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Abstract [Objectives] This study was conducted to explore how can we better break the dormancy of velvetleaf seeds and the best method to promote seed germination. [Methods] With the seeds of velvetleaf （Abutilon theophrasti） as experimental materials， the dormancy of velvetleaf seeds was revealed through seed vigor determination， water absorption rate determination and germination tests， and the seeds of velvetleaf were treated by physical and chemical methods to explore the best method for breaking dormancy of velvetleaf seeds. [Results] Poor water permeability of seed coat and endogenous inhibitory substances present in the seeds were the main reasons leading to the dormancy of velvetleaf seeds. The shell-breaking treatment， 98% concentrated sulfuric acid treatment and 40% NaOH strong alkali treatment all could break the dormancy obstacle of velvetleaf seed coat. Compositing the above measures with 200 mg/ml gibberellin solution could further improve the germination ability of seeds. Among them， compositing the mechanical shell-breaking treatment with gibberellin had the best effect， followed by the composite of 10 and 15 min of 98% concentrated sulfuric acid treatment with gibberellin.[Conclusions] This study lays a foundation for the utilization and development and the comprehensive prevention and control research of velvetleaf seeds.
Key words Velvetleaf; Dormancy breaking; Seed germination; Treatment
Velvetleaf （Abutilon theophrasti） is an annual herbaceous plant in Abutilon of Malvaceae， which has strong resistance and wide adaptability and is commonly distributed in roadsides， wasteland and fields. It is widely distributed in all provinces and regions except for the Qinghai-Tibet Plateau in China[1]. Velvetleaf contains phenols， flavonoids， polysaccharides， coumarins， volatile oils， saponins， steroids， terpenes and other components[2]. It is used as a whole herb and has the effect of clearing away heat and toxic materials. It mainly treats dysentery， otitis media， tinnitus， deafness， arthralgia， carbuncles， etc.[1]. The oil content of its seeds is commonly used in soap making， industrial lubricants and energy plants[3]. The stem bark has strong salt and water tolerance and can be used to make ropes[4]， and has important medicinal and economical utilization value. Velvetleaf is cultivated in some areas of Northeast China. The aboveground and underground parts of velvetleaf have a good effect on lead enrichment and transfer. Velvetleaf has the characteristics of strong lead accumulation ability and high low-concentration lead pollution remediation efficiency， and can be used as a potential species for the remediation in lead-contaminated areas[5]. Meanwhile， with the change of farming system， the occurrence of velvetleaf tends to increase. It is now the main weed in cotton， corn and bean farmland[6]， and it is also one of the main trap plants for cotton bollworms[7]. Velvetleaf is propagated with seeds， which have dormancy[5]. Breaking seed dormancy is the foundation of cultivation and research. Seed dormancy is mainly caused by factors such as seed coat obstacle， morphological after-ripening， physiological after-ripening， presence of inhibitory substances， and bad environment[8-9]， and different factors correspond to different dormancy breaking methods. Studies have reported that soaking seeds in warm water at 30-60 ℃[4] and low-temperature treatment of seeds[3] can significantly increase the germination rate of velvetleaf seeds， in which the seed coat obstacle is one of the important factors for velvetleaf seed dormancy[5]. In this study， with velvetleaf seeds collected in the wilderness of northern Suzhou as the research object， how can we break the dormancy of velvetleaf seeds better and best promote seed germination was investigated by physical shell breaking， acid and alkali treatment of seed coat and plant hormone treatment， aiming to lay a foundation for the utilization and development and the comprehensive prevention and control research of velvetleaf.
Materials and Methods
Experimental materials
The velvetleaf seeds used for the experiment were sourced from the wilderness of northern Suzhou. They were collected during the maturation stage of wild velvetleaf plants in autumn 2019， and were air-dried under indoor conditions and stored for future use. Full seeds with uniform size were used as experimental materials. Before the experiment， the surface of the seeds was first sterilized with a potassium permanganate solution with a mass concentration of 0.5%， and then rinsed with distilled water several times.
Experimental methods
Different treatment methods of seed coat
Physical shell-breaking treatment： A knife was used to cut the seed coat on the back of the treatment seeds （PB） without damaging embryos of the seeds. After cutting， the seeds were sterilized with 75% ethanol for 15 s， and then rinsed repeatedly with distilled water for use.
Strong acid treatment： The seeds were added in a beaker， into which 98% concentrated sulfuric acid was added for acid erosion treatment. The treatment was performed with stirring for 5 （SA5）， 10 （SA10） and 15 min （SA15）， respectively. After the treatment， the seeds were immediately rinsed repeatedly with distilled water for use.
Strong alkali treatment： The seeds were placed in a beaker， and added with 40% sodium hydroxide solution. The seeds were soaked for 10 （SH10）， 20 （SH20） and 30 （SH30） min， respectively. After the treatment， the seeds were rinsed repeatedly with distilled water for use. Determined indexes
Determination of seed vigor： Seed viability was measured by the TTC （2，3，5-triphenyltetrazolium chloride） staining method[10]. Fifty seeds were taken in three repetitions， which were soaked in distilled water at room temperature for 24 h. The seed coat was peeled off and cut longitudinally. The seeds were soaked in 0.5% TTC solution， and placed at 30 ℃ in the dark for 12 h， for color development. The embryo and endosperm staining status were observed. The seed vigor was indentified， and the color development results were recorded. The seed viability calculation formula was： Seed viability （%）=Viable seeds/Total number of seeds tested×100.
The law of seed germination and water absorption： After the seeds were treated according to "Different treatment methods of seed coat"， they were put into beakers at 25 ℃ room temperature， with velvetleaf seeds without any treatment as the control （CK）. Distilled water was added to the beakers to soak the seeds， the weight of which was measured every 2 h. Each treatment included 100 seeds， with three replicates. When weighing， absorbent paper was used to absorb the surface moisture， and the seeds were accurately weighed on an electronic balance （accurate to 1/10 000）， followed by recording the data. The calculation formula of seed water absorption rate was as follows：
Seed water absorption rate （%）=[（Seed weight after water absorption–Seed weight before water absorption）/Seed weight before water absorption]×100%
Seed germination index and seedling survival status： The seeds treated under "Different treatment methods of seed coat" and the seeds treated with gibberellin （GA3， 200 mg/ml） after the treatment under "Different treatment methods of seed coat" were taken randomly， 50 seeds for each treatment， in three replicates. The seeds were placed in petri dishes with filter paper. The semi-submerged seed soaking method was used to induce germination. Each culture dish was placed in a constant temperature incubator at 25 ℃ for 7 d. Distilled water was added to the dish every day， under the standard of submerging half of the seeds. The number of seeds germinated was recorded every 24 h， with the radicle extending from the seed coat as the standard for seed germination. The germination vigor （Gv） was calculated on the third day. On the seventh day， the germination rate （G） and germination index （GI） were calculated; the seed embryo axis length， radicle length and fresh weight were measured; and the vital index was calculated. The calculation formulas were as follows： Wherein Gt is the number of seeds that germinated normally in t days; Gn is the number of seeds tested; Ga is the the number of all seeds normally germinated at the end of germination; Dt is the number of days corresponding to Gt; and S is the total length of velvetleaf plant.
Data analysis and treatment
The test data was statistically analyzed using SPSS12.0 statistical software.
Results and Analysis
Seed vigor determination
Seed viability is the potential ability of seeds to germinate or the ability of embryos to become seedlings， but seeds with viability may not all germinate. In this study， the seed vigor was measured by the TTC staining method， and the results showed that 96.67% of the seed embryos were dyed red， that is， the seeds had viability.
Changes of water absorption of velvetleaf seeds under different treatments
The shell-breaking treatment， the concentrated sulfuric acid treatment and NaOH treatment were tested for water absorption rate， with the complete untreated seeds of velvetleaf as the CK. The effects of the four types of treatment methods are shown in Fig. 1. After the seeds absorbed water for 24 h， the water absorption rate reached 168.50% in the shell breaking treatment; the water absorption rate of the untreated intact seeds was 47.61%; the water absorption rate of the seeds treated with concentrated sulfuric acid for 15 min reached 125.90%， and the water absorption rates of the seeds treated with concentrated sulfuric acid for 5 and 10 min were 100.27% and 114.79%， respectively; and the water absorption rate of seeds treated with NaOH for 20 min was 113.74%， and the water absorption rates of seeds treated with NaOH for 10 and 30 min were 90.63% and 117.71%， respectively. It showed that the permeability of velvetleaf seed coat was poor， and eroding the seed coat with acid and alkali could improve the water absorption rate of seeds.
Seed germination of velvetleaf seeds under different treatments
As shown in Table 1， after 24 h， in all kinds of composite treatments with gibberellin， 17 seeds （11.33%） of the seeds of the shell-breaking treatment were germinated in the three replicates in total. The seeds of acid and alkali treatments sporadically germinated. The seeds of the CK did not germinate. Basically no seeds germinated in the various treatments without being composited with gibberellin.
Kuojie XIAO et al. Study on Dormancy Mechanism and Dormancy-breaking Method of Velvetleaf （Abutilon theophrasti） Seeds It could be seen from Table 2 that different treatment methods had different effects on the seed germination index of velvetleaf. The seed germination vigor， germination rate and germination index were relatively large in the shell-breaking treatment and its gibberellin composite treatment， showing that the seeds germinated early and the seedling emerged most neatly， followed by the composite treatments of gibberellin with the concentrated sulfuric acid treatment （10 and 15 min）. NaOH treatment could also promote seed germination to a certain extent， and the NaOH treatment was the best for 20 min， but the germination vigor， germination rate and germination index of alkali-treated seeds were lower than the acid treatments.
Seed survival of velvetleaf seeds under different treatments
It could be seen from Table 3 that the effects of different treatments on the growth of velvetleaf seedlings were slightly different. The shell-breaking treatment was conducive to the elongation and growth of the radicle and hypocotyl of velvetleaf seeds， and the root length， stem length and fresh weight were the largest in the composite treatment of gibberellin with shell breaking; and the stem length was the largest in the composite treatment of gibberellin with shell breaking， followed by the shell-breaking treatment. In the composite treatments with gibberellin， the root length， stem length， fresh weight and vital index of various treatments increased to varying degrees.
Conclusions and Discussion
Seed germination begins with imbibition. When the seed coat is waxy， gelatinous or leathery， moisture and oxygen cannot easily pass through the seed coat， which also prevents the inhibitors from overflowing from the seed coat， resulting in dormancy[5]. In the germination test， the untreated seeds had a germination rate of about 38.00% after 7 d， and 96.67% of the seeds had viability in the seed vigor determination test， indicating that there is a phenomenon of dormancy in velvetleaf. In the water absorption test of velvetleaf seeds， the water absorption rate of the untreated intact seeds after absorbing water for 24 h was about 47.61%， which was 119.89% different from the water absorption rate of the seeds of the shell-breaking treatment. The seed coat largely prevented the embryos from absorbing water， indicating that the hard seed coat of velvetleaf leads to dormancy as an exogenous barrier. The inhibitor action mechanism of seed dormancy believes that seed dormancy is due to the presence of inhibitors in different parts of the seeds， such as aldehydes， phenols， organic acids， abscisic acid， alkaloids， etc.[12]. In the process of seed dormancy and germination， plant hormones can respond to various physiological changes in seeds through signal transmission， regulate the metabolism of a series of proteins and enzymes， and thus regulate the dormancy and germination of seeds[13]. Many studies have shown that exogenous additions such as gibberellin can promote the germination of dormant seeds[14-18]. In this study， gibberellin was added to the velvetleaf seeds treated by shell breaking and acid and alkali， and their germination vigor， germination rate， germination index， vital index and fresh weight of seedlings were further improved. In summary， it is inferred that the dormancy of velvetleaf seeds is a comprehensive dormancy combining poor permeability of exogenous seed coat and endogenous inhibitory substances within the seeds. When the seeds have comprehensive dormancy， a comprehensive method should be used to break the seed dormancy. Both physical shell breaking and chemical corrosion could improve the imbibition of velvetleaf seeds and increase the germination rate of seeds， and after the addition of gibberellin， gibberellin entered the seeds， breaking dormancy and further enhancing the seed germination ability. The composite treatment of physical shell breaking with gibberellin was best， followed by the composite of 10 or 15 min of 98% concentrated sulfuric acid treatment with gibberellin. The shell breaking treatment was time-consuming and labor-intensive， while the treatment with concentrated sulfuric acid was easier to operate and the treatment was more uniform. Therefore， when treating the seeds of velvetleaf， if the treated seeds are less， artificial shell breaking could be adopted. If the number of seeds treated is large， we can choose 98% concentrated sulfuric acid to soak the seeds for 10 or 15 min， while combining with gibberellin treetment.
References
[1] ZHAO WY， ZHAO CJ， LI CY， et al. Study on antiodidant activities of velvetleaf （Abutilon theophrasti Medicus） extracts[J]. Heilongjiang Medicine Journal， 2018， 31（6）： 1187-1189. （in Chinese）
[2] SHI KM， LI CY， LI C， et al. Progress of study on constituents of Abutilon theophrastii Medic[J]. Heilongjiang Medicine Journal， 2015， （46）： 223-227. （in Chinese）
[3] DENG TF， WANG X. Effect of different plant growth regulators and temperature treatment on germination of velvetleaf seeds[J]. Journal of Henan Institute of Science and Technology： Natural Science Edition， 2015， 43（1）： 20-24. （in Chinese）
[4] WANG JS. Effect of the light and temperature on the germination characteristics of the velvetleaf seed[J]. Northern Horticulture， 2012（1）： 50-51. （in Chinese）
[5] CHANG CS， ZHANG LX， LIU J， et al. The dormancy mechanism and dormancy-breaking method of the seeds of velvetleaf[J]. Plant Physiology Communications， 2016， 52（6）： 967-974. （in Chinese）
[6] KONG XQ， MA KM， LI YP， et al. Effect of bromoxynil on broad-leaved weeds in corn field[J]. Weed science， 2004， 22（4）： 36-37. （in Chinese）
[7] JIANG CG. Optimized study of Abutilon theophrasti trapping Helicoverpa armigera in cotton field[J]. Journal of Anhui Agricultural Sciences， 2015， 43（28）： 81-83， 127. （in Chinese）
[8] ZHAO WQ， JI L. Studies on seed dormany and dormancy breaking[J]. Journal of Shanxi Agricultural Sciences， 2017， 45（3）： 477-481. （in Chinese） [9] BASKIN CC， BASKIN JM. Seeds： Ecoloyg， biogeography， and evolution of dormancy and germination[M]. San Diego： Academic Press， 1998.
[10] MU XQ， SHI L， ZHAO YQ， et al. Seed dormancy mechanism and dormancy breaking methods of Datura stramonium L.[J]. Acta Botanica Boreali-Occidentalia Sinica， 2011， 31（4）： 683-689. （in Chinese）
[11] YANG QH， YE WH， SONG SQ， et al. Summarization on causes of seed dormancy and dormancy polymorphism[J]. Acta Botanica Boreali-Occidentalia Sinica， 2003， 23（5）： 837-843. （in Chinese）
[12] MU XQ， SHI L， ZHAO YQ， et al. Seed dormancy mechanism and dormancy breaking methods of Datura stramonium L.[J]. Acta Botanica Boreali-Occidentalia Sinica， 2011， 31（4）： 683-689. （in Chinese）
[13] ZHAO WQ， LI L. Studies on seed dormancy and dormancy breaking[J]. Journal of Shanxi Agricultural Sciences， 2017， 45（3）： 477-481. （in Chinese）
[14] HUARTE HR， ZORRAQUN MRP， BURSZTYN EM， et al. Effects of environmental factors on seed germination and seedling emergence of common teasel （Dipsacus fullonum）[J]. Weed Science， 2016， 64（3）： 421-429.
[15] NORSAZWAN MG， PUTEH AB， RAFII MY. Oil palm （Elaeis guineensis） seed dormancy type and germination pattern[J]. Seed Science and Technology， 2016， 44（1）： 1-12.
[16] KOODZIEJEK J. Seed dormancy in cereal weed Adonis flammea Jacq. （Ranunculaceae）[J]. Journal of Agricultural Science and Technology， 2018， 20（1）： 109-120.
[17] ZHANG SY， TANG H， HE LX. Effect of gibberellic acid on seed hypocotyl germination and cell structure of seed coat and endosperm of Paeonia rockii[J]. Acta Botanica Boreali-Occidentalia Sinica， 2019， 39（12）： 2187-2196. （in Chinese）
[18] SHAN XD， ZHANG R， MAIWULIDAI·KAHAR， et al. Effect of gibberellin soaking on seed germination of perennial ryegrass under polyethylene glycol simulated drought conditions[J]. Pratacultural Science， 2019， 36（9）： 2304-2311. （in Chinese）
Key words Velvetleaf; Dormancy breaking; Seed germination; Treatment
Velvetleaf （Abutilon theophrasti） is an annual herbaceous plant in Abutilon of Malvaceae， which has strong resistance and wide adaptability and is commonly distributed in roadsides， wasteland and fields. It is widely distributed in all provinces and regions except for the Qinghai-Tibet Plateau in China[1]. Velvetleaf contains phenols， flavonoids， polysaccharides， coumarins， volatile oils， saponins， steroids， terpenes and other components[2]. It is used as a whole herb and has the effect of clearing away heat and toxic materials. It mainly treats dysentery， otitis media， tinnitus， deafness， arthralgia， carbuncles， etc.[1]. The oil content of its seeds is commonly used in soap making， industrial lubricants and energy plants[3]. The stem bark has strong salt and water tolerance and can be used to make ropes[4]， and has important medicinal and economical utilization value. Velvetleaf is cultivated in some areas of Northeast China. The aboveground and underground parts of velvetleaf have a good effect on lead enrichment and transfer. Velvetleaf has the characteristics of strong lead accumulation ability and high low-concentration lead pollution remediation efficiency， and can be used as a potential species for the remediation in lead-contaminated areas[5]. Meanwhile， with the change of farming system， the occurrence of velvetleaf tends to increase. It is now the main weed in cotton， corn and bean farmland[6]， and it is also one of the main trap plants for cotton bollworms[7]. Velvetleaf is propagated with seeds， which have dormancy[5]. Breaking seed dormancy is the foundation of cultivation and research. Seed dormancy is mainly caused by factors such as seed coat obstacle， morphological after-ripening， physiological after-ripening， presence of inhibitory substances， and bad environment[8-9]， and different factors correspond to different dormancy breaking methods. Studies have reported that soaking seeds in warm water at 30-60 ℃[4] and low-temperature treatment of seeds[3] can significantly increase the germination rate of velvetleaf seeds， in which the seed coat obstacle is one of the important factors for velvetleaf seed dormancy[5]. In this study， with velvetleaf seeds collected in the wilderness of northern Suzhou as the research object， how can we break the dormancy of velvetleaf seeds better and best promote seed germination was investigated by physical shell breaking， acid and alkali treatment of seed coat and plant hormone treatment， aiming to lay a foundation for the utilization and development and the comprehensive prevention and control research of velvetleaf.
Materials and Methods
Experimental materials
The velvetleaf seeds used for the experiment were sourced from the wilderness of northern Suzhou. They were collected during the maturation stage of wild velvetleaf plants in autumn 2019， and were air-dried under indoor conditions and stored for future use. Full seeds with uniform size were used as experimental materials. Before the experiment， the surface of the seeds was first sterilized with a potassium permanganate solution with a mass concentration of 0.5%， and then rinsed with distilled water several times.
Experimental methods
Different treatment methods of seed coat
Physical shell-breaking treatment： A knife was used to cut the seed coat on the back of the treatment seeds （PB） without damaging embryos of the seeds. After cutting， the seeds were sterilized with 75% ethanol for 15 s， and then rinsed repeatedly with distilled water for use.
Strong acid treatment： The seeds were added in a beaker， into which 98% concentrated sulfuric acid was added for acid erosion treatment. The treatment was performed with stirring for 5 （SA5）， 10 （SA10） and 15 min （SA15）， respectively. After the treatment， the seeds were immediately rinsed repeatedly with distilled water for use.
Strong alkali treatment： The seeds were placed in a beaker， and added with 40% sodium hydroxide solution. The seeds were soaked for 10 （SH10）， 20 （SH20） and 30 （SH30） min， respectively. After the treatment， the seeds were rinsed repeatedly with distilled water for use. Determined indexes
Determination of seed vigor： Seed viability was measured by the TTC （2，3，5-triphenyltetrazolium chloride） staining method[10]. Fifty seeds were taken in three repetitions， which were soaked in distilled water at room temperature for 24 h. The seed coat was peeled off and cut longitudinally. The seeds were soaked in 0.5% TTC solution， and placed at 30 ℃ in the dark for 12 h， for color development. The embryo and endosperm staining status were observed. The seed vigor was indentified， and the color development results were recorded. The seed viability calculation formula was： Seed viability （%）=Viable seeds/Total number of seeds tested×100.
The law of seed germination and water absorption： After the seeds were treated according to "Different treatment methods of seed coat"， they were put into beakers at 25 ℃ room temperature， with velvetleaf seeds without any treatment as the control （CK）. Distilled water was added to the beakers to soak the seeds， the weight of which was measured every 2 h. Each treatment included 100 seeds， with three replicates. When weighing， absorbent paper was used to absorb the surface moisture， and the seeds were accurately weighed on an electronic balance （accurate to 1/10 000）， followed by recording the data. The calculation formula of seed water absorption rate was as follows：
Seed water absorption rate （%）=[（Seed weight after water absorption–Seed weight before water absorption）/Seed weight before water absorption]×100%
Seed germination index and seedling survival status： The seeds treated under "Different treatment methods of seed coat" and the seeds treated with gibberellin （GA3， 200 mg/ml） after the treatment under "Different treatment methods of seed coat" were taken randomly， 50 seeds for each treatment， in three replicates. The seeds were placed in petri dishes with filter paper. The semi-submerged seed soaking method was used to induce germination. Each culture dish was placed in a constant temperature incubator at 25 ℃ for 7 d. Distilled water was added to the dish every day， under the standard of submerging half of the seeds. The number of seeds germinated was recorded every 24 h， with the radicle extending from the seed coat as the standard for seed germination. The germination vigor （Gv） was calculated on the third day. On the seventh day， the germination rate （G） and germination index （GI） were calculated; the seed embryo axis length， radicle length and fresh weight were measured; and the vital index was calculated. The calculation formulas were as follows： Wherein Gt is the number of seeds that germinated normally in t days; Gn is the number of seeds tested; Ga is the the number of all seeds normally germinated at the end of germination; Dt is the number of days corresponding to Gt; and S is the total length of velvetleaf plant.
Data analysis and treatment
The test data was statistically analyzed using SPSS12.0 statistical software.
Results and Analysis
Seed vigor determination
Seed viability is the potential ability of seeds to germinate or the ability of embryos to become seedlings， but seeds with viability may not all germinate. In this study， the seed vigor was measured by the TTC staining method， and the results showed that 96.67% of the seed embryos were dyed red， that is， the seeds had viability.
Changes of water absorption of velvetleaf seeds under different treatments
The shell-breaking treatment， the concentrated sulfuric acid treatment and NaOH treatment were tested for water absorption rate， with the complete untreated seeds of velvetleaf as the CK. The effects of the four types of treatment methods are shown in Fig. 1. After the seeds absorbed water for 24 h， the water absorption rate reached 168.50% in the shell breaking treatment; the water absorption rate of the untreated intact seeds was 47.61%; the water absorption rate of the seeds treated with concentrated sulfuric acid for 15 min reached 125.90%， and the water absorption rates of the seeds treated with concentrated sulfuric acid for 5 and 10 min were 100.27% and 114.79%， respectively; and the water absorption rate of seeds treated with NaOH for 20 min was 113.74%， and the water absorption rates of seeds treated with NaOH for 10 and 30 min were 90.63% and 117.71%， respectively. It showed that the permeability of velvetleaf seed coat was poor， and eroding the seed coat with acid and alkali could improve the water absorption rate of seeds.
Seed germination of velvetleaf seeds under different treatments
As shown in Table 1， after 24 h， in all kinds of composite treatments with gibberellin， 17 seeds （11.33%） of the seeds of the shell-breaking treatment were germinated in the three replicates in total. The seeds of acid and alkali treatments sporadically germinated. The seeds of the CK did not germinate. Basically no seeds germinated in the various treatments without being composited with gibberellin.
Kuojie XIAO et al. Study on Dormancy Mechanism and Dormancy-breaking Method of Velvetleaf （Abutilon theophrasti） Seeds It could be seen from Table 2 that different treatment methods had different effects on the seed germination index of velvetleaf. The seed germination vigor， germination rate and germination index were relatively large in the shell-breaking treatment and its gibberellin composite treatment， showing that the seeds germinated early and the seedling emerged most neatly， followed by the composite treatments of gibberellin with the concentrated sulfuric acid treatment （10 and 15 min）. NaOH treatment could also promote seed germination to a certain extent， and the NaOH treatment was the best for 20 min， but the germination vigor， germination rate and germination index of alkali-treated seeds were lower than the acid treatments.
Seed survival of velvetleaf seeds under different treatments
It could be seen from Table 3 that the effects of different treatments on the growth of velvetleaf seedlings were slightly different. The shell-breaking treatment was conducive to the elongation and growth of the radicle and hypocotyl of velvetleaf seeds， and the root length， stem length and fresh weight were the largest in the composite treatment of gibberellin with shell breaking; and the stem length was the largest in the composite treatment of gibberellin with shell breaking， followed by the shell-breaking treatment. In the composite treatments with gibberellin， the root length， stem length， fresh weight and vital index of various treatments increased to varying degrees.
Conclusions and Discussion
Seed germination begins with imbibition. When the seed coat is waxy， gelatinous or leathery， moisture and oxygen cannot easily pass through the seed coat， which also prevents the inhibitors from overflowing from the seed coat， resulting in dormancy[5]. In the germination test， the untreated seeds had a germination rate of about 38.00% after 7 d， and 96.67% of the seeds had viability in the seed vigor determination test， indicating that there is a phenomenon of dormancy in velvetleaf. In the water absorption test of velvetleaf seeds， the water absorption rate of the untreated intact seeds after absorbing water for 24 h was about 47.61%， which was 119.89% different from the water absorption rate of the seeds of the shell-breaking treatment. The seed coat largely prevented the embryos from absorbing water， indicating that the hard seed coat of velvetleaf leads to dormancy as an exogenous barrier. The inhibitor action mechanism of seed dormancy believes that seed dormancy is due to the presence of inhibitors in different parts of the seeds， such as aldehydes， phenols， organic acids， abscisic acid， alkaloids， etc.[12]. In the process of seed dormancy and germination， plant hormones can respond to various physiological changes in seeds through signal transmission， regulate the metabolism of a series of proteins and enzymes， and thus regulate the dormancy and germination of seeds[13]. Many studies have shown that exogenous additions such as gibberellin can promote the germination of dormant seeds[14-18]. In this study， gibberellin was added to the velvetleaf seeds treated by shell breaking and acid and alkali， and their germination vigor， germination rate， germination index， vital index and fresh weight of seedlings were further improved. In summary， it is inferred that the dormancy of velvetleaf seeds is a comprehensive dormancy combining poor permeability of exogenous seed coat and endogenous inhibitory substances within the seeds. When the seeds have comprehensive dormancy， a comprehensive method should be used to break the seed dormancy. Both physical shell breaking and chemical corrosion could improve the imbibition of velvetleaf seeds and increase the germination rate of seeds， and after the addition of gibberellin， gibberellin entered the seeds， breaking dormancy and further enhancing the seed germination ability. The composite treatment of physical shell breaking with gibberellin was best， followed by the composite of 10 or 15 min of 98% concentrated sulfuric acid treatment with gibberellin. The shell breaking treatment was time-consuming and labor-intensive， while the treatment with concentrated sulfuric acid was easier to operate and the treatment was more uniform. Therefore， when treating the seeds of velvetleaf， if the treated seeds are less， artificial shell breaking could be adopted. If the number of seeds treated is large， we can choose 98% concentrated sulfuric acid to soak the seeds for 10 or 15 min， while combining with gibberellin treetment.
References
[1] ZHAO WY， ZHAO CJ， LI CY， et al. Study on antiodidant activities of velvetleaf （Abutilon theophrasti Medicus） extracts[J]. Heilongjiang Medicine Journal， 2018， 31（6）： 1187-1189. （in Chinese）
[2] SHI KM， LI CY， LI C， et al. Progress of study on constituents of Abutilon theophrastii Medic[J]. Heilongjiang Medicine Journal， 2015， （46）： 223-227. （in Chinese）
[3] DENG TF， WANG X. Effect of different plant growth regulators and temperature treatment on germination of velvetleaf seeds[J]. Journal of Henan Institute of Science and Technology： Natural Science Edition， 2015， 43（1）： 20-24. （in Chinese）
[4] WANG JS. Effect of the light and temperature on the germination characteristics of the velvetleaf seed[J]. Northern Horticulture， 2012（1）： 50-51. （in Chinese）
[5] CHANG CS， ZHANG LX， LIU J， et al. The dormancy mechanism and dormancy-breaking method of the seeds of velvetleaf[J]. Plant Physiology Communications， 2016， 52（6）： 967-974. （in Chinese）
[6] KONG XQ， MA KM， LI YP， et al. Effect of bromoxynil on broad-leaved weeds in corn field[J]. Weed science， 2004， 22（4）： 36-37. （in Chinese）
[7] JIANG CG. Optimized study of Abutilon theophrasti trapping Helicoverpa armigera in cotton field[J]. Journal of Anhui Agricultural Sciences， 2015， 43（28）： 81-83， 127. （in Chinese）
[8] ZHAO WQ， JI L. Studies on seed dormany and dormancy breaking[J]. Journal of Shanxi Agricultural Sciences， 2017， 45（3）： 477-481. （in Chinese） [9] BASKIN CC， BASKIN JM. Seeds： Ecoloyg， biogeography， and evolution of dormancy and germination[M]. San Diego： Academic Press， 1998.
[10] MU XQ， SHI L， ZHAO YQ， et al. Seed dormancy mechanism and dormancy breaking methods of Datura stramonium L.[J]. Acta Botanica Boreali-Occidentalia Sinica， 2011， 31（4）： 683-689. （in Chinese）
[11] YANG QH， YE WH， SONG SQ， et al. Summarization on causes of seed dormancy and dormancy polymorphism[J]. Acta Botanica Boreali-Occidentalia Sinica， 2003， 23（5）： 837-843. （in Chinese）
[12] MU XQ， SHI L， ZHAO YQ， et al. Seed dormancy mechanism and dormancy breaking methods of Datura stramonium L.[J]. Acta Botanica Boreali-Occidentalia Sinica， 2011， 31（4）： 683-689. （in Chinese）
[13] ZHAO WQ， LI L. Studies on seed dormancy and dormancy breaking[J]. Journal of Shanxi Agricultural Sciences， 2017， 45（3）： 477-481. （in Chinese）
[14] HUARTE HR， ZORRAQUN MRP， BURSZTYN EM， et al. Effects of environmental factors on seed germination and seedling emergence of common teasel （Dipsacus fullonum）[J]. Weed Science， 2016， 64（3）： 421-429.
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