| As far as the use of heterosis concerned soybean lagged behind the other field crops such as corn,sorghum,rice,rape seeds and so on.The major obstacles came from the lack of optimum mechanism to avoid self pollination on female parent.And the hybrid seeds production was also relatively difficult because of low efficient pollen transfer from male to female parent.Breakthrough happened in 1993 when the first cytoplasmic male-sterile line was developed by Sun et al.The tree lines system that could be used in hybrid soybean breeding was soon established later.The fist hybrid soybean “HybSoy 1” was released in Jilin Provence in 2002.A total 16 hybrid are available in China at present.The researches on seed production technologies have achieved great progresses.China is leading the world in this field.However the low seed setting rate on female parent in seed production field or low outcrossing rate in another word is still the bottleneck of commercialization of hybrid soybean.The unsatisfied seed yield of those hybrids that showed very good performance in the production field leads to higher seed price and than the farmer are reluctant to buy the hybrid seeds.Outcrossing rate is influenced by multiple factors including the species and amount of pollination insect vectors,the environment of seed production area and microclimate in seed production field,the genotypes of parents and the allocation pattern and growth status in the field.The genotypes of parents,especially the characters related to the attraction to pollination insects are the basic one in determining the outcrossing rate among above mentioned factors.Experienced for years the regions that are favorable to the hybrid seeds production have been identified and the wild bees and trained honey bees have been found to pollinate soybean very well.In general,the technical system for producing hybrid seeds has been established.In certain conditions the yield of hybrid seeds reached 1,300Kg/hm for specific hybrid combination.In this case the development of CMS three lines with high outcrossing characteristics becomes the key to improve the hybrid seeds production.What characters are closely related to outcrossing rate? How to identify or evaluate male-sterile and male-fertile lines for the high outcrossing rate in breeding populations will addressed in this study.264 CMS three lines were used in this experiment.Based on the general understanding of the effect of characters and flowering biology on outcrossing rate,this study was mainly focused on the secretion rhythm of nectar and aromas or volatiles among the three lines with varied outcrossing rate.The correlations between data of outcrossing rate taken in caged plots and that taken in open field plots were calculated.The preferential visiting behavior of leafcutter bees was performed in this study.A micro plot design was used in the open field to evaluate outcrossing rate.A method was developed by the use of micro plots to high efficiently evaluate the outcrossing rate in a large scale in open field.This system provided basic theory and techniques support for the breeding of three lines with high outcrossing rate may boost the commercialization of hybrid soybean in turn.The main results are as folloows:1.Some characters of male-sterile lines and maintainers such as stem termination pattern,leaf shape,flower color,origin of parents had no any relations with outcrossing rate.However the flowering time showed obvious influence on outcrossing rate.The outcrossing rate of a group male-sterile lines flowering on June 15-20 was obviously higher than that of later flowering genotype.No differences were detected among other groups.Maturity time was also found to have impact on outcrossing rate.The outcrossing rate was much higher in an early maturing malesterile lines flowering on August 20-25 than that in other groups,and no obvious difference was revealed among other groups.2.The flowering characteristics of male-sterile lines and their maintainers within the same maturity group gave great impact on outcrossing rate.Those characters included total number of flowers,days of flowering,daily flowering number,the size of flower(length,width,diameter of receptacle,and the area of flower).The total numbers of flower,days of flowering,flowering numbers per day,width of flower and area of flower in restorer lines expressed obvious effect on the seed setting in male-sterile lines of the female parent.But such kind of effect was not detected for daily flowering number and diameter of receptacle.3.The nectar secretion was found in all CMS three line in soybean and it seemed to have the following rhythm: Secretion initiated at about 06:00 a.m.for most materials and reached a peak at 07:00-08:30.after flower opened,then the nectar secretion decreased gradually.A sub-peak appeared at about 13:00.while the nectar could not be detected at 17:00.Nectar secretion was greatly influenced by the weather conditions.The amount of nectar secretion increased gradually over time during periods of high temperature and no rainfall for several days.Rainy weather and low temperatures inhibited nectar secretion.There were obvious variations of nectar amount among different genotypes tested.Significant nectar variation within a genotype among years was also observed,and the highest nectar secretion was 3-fold higher than the lowest.The amount of nectar secretion from restorers was significantly higher than that of male-sterile lines and maintainers.There was no significant difference in nectar secretion between male-sterile lines and maintainers.male-sterile lines and maintainers with higher outcrossing rates secreted more nectar.The amount of nectar secretion of male-sterile lines and maintainers were significant positively correlated with the outcrossing rate of male-sterile lines.4.As many as 31 kinds of volatiles including 1-Octen-3-ol?Methanol?Acetaldehyde?Caproaldehyde?3-Octanone et al.were detected in CMS three lines of soybean.The types of volatiles were the same at different sampling time,but the relative percentage of each volatile was differentiated greatly.The similar situation was observed in all CMS three lines.The ratio of each of 8 types of volatiles including formaldehyde was different between male-sterile lines and maintainer lines.The ratio of each of 15 types of volatiles including Methanol was different between male-sterile lines and restorer lines.Only 3 types of volatiles were differed between maintainer lines and restorer lines.They were 3-Methyl butanal,Trans-2-hexenal and 2-Pentylfuran.The contents of 5 types of volatiles were found obviously different between high outcrossing CMS A-lines and low outcrossing CMS A-lines.Those compounds were 3-Methyl butane,Valeraldehyde,Trans-2-Pentenal,Methyl acetate and Nonana.The relative content of two compounds of acetaldehyde and n-propanol were found obviously different.The contents of all kinds of volatiles were found no deference between high outcrossing CMS A-lines and their maintainers and the same were also found between low outcrossing rate CMS A lines and their maintainers.5.Compared to their maintainers(iso-lines)the female fertility of high outcrossing CMS Alines was normal.On the contrary the female fertility of low outcrossing CMS A-line was lowed down compared to their maintainer CMS B-lines.The female fertility of high outcrossing CMS Alines was better than that of low outcrossing CMS A-lines.No obvious differences of female fertility were detected among high outcrossing rate CMS B-lines,medium outcrossing rate CMS B-lines and low outcrossing rate CMS B-line.6.The outcrossing rate of CMS A-line was significant positively correlated with the visiting frequency to CMS A-line,accumulated visiting time to CMS A-line and visiting frequency to CMS B-line by pollinators and the correlation coefficient was 0.726,0.664 and 0.602 respectively.The outcrossing rate of CMS A-line correlated positively with accumulated visiting time to CMS B-lines and the correlation coefficient was 0.513.The visiting frequency to CMS A-lines was significant positively with the visiting frequency to CMS B-lines and the correlation coefficient was 0.676.7.The outcrossing rate measured in the open field was significant positively correlated with that in the caged plots and the correlation coefficient was as high as 0.855.8.The outcrossing rate of male-sterile lines was evaluated in three different forms of micro plots in the field.The results were coincided very well with the data gathered from caged plots.The method used here could be practiced for the evaluation of outcrossing rate of male-sterile lines.As far as the effect of pollen transfer concerned treatment I and III were better to treatment II.Considering the similarity of phenotypes of CMS A lines and their isolines B-lines it might be little bit difficult to exactly identify A-lines from B-lines in harvesting time the treatment II should be the first choice.Owing to the alternatively planting of A-lines and B-lines in the field it will make the sampling operation easy,convenient and less mistakes.9.In the micro plots,high outcrossing rate A-line was used for common female parent,unevaluated fertile germplasm were used for male parent,known outcrossing rate B-lines were use for male parent check,through natural insect pollination,it can be used to evaluate the outcrossing ability of the fertile germplasm by investigating the difference in the number of female parent's pods.This method can evaluate any cultivated soybean fertile resources effectively,and it has been applied in the practice of hybrid soybean breeding. |
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