| The water-management method of "storing clear water and discharging muddy water" has been adopted after completing the Three Gorges Dam,resulting in a water-level-fluctuation zone with a vertical drop of 30 m on both sides of the reservoir.The water-level-fluctuation zone in the Three Gorges Reservoir area is characterized by flooding in non-growing season,prolonged flooding,and sizeable submerging depth.The significant change of the growing environment and the frequent disturbance of unseasonal flooding caused most original plant species’ gradual disappearance.The water-level-fluctuation zone of the Three Gorges Reservoir is facing a series of problems such as reduction of biodiversity,severe soil erosion and,degradation of the ecosystem.Vegetation is an essential part of the ecosystem,which plays a vital role in reducing soil erosion,enhancing slope stability,and protecting biodiversity.Therefore,the restoration and reconstruction of vegetation in the water-level-fluctuation zone are of great significance for managing the ecological environment in the Three Gorges Reservoir area.The selection of amphibious plant species that are tolerant to both flooding and drought is the key and prerequisite for the restoration of vegetation in the water-level-fluctuation zone of the Three Gorges Reservoir area.The results show that Taxodium distichum var.imbricatum can withstand the prolonged and frequent unseasonal flooding caused by the impoundment of the Three Gorges Reservoir.Therefore,pond cypress is considered as a suitable woody plant in the water-level-fluctuation of the Three Gorges Reservoir area.In addition to water-submersion resistance characteristics,pond cypress also has the advantages of salt-alkali resistance,strong adaptability,rapid growth,good wood,and high ornamental value.It is an excellent tree species for vegetation restoration in water-level-fluctuation zone and river wetland afforestation.Therefore,it is of great ecological and economic value to study its production and propagation technology.Seed propagation is the most common method of plant reproduction.However,there are some problems in the direct sowing of untreated pond cypress seeds,such as slow germination,uneven emergence of seedlings,long continuous germination time,large seed consumption,and low seeding rate,which bring some difficulties to the production management.In order to promote the effective utilization of pond cypress seeds germplasm resources,the seed dormancy of pond cypress seeds was studied by scanning electron microscope(SEM)observation of seed coat structure,determination of seed coat water permeability,extraction,and determination of seed endogenous inhibitors.Different storage treatments,different acid-scarified treatment+GA treatment,different temperature treatment,different concentrations of citric acid treatment,and different time ultrasonic treatment were used to explore the effective measures to release the seed dormancy of pond cypress seeds.In order to explore and understand the release process of seed dormancy,the contents of soluble sugar,starch,and soluble protein and the activities of acid phosphatase,peroxidase,and glucose-6-phosphate dehydrogenase were measured.It is hoped that the research will provide a theoretical basis for the measures taken in production to increase the germination rate and regularity of pond cypress seeds and can provide a theoretical basis and technical guidance for the production of seedlings required for vegetation restoration in the water-level-fluctuation zone of the Three Gorges Reservoir.The main research results obtained are as follows:(1)In this experiment,the seed viability of the newly harvested pond cypress was 80.95%,but it did not germinate and had significant dormancy.The seed coat of pond cypress from outer to inner is epicuticle,sarcotesta,sclerotesta,and endotesta.Compared with the control,the hot water treatment could promote the abscission of the epicuticle but had no obvious effect on the structure of the seed coat.The seed coat permeability had no noticeable change.The corrosion cracks and pits appeared on the seed coat surface after acid-scarified treatments.With the extension of treatment time,the corrosion cracks and pits increased,the seed coat structure was further damaged,and the seed coat permeability increase.Although the seed coat of pond cypress is thick and hard,it has certain water permeability.The gametophyte and embryo can absorb part of the water through the seed coat.The acid-scarified treatments were beneficial to promote the water absorption of gametophytes and embryos.However,the hard seed coat hinders the water absorption and expansion of the gametophyte and embryo to a certain extent.It may produce mechanical resistance to the elongation and growth of the radicle.(2)The seed coat of pond cypress contains endogenous inhibitors.The inhibition intensity of methanol extracts on seedling and root growth was as follows: seed coat > gametophyte and embryo,which indicated that the inhibitory substances in seed mainly existed in the seed coat.The results of the separation and bioassay of methanol extract from the seed coat of pond cypress showed that all the separated phases of methanol extract from the seed coat had different inhibitory effects on the growth of Brassica pekinensis.Among them,petroleum ether had the most substantial inhibitory effect on seedling height and root length of Brassica pekinensis.It can be concluded that the inhibitory substances are mainly concentrated in the petroleum ether phase,and they may be some substances with less polarity.Therefore,the structure and endogenous inhibitory substance of the seed coat of pond cypress are the main reasons for seed dormancy.The seed of pond cypress is a comprehensive dormancy with both physical dormancy and physiological dormancy.(3)The single physical or chemical measures,such as temperature treatment,ultrasonic treatment,and citric acid treatment,had a certain effect on releasing the seed dormancy.However,there were still a lot of seeds not germinating.The combination of acid-scarified treatment and gibberellin soaking treatment could obtain a better germination effect in a short time.The highest germination rate was 36.52% after acid-scarified treatment for 30 min and soaking in gibberellin 300 ppm for 7 days.In addition,acid-scarified treatment significantly promoted seed germination,which indicated that the mechanical constraint of seed coat had a serious hindrance to seed germination.The results showed that low-temperature stratification,low temperature to warm temperature stratification,and water immersion treatment could effectively release seed dormancy.The germination rate of waterlogged seeds could reach 72.17% after 60 days.The seeds germinated quickly and orderly,and the germination effect was ideal.(4)The decomposition and transformation of macromolecular nutrients play an important role in releasing seed dormancy.The contents of soluble sugar,starch,and soluble protein in the seeds of pond cypress had apparent changes during the process of dormancy release.With the extension of storage time,the contents of soluble sugar and soluble protein in the treated seeds generally showed a first increasing trend,then decreasing and then increasing.Although the seed starch content of each treatment fluctuated slightly,the whole showed a gradually decreasing trend.It was concluded that the changes in nutrients were closely related to the release of seed dormancy.(5)With the gradual release of seed dormancy,the key enzymes in the seed were gradually activated.The activities of acid phosphatase,peroxidase,and glucose-6-phosphate dehydrogenase were significantly increased with the prolonging of storage time.Different storage conditions had different effects on the peroxidase activity of seeds,and the overall fluctuation of each treatment was different.Still,the enzyme activity tended to increase with the prolonging of storage time.The activities of glucose-6-phosphate dehydrogenase and acid phosphatase showed an“up-down-up-down trend”.The results indicated that these three enzymes are closely related to the release of seed dormancy and can be used as indicators of the seed dormancy release process.In conclusion,seed dormancy of pond cypress is mainly caused by mechanical restraint of seed coat and inhibitory substances in the seed coat,which belongs to the type of comprehensive dormancy.The combination of acid-scarified treatment and gibberellin soaking treatment could release the dormancy of some seeds.Low-temperature stratification,low temperature to warm temperature stratification,and flooding treatment can release most of the dormancy of seeds.In the storage process,the decomposition and transformation of the main storage substances,such as soluble sugar,starch,soluble protein,and the gradual activation of key enzymes,such as peroxidase,acid phosphatase,and glucose-6-phosphate dehydrogenase,play a crucial role.Stratification and flooding are easy to operate and pollution-free,which are ideal measures to release pond cypress seed dormancy and can be widely used in seedling production of pond cypress. |
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