Study On The Reproductive Biology Of The Critically Endangered Plant Magnolia Zenii ’Cheng’

In this paper,Magnolia zenii’Cheng’in Jiangsu Vocational Technical College of Agriculture and Forestry was selected as the study material.The reproduction embryology,flower buds differentiation and development of this species were studied systematically for the first time.Male and female gametophyte development,embryogenesis and nutrients,oxidation process of seed development,and dynamic of endogenous hormones were studied to explore the sexual characteristics and seed development of the species.We aimed to provide scientific information for conservation of this endangered species.The main research conclusions are as follows.1.The process of flower bud differentiation was divided into the following four stages.I.Flower bud differentiation phase.The morphological changes of flower bud differentiation could be divided into the undifferentiation phase,bud scale differentiation phase,the petal primordium differentiation stage,the stamen primordium differentiation phase,and the pistil differentiation phase.II.Bud dormancy phase when buds growth almost stopped growing.III.Flower bud development phase.Flower buds began to develop,and there were obvious morphological changes in the stamens and pistils.IV.Flowering phase.The flowering duration was about half a month.The stamens consists of 65-73 stamens,and the pistil consists of 135-150 carpels.2.There were four pollen sacs in the mature anthers of Magnolia zenii‘Cheng’,the anther wall was consisted of epidermis,endothecium,middle layer and glandular tapetum.The tetrad of microspore was tetrahedral shape,symmetric type or"T"type.Mature pollen grains were mostly two-celled with one germinal aperture.Two ovules were born on the ventral wall of carpels,anatropous,double integument and thick nucellus.The megaspore was monosporogenesis,four megaspores arranged in a line,and the micropylar megaspore was the functional one.The female gametes matured about 30 days later than the male gametes.The mature embryo was 7-cell 8-nuclear polygonum type.The abnormal development of female gametophyte occurred from the stage of dikaryotic embryo to the stage of 8-nucleus embryo.The abnormal development of male gametes mainly occurred in the tetrad stage,which was one of the factors leading to abortion.3.The fruit of Magnolia zenii’Cheng’was a follicle,pericarp gradually changed from green to red,fruit splits in early to mid-September,seeds fall off,hanging on fruit by white stipe silk.The whole seed of Magnolia zenii’Cheng’was nearly round,and the seed coat changed from green to to orange.The seed of removed the seed coat was heart-shaped.The seed coat gradually changed from brown to black,and the dry weight and fresh weight of the seeds showed a trend of slow increase.The transverse and longitudinal diametral of seeds increased rapidly within 75 days after flowering,but the transverse and longitudinal changes of seeds were not significant after 120 days after flowering.The seed development lasted for 165d,which divided into three stages:I.Rapid seed development stage（30-90 days after flowering）,early seed development stage,rapid seed development speed,rapid endosperm development,high seed water content,rapid water accumulation in the seed.II.Rapid embryo development stage（90-135 days after flowering）,seed embryo development experienced proembryo,spherical embryo,heart type embryo,torpedo type embryo,cotyledon embryo and seed water content decreased continuously,and the water content was 20.79%when the seed embryo was fully mature.III.Seed dormancy stage（135-165 days after flowering）,the seed was the dormant stage,the cotyledon embryo morphologically developed completely,the length was only 1.88mm,the moisture content stopped to decline,until the seed was naturally mature,the moisture content of was 20.51%,showing intermediate seed.4.With the continuous development of seed,mitochondria and endoplasmic reticulum and other organelles and powder plastid degradation disappear gradually,the nucleus also gradually narrowed,lipid droplets and protein is increasing in the endosperm cells of Magnolia zenii’Cheng’.The protein body with large size,deep dyeing and irregular shapes was first formed in the vacuole,with the continuous development of the endosperm,the protein body was constantly increasing in quantity and volume,and vacuole gradually disappeared.When the seeds mature,the protein body dissolved and the endosperm was filled with a large number of lipid droplets.At 135day after flowering,when the morphology of seed embryo was fully developed,a light-colored sphere appeared in the endosperm.With the gradual maturity of seeds,the number of spheres increased.It was speculated that this substance was closely related to the formation of seed dormancy of Magnolia zenii’Cheng’.5.The content of total soluble sugar,soluble starch,soluble protein and crude fat in seeds showed an overall trend of increase,and the crude fat content accounted for the largest proportion,showing the characteristics of lipid seeds.90 days after flowering,the four nutrients showed obvious fluctuations,and the activities of NADP-MDH,G-6-PDH and 6-PGDH enzymes all decreased to the lowest level.135 days after flowering,the content of soluble total sugar,soluble protein and crude fat did not change significantly,but the soluble starch decreased sharply to the peak value,the activity of NADP-MDH enzyme was maintained in a stable state,and the activity of 6-PGDH enzyme reached the peak value.Combined with the morphological development of seed embryos,it was believed that the morphological development of seed embryos was completed at 135 days after flowering.Until 165 days after flowering,the seeds matured and fell off,and the activities of G-6-PDH enzyme and 6-PGDH enzyme rapidly decline d to the lowest value in the whole seed development process,indicating that the physiological activities of seeds were rapidly weakened and the seeds turned to a dormant state.6.The content of ABA was always at a high level during the seed development,the content of IAA and ZT followed,and the content of GA₃ was the lowest.The content of ABA in the seeds of Magnolia zenii’Cheng’increased continuously during seed development,while the content of GA₃,IAA and ZT reached the peak at the gelatin-like endosperm development in the early stage of development,and then decreased to the minimum rapidly until seed maturity and shedding.In the rapid development stage of seed embryo,the content of ABA increased rapidly and the content of GA₃ decreased rapidly,indicating that ABA had a promoting effect on the development of seed embryo.ABA played a very important role in the seed development and maturation of Magnolia zenii ’Cheng’.