| Sassafras tzumu mostly grows loosely or forms mixed forest,which is a common tree species in the middle and low mountains of Yangtze valley and its south.The S.tzumu grows fast in the early stage.The tree is tall and straight,with beautiful and decay-resistant wood quality and the scenery of flowers in spring and leaves in autumn.It is a valuable tree species with high quality wood and landscape.The fruit of S.tzumu ripens in summer and it is difficult for seeds to maintain the vitality for a long time under high temperature,and the seeds are dormant.Therefore,the appropriate seed storage,release of dormancy,faster and uniform germination are the key to seedling production.In this study,the viability after seed harvest,characteristics and causes of seed dormancy were studied.In addition,the physiological and molecular basis of dormancy release was explored by the analysis of endogenous hormones,nutritional metabolism,transcriptomics and proteomics.The results above can deepen the understanding of S.tzumu seed dormancy,and provide theoretical and technical support for the seeding breeding.The main conclusions of the study are as follows:（1）The fruit of S.tzumu is a berrylike drupe that ripens in late June to late July with the moisture content of 47.98%.The seed does not have endosperm and the viability is about 78%.The fruit loses water quickly under high temperature after harvest and the seeds lose viability with the temperature above 32℃within 3-5 days.Therefore,the pericarp shall be removed from the fruit immediately after harvested to avoid loss of seed quality caused by water loss under high temperature.（2）The physiological dormancy of S.tzumu seeds is mainly caused by the inhibitory substances in the seed coat and embryo,moreover,there is shallow dormancy in the decotyledoned embryos.The GC-MS results show that there are a variety of compounds in seed,including alkanes,esters,ketones,alcohols and organic acids,among which the dibutyl phthalate,methyl octadecenoate,methyl linoleate,oleic acid,ethyl propionate,oleoylglycerol,octadecenoic acid and palmitic acid have been proved to have the function of inhibiting germination in other plant seeds.（3）The S.tzumu seed could be safe storage for more than 8 months under low temperature with the characteristics of low moisture content and high viability.When the seeds were stored at 4℃,the contents of IAA,ABA and JA decreased firstly and then increased,the contents of GA3 continuously increased,and the contents of ZR remained relatively stable.The ratios of IAA/ABA decreased firstly and then increased,GA3/ABA increased slowly,ZR/ABA increased firstly and then decreased.The contents of soluble sugar and crude fat decreased,and the free amino acids increased.The germination percentage of seeds stored at 4℃was significantly positively correlated to GA3 and free amino acids content（p<0.01）,positively correlated to GA3/ABA（p<0.05）,and negatively correlated to soluble sugar（p<0.05）.（4）The treatment cold stratification could promote the dormancy release of S.tzumu seeds.About 54.44%seeds germinated after 8 months of stratification at 4℃,and the germination percentage of seeds increased to 69.33%treated with concentrated sulphuric acid for 30 s and soaked with 0.1%GA3 for 8 h after cold stratification.（5）RNA-seq technology was used to study the transcriptome of S.tzumu seeds dormancy release.A total of 3962 and 12426 DEGs were identified during seed cold stratification and spouting stages,respectively.The classification and metabolic pathways of DEGs were similar,but the genes expression patterns were different.The DEGs relevant to dormancy pathway were mainly down-regulated during seed cold stratification.The activities of glycolysis and tricarboxylic acid cycle in seeds were low,the sensitivity to IAA was decreased,while the GA and ABA were increased.The level of protein translation was low,and the metabolism speed was slow.The DEGs were mainly up-regulated during seed spouting stage,the glycolysis activity was enhanced,the sensitivity to GA and ABA was weakened,the metabolism of amino acids and ribosomes was vigorous,and the level of oxidative phosphorylation was enhanced,which promoted the transformation of seeds to germination.（6）The proteomics of S.tzumu seeds during dormancy release were analyzed by iTRAQ and mapped with Unigene as the protein database.A total of 268 non-redundant proteins were identified.Among them,100 and 74 EDPs were identified in cold stratification and spouting stages of S.tzumu seeds,respectively,which mainly involved carbon metabolism,amino acid biosynthesis,pyruvate metabolism,fatty acid synthesis.The EDPs analysis showed that the EMP-TCA existed in the respiratory metabolism of seeds dormancy release,and the respiration was enhanced during the spouting stage,moreover,amino acid biosynthesis and HSP70 protein may play an important role in regulating the dormancy release of S.tzumu seeds. |
