Study Of Seed Dormancy Formation And Breaking In Legume Species

Most temperate and arctic-zone species of Fabaceae have an impermeable seed coat that imposes a physical exogenous dormancy on the seed.However,mechanisms underlying dormancy formation and release in seeds of legumes are generally poorly understood.Present study examined seed dormancy formation and release,and dormancy release mechanisms in several wild and cultivated legume species by a series of field and lab experiments,the results are summarized below:1)Similar to most legume species,Sophora alopecuroides seed developmental process showed three semi-independent phases which were rapid seed growth,food reserve accumulation and seed maturation stages.However,in compared to other legume species,a longer seed growth stage which started from flowering to 30 days after flowering was observed in S.alopecuroides seed.In addition,there was a flat phase in food reserve accumulation stage which lasted for 6-9 days in the S. alopecuroides seed developmental process.In compared to drought habitat,plant in the wet habitat attained significantly higher thousand seed weight,however,the time formed viable seed was delayed.2)Seed dormancy begin to form by the end of food reserve accumulation stage, and the percentage of dormant seed increased dramatically as the seed water content declined to 20%;and arrived 100%when the seed water content declined to about 10%.Consistent with seed dormancy formation,seed coat palisade cell have formed by the end of rapid growth stage and the cell layer become dense at the food reserve accumulation stage.It indicated that the structure change in the palisade cell layer during dehydration process is one of the most important factors for seed impermeability.3)The response of S.alopecuroides seed to burial depth and micro-environments varied with population.E.g.the dormancy release rate of S.alopecuroides seed from Ejina County declined as burial depth increased;however,for seed from Zuo County, the fastest dormancy release rate was observed in the 2 cm burial depth,but slowest when seed exposed to soil surface.The results from lab simulated experiments indicated that high temperature is the most important factor for dormancy release for seed from Ejina County,however,dormancy release for seed from Zuo County required seeds exposed to wet heat condition.In compared to seed from Ejina County, seed from Zuo County is smaller,seed coat is darker,and the seed coat thickness beneath hilum is significantly lower.These differences may contribute to the difference of dormancy release requirements between populations.4)After dormancy breaking treatment with sulfuric acid,hot water and liquid nitrogen on Medicago sativa,Melilotus alba,Lotus corniculatus,Coronilla varia,Vicia unijuga,Sophora alopecuroides,Sesbania sesban,Vigna oblongifolia,Vigna membranacea,the results showed sulfuric acid treatment was the most effective method to release seed dormancy,but the optimal treatment time varied with species, longer treatment time may increase dead seed percentage.Refered to hot water treatment,80℃was the optimal temeperature for dormancy breaking,and 60℃showed less effect,while 95℃leaded most seeds to death.Effects of liquid nitrogen treatment were closely correlated with seed size,e.g.liquid nitrogen effectively released dormancy of small size seed such as Medicago sativa,but showed no effects on dormancy of middle size seed such as Sesbania sesban,and harm some large size seed such as Sophora alopecuroides.In addition to that,the time dipped into liquid nitrogen showed no effects on dormancy breaking.5)The seed dormancy breaking process could be classified different stages,e.g. slow permeable and fast permeable stage.However,the key sites determining seed entered each stage varied with species,e.g.hilum was the key site made Sophora alopecuroides seed enter slow permeable stage,but the key site for Sesbania sesban seed was lens.The primary site of water entry into seed after dormancy breaking varied with species and dormancy breaking methods.E.g.after sulfuric acid treatment, the primary site of water entry into Sophora alopecuroides seed is hilum;however, after hot water treatment,the sites were hilum and extrahilar region.And also,for Sesbania sesban seed,the primary site of water entry into seed was lens whatever the treatments.Again,test time significantly affected determing of primary site of water entry into seed after dormancy breaking.E.g.less than 5 days test time obviously underestimated seed number entering into slow permeable stage,and 14 days or longer time is better for water imbibition site test.6)Four independent experiments were designed to investigate the effects of the pericarp on seed imbibition,dehydration,germination,seedling establishment and seed longevity in the field in seeds of Hedysarum scoparium.The results showed that the presence of the pericarp significantly decreased initial seed imbibition rates and germination percentage in petri dishes and pot experiments.However,pericarp significantly increased final seed water content after fully imbibition and seedling survival rate after seed germination in the drought condition.It also significantly increased seed longevity under field condition.