Seed Development,Dormancy And Germination Characteristic Of Paeonia Ostii

All species in section Moutan of Paeonia(Paeoniaceae)are collectively called tree peony.Aside from its ornamental value as the "king of flowers," its root have been used in traditional medicines to treat convulsions and for analgesic uses.Meanwhile,the seeds contain unsaturated fatty acids,which are one of the most promising types of healthy edible oils.Seeds provide an important method for propagating tree peony species.However,seeds of tree peonies have epicotyl dormancy and undeveloped embryos,which create many problems to seedling cultivation and management.Therefore,this thesis used Paeonia ostti,a main tree peony species cultivated for oil production,as a material for studying the morphology,anatomy,seed germination,and storage compounds of P.ostii seeds during development and to reveal the developmental characteristic of P.ostii seeds.In addition,by studying the dormancy-related responses of freshly matured seed to habitat conditions(temperature and light)and germination phenology to understand the relationships between seed dormancy/germination and environmental factors.On this basis,the thermal and hydrotime models of seed dormancy and germination were established to quantify the temperature and water requirements related to seed germination.The main results are shown as follows:(1)Paeonia ostii requires about 100 days from flowering to seed maturity.The seeds gain the ability to germinate after 60-70 days of development.During development,the water content of the seeds gradually decreases,while the dry weight and the content of storage compounds(including sugar,starch,and protein)gradually increases.The length,width,height,and fresh weight of the seeds increased during development and then decreased,with peaking at 65 days(2)During seed development,the cells formed in the initial division of the zygotes did not form cell walls,but instead appeared with a free nucleus stage which is similar to the early stage of development of seed embryos in gymnosperms.After 25 days of development,the free nuclei begins to develop into cells.Next,the development of the seed embryo is similar to that of other angiosperms.Spherical,heart-shaped,and cotyledon-shaped embryos were observed after 45,60,and 70 days,respectively.Paeonia ostii endosperm development follows the nuclear type,which develops earlier than the seed embryo.About 35 days passed from the division of the primary endosperm nucleus to the beginning of the endosperm cellularization,and the endosperm changed from liquid to solid after 70 days.From the exterior to the interior,the seed coat consisted of the following four layers:cuticle,palisade layer,osteosclerid cells layer,and the parenchymatous layer.(3)Freshly matured seeds of P.ostii have deep simple epicotyl morphophysiological dormancy,i.e.seeds of P.ostii had embryos that were differentiated but underdeveloped(small),and thus have morphological dormancy.Freshly matured seeds do not have an epicotyl,which begins to differentiate when the radicle begins to break through the seed coat.Moreover,the radicle and epicotyl themselves have physiological dormancy.Germination of P.ostii seeds requires warm stratification for embryo growth and radicle protrusion followed by cold stratification for epicotyl growth.Gibberellin acid can promote a release from dormancy Seeds that are breaking shoot dormancy may enter a secondary dormancy under conditions that are inappropriate for shoot germination.The entire dormancy breaking process takes 100 days,seeds with root lengths of 3-5 cm take less time to germinate than seeds with root lengths of 1-2 cm(4)Variations exist in temperature thresholds and basal water potential for individual seed germination within a population.Based on the germination rate(1/t50),the three critical points of temperature required to stimulate the germination of P.ostii shoot are:a base,optimal,and ceiling temperatures of-2.52℃,1.41℃,and 11.8℃,respectively;the thermal time constant was 189.03℃.d.The three critical points of temperature required for germination of the radicle are:the base,optimal,and ceiling temperatures of 14.56℃,19.42℃,and 30.58℃,respectively;the thermal time constant was 162.61℃.d.Based on the germination rate(1/t50),it was determined that the basic water potential required for the germination of P.ostii shoots was-0.94 MPa;the hydrotime constant was 84.99 MPa.d.Meanwhile,the basic water potential of the radicle was-0.44 MPa,and the hydrotime constant was 17.11 MPa.d.The thermal and hydrotime model could accurately predict the germination time course under different temperatures and water potentials.