| There is a reservoir in the upstream of Three Gorges Dam(TGD)after impounding which the length is more than 660 km.The water level is fluctuated by human regulating.Currently,impoundment begins every year's September,the water level raise to the maximum in the mid-October,and gradually begin to decline in January the next year,finally in May dropped to the lowest water level :145 m.The impoundment make the drawdown zone between them can get to 30 m vertically.The original water-level fluctuation rhythm of Yangtze River is changed dramatically because of impoundment.As a result,the drawdown area appeared named Reservoir drawdown area of the Three-Gorges Reservoir(Reservoir Drawdown Zone,RDZ)which submerged in all winter.Furthermore,the Yangtze River riverbank in the upper reaches of Jiangjin has not been regulated by impoundment,only influenced by the annual summer flood,it is Natural Riparian Zone(NRZ).At present,the vegetation in the water-level-fluctuating zone is mainly composed of annual plants,NRZ widely distributed winter annuals,and RDZ is dominated by summer annuals.The seeds of these annuals could tolerate submergence in the water-level-fluctuating zone.For a large number of seeds,long time submergence is a serious environmental stress.Why the seeds of these annuals could tolerate submergence in the water-level-fluctuating zone? Physical dormancy,endogenous dormancy,induced dormancy and the others may be the reasons that these seeds could tolerate submergence,therefore,in order to clarify the submergence tolerant mechanism of seeds in different water rhythm,we collected the seeds(fruits)of one year dominant annuals in NRZ: Rumex dentatus L.,Stellaria media(L.)C yr.,Torilis scabra(Thunb.)DC.,Vicia angustifolia L.which all winter annuals,these seeds were treated differently: freshly harvested intact seeds(f);freshly harvested and then scratched seeds(s);freshly harvested and then stored to winter(aw),we conducted the germination,rot ratio,intactness ratio,as well as the germination dynamics during water submergence of summer(23 oC,90 d),we also compared the seeds were submerged in suitable germination condition(13 oC,90 d)as control.We also collected the seeds(fruits)of dominant annuals in RDZ: Echinochloa crusgalli(L.)Beauv.,Eleusine indica(L.)Gaertn.,Bidens tripartita L.,Celosia argentea L.,Polygonum lapathifolium L.,which all summer annuals,these seeds were treated differently: freshly harvested intact seeds(f);scratched seeds(s);freshly harvested and then stored 1 year;freshly harvested and then stored 2 years,we conducted the germination,rot ratio,intactness ratio,as well as the germination dynamics during water submergence of winter(13 oC,90 d),we also compared the seeds were submerged in suitable germination condition(23 oC,90 d)as control.The results were as follow:Topic 1(NRZ): the seeds(fruits)could keep a high proportion of intactness ratio after summer submergence;the fresh fruits(f)germination of Rumex dentatus is 12.22%,which was significantly lower than fruits were stored to winter(aw)of 47.78%;seeds germination of Stellaria media were low at different treatments,however,germination at 13 oC(97.78%)was significantly higher than that of other treatments;the fruits germination of Torilis scabra were lower than 85% at different treatments,which was significantly lower than that of 13 oC(94.44%);seeds germination of Vicia angustifolia were low at different treatments,however,germination of scratched seeds(s)(97.78%)was significantly higher than that of other treatments,furthermore,there was significant difference in water content between the seeds without submerged and the waterlogged seeds.In summary: that was after ripening effect was not completed caused fruit of Rumex dentatus could tolerate submergence;the main reason why the seeds(fruits)of Stellaria media and Torilis scabra could tolerate submergence was that they were in induced dormancy;the protective effect of seed coat(physical dormancy)is the main reason that the seeds of Vicia angustifolia could tolerate submergence.Topic 2(NRZ): the seeds(fruits)could keep a high proportion of intactness ratio after winter submergence;the fresh fruits(f)germination of Echinochloa crusgalli were low at 13 oC and 23 oC;germination of fruits that stored 1 year and stored 2 years were significantly higher than that of other treatments,what's more germination at 23 oC were significantly higher than that of 13 oC;the fruits germination of Eleusine indica at different treatments(fresh fruits(f),stored 1yeae fruits(1year)and stored 2years fruits were significantly lower than that of scratched fruits(s),furthermore,the scratched fruits(s)germination at 13 oC were significantly lower than that of 23 oC;the fresh fruits(f)germination of Bidens tripartite was low at 13 oC(0.5%),germination increased to 62% at 23 oC,the same result as scratched fruits(s)germination at 13 oC and 23 oC(15%,95%),and that it was significantly higher than that of fresh fruits(f);the germination ratio of freshly harvested seeds of Celosia argentea was 0% at 13 oC,which was significantly lower than that under 23 oC(74.5%);freshly harvested fruits of Polygonum lapathifolium did not germinate at 13 oC and 23 oC,after scratched the seedcase,the seed germination increased,and the fruit germination at 23 oC was higher than that of 13 oC.In conclusion: it was the coincidence of endogenous dormancy and induced dormancy that the fruit of Echinochloa crusgalli fruit could resist submergence;the seedcase of Eleusine indica,Bidens tripartite and Polygonum lapathifolium had a mechanical restraining effect and contained germination inhibiting substances,what's more,the low temperature of winter submergence induced the dormancy of fruits;Induced dormancy is the main reason that the seeds of Celosia argentea could tolerate submergence. |
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