the vigor of seeds of all grades. Conclusion and Discussion
The tests results show that the twofactor treatment of the processing intensity and time of corona field has a certain interaction effect. Both the low field intensity with long processing time and high field intensity with short processing time could significantly increase seed vigor. Germination potential， germination rate， germination index， and vigor index are the most sensitive macroscopic indicators in the germination period after electric field treatment. The results of this study indicate that appropriate corona field treatment can significantly improve the various indicators of seed vigor， which is consistent with the findings of Wang et al.[9]. The seed vigor increases significantly in the corona field treatment under the field intensity of 200 KV/m for 4 min， and the increases of various seed vigor indicators are greater for the seeds with low vigor of the same variety， which is consistent with the results of Xu et al.[10]. Dielectric separation can better classify hybrid rice seeds according to the vigor size. After separation， the seed vigor indicators of seeds of different grades show significant differences， and the separation effect is the best at the drum rotating speed of 30 r/min with separating voltage of 4 500 V. After processing with the combination of optimized corona field and dielectric separation， the seeds of Grade I， II and III not only show significant differences in seed vigor， but also show significant increases in the vigor from the seeds separated from dielectric separation alone.
The test results show that corona field processing and dielectric separation cannot separate seeds with germination rates of higher than 80% from the hybrid rice seeds with the germination rates of lower than 60%， while the corona field processing and dielectric separation can separate seeds with germination of higher than 80% from the seeds with the germination rates of 65%-79%. For the seeds of Wangliangyou 6 （2015）， N Liangyouhuazhan （2015） and Wangliangyou 6 （2016）， the germination rates of seeds of Grade I separated from the combined treatment are increased by 20， 15 and 20 percentage points from the control. For the seeds of N Liangyouhuazhan （2016） and Y Liangyou 9918（2016） which have the germination rates of higher than 80%， the germination rates of seeds of Grade I separated from the combined treatment are increased by 3 and 9 percentage points respectively compared to the control， and the separation effect is relatively insignificant. The conductivity of the seed leachate can reflect the degree of damage of the seed membrane system[11]. The test results show that appropriate corona field treatment can significantly reduce the seed leachate conductivity， and the membrane system can be repaired to a certain extent after the seed is treated by the electric field. MDA content is an important index reflecting the strength of membrane peroxidation and the degree of plasma membrane destruction[12]. This study shows that after the seeds are processed with appropriate corona field treatment， the MDA content of seedlings is significantly reduced， which is consistent with the results of Gao et al.[13]. Proline is an effective osmotic regulator in plants. Adversity causes massive accumulation of free proline in the plant[14]. This study shows that after the seeds are processed with appropriate corona field treatment， the proline content of seedlings increase significantly， indicating that after the electric field treatment to seeds， the seedlings increase the stress resistance through the accumulation of proline. POD can remove the toxic H2O2， and it is an important antioxidant enzyme in the enzymatic defense system of membrane lipid peroxidation in plants[15]. The results of this study indicate that the appropriate corona field treatment to seeds can significantly increase the POD content of seedlings， so that the seedlings can maintain normal metabolic levels. The activity of the root system is an important physiological indicator of plant growth. It directly affects the growth and nutritional status of the aboveground parts and the yield level[16]. This study shows that suitable corona field treatment to seeds can significantly improve the root activity of seedlings， which is consistent with the results of Gao et al.[17].
The corona field treatment of the seed can repair the seed membrane system， enhance the stress resistance， increase the internal enzyme activities of the seed， and thus the improve the seed vigor. It is this kind of repair effect that makes the corona field treatment better improve the vitality of the seed with low vigor. Dielectric separation can separate the seeds of hybrid rice according to the level of their vitality. Processing with corona field treatment before separation to improve seed vigor can significantly improve the vigor of seeds of all grades after separation， which provides a new way to meet the requirements of current mechanized production for seeds with high vitality， and also provides a reference for the seed companies to process aged seeds. References
[1]HUANG HY， DU N， HAN Z. Effects of wheat seedling cold resistance through treatment of seeds with high voltage electrostatic field[J]. Seed， 2016， 35（8）： 24-26， 31.
[2]BISWAS D， SADHU TK， BHATTACHARYYA R. Effect of presowing electric field treatment of the seeds on some physiological parameters of Phaseolus mungo L.[J]. Indian Journal of Plant Physiology，2016，21（3）： 1-4.
[3]ZHAO JP. The new advances of study on seed dielectric separation technology[J]. Scientia Silvae Sinicae， 2000（S1）： 97-103.
[4]LU CM. Preliminary analysis on reuses of stale cotton seeds[J]. Seed， 2012，31（5）： 98-100.
[5]LI FD， ZHANG XG， LIU XZ， et al. Effects of electric field processing and dielectric separation on cotton seed germination rate and seedling mass[J]. Transactions of the Chinese Society of Agricultural Engineering， 2010， 26（9）： 128-132.
[6]ZHANG CQ， WANG JH. Seed inspection[M]. Beijing： China Education Press， 2006： 51-58.
[7]DUAN YH， LI XX， LI WH. Determination of seed activity of hybrid rice using conductivity method[J]. Hunan Agricultural Sciences， 2010（23）： 17-19.
[8]XIAO LT， WANG SG. Experimental technique of plant physiology[M]. Beijing： China Agriculture Press， 2005： 65-221.
[9]WANG QY， LU GZ， ZHAO YQ. Experimental of bourgeoning paddy rice seeds with high voltage electrostatic field[J]. Journal of Yunnan Agricultural University， 2005（01）： 147-150.
[10]XU J， TAN M， ZHANG CQ， et al. Improving paddy seed vigor by corona discharge field processing and dielectric separation[J]. Transactions of the Chinese Society of Agricultural Engineering， 2013，29（23）： 233-240.
[11]WANG ZB， WANG SC， XU GL. Method for determination of seed vitalityconductivity method[J]. Seed， 1998（03）： 70.
[12]YANG CX， LI XL， GAO DS， et al. Effects of chilling stress on membrane lipid peroxidation and activities of protective enzymes in nectarine flower organs[J]. Journal of Fruit Science， 2005， 22（1）： 69-71.
[13]GAO WN， GU XQ， QIAO XL， et al. Effect of highvoltage electrostatic field on the seed vigor of aged rice seeds[J]. Acta Agriculturae BorealiOccidentalis Sinica， 2007（2）： 37-40.
[14]ZHAO HW， WANG XP， YU MF， et al. Effects of drought stress and rewatering on antioxidant system and proline content in rice during tillering stage[J]. Journal of Northeast Agricultural University， 2016，47（2）： 1-7.
[15]HUANG SM， ZHANG HB， YU HZ. Relation of the activities of SOD， POD， CAT and the content of MDA in leaf to grainleaf ratio of rice[J]. Hybrid Rice， 2017，32（4）： 76-80.
[16]XU GW， SUN HZ， LU DK， et al. Differenced in root ultrastructure and activity of rice roots under different irrigation and nitrogen supply levels[J]. Plant Nutrition and Fertilizer Science， 2017， 23（3）： 811-820.
[17]GAO WN， WANG GX， ZHANG H， et al. Effect of high voltage electrostatic field on the sprout roots of aging rice seeds[J]. Journal of Northwest SciTech University of Agriculture and Forestry （Natural Science Edition）， 2006（11）： 83-86.
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