Study On Pollen Limitation And Sexual Reproduction Abortion Of Symphytum Officinale (Boraginaceae)

As a high-quality forage grass,the phenomenon of ’more flowers and less fruits’ has generally existed since the introduction of Symphytum officinale.The reasons for the low seed setting rate and fruit setting rate are unknown,leading to an extreme lack of basic theoretical support in seed production.Consequently,its excellent characteristics have not been scientifically developed and utilized.To solve the above problems,the key is to find out the reasons that lead to the low seed setting rate.Therefore,in this paper,a supplementary pollination experiment was carried out on natural population of S.officinale to explore whether its seed setting was limited by pollen.Using paraffin section,optical section(laser scanning confocal microscope),scanning electron microscope and other methods,the reproductive biology related problems such as the perfection of anther,stigma,pollen and embryo sac development were studied,and then the reasons for the low seed setting rate of the natural population of polymer grass were revealed,which laid the foundation for the scientific development and efficient development of S.officinale.The main results are as follows:(1)The seed-setting rate and fruit-setting rate of S.officinale were significantly increased after supplementary pollination with cross-pollinated pollen,compared with those after natural pollination.It was found that the source of pollen significantly affected the seed yield of S.officinale,and the seed-setting of natural population of S.officinale was limited by the quality of pollen;(2)The pollen of S.officinale viability was detected to be low.The morphology of pollen grains of S.officinale was observed under a scanning electron microscope.The pollen grains of S.officinale were long-spherical,and there existed a large number of serious deformation and shrinkage of pollen grains.There were no significant differences or defects in the exine ornamentation of the long-spherical pollen grains and the deformed and shrunken pollen grains;During the meiosis of S.officinale pollen mother cells,there existed abnormal behaviors of delayed chromosomes and chromosome bridges in the anaphase of meiosis anaphase I and meiosis anaphase II.There were abnormalities in the cytoplasmic division of microspores.monozygote,diad,triads,diad with a micronucleus,tetrads with a micronucleus and polyad were observed at the tetrad stage;the above abnormal meiosis and cytokinesis lead to a large number of pollen abortion;(3)The paraffin section showed that the anther wall of S.officinale would fully differentiate in the stage of pollen mother cells and formed a pollen sac with a large number of pollen mother cells inside.The microspore undergoes deformation and shrinkage after being released from the tetrad,the phenomenon of pollen deformation and shrinkage may be related to the degradation time line of tapetum;(4)Stigma receptivity of S.officinale is strong and the duration is long;The mastoid cells on the stigma surface of S.officinale were papillary and plump in shape,and there was no significant development defect.The stigma of S.officinale had no secretion and belonged to dry stigma;When the stigma of S.officinale received fertile pollen,the pollen could normally germinate and extend into the ovary,and the absence of pollen tube in part of the style might be due to poor fertility of the received pollen;The embryo sac was observed under a laser scanning confocal microscope using the technique of integral ovary transparency.No significant defects were observed during the development of the embryo sac in S.officinale,which allowed for the formation of a normal mature embryo sac.The influencing factors of low seed-setting rate of natural population of S.officinale are diversified.The seed-setting of natural populations of S.officinale is limited by pollen,so it is difficult for it to receive cross-pollinated pollen,which leads to its low seed setting rate.As the male gametophyte of plants,the fertility of pollen is crucial for seed setting.However,there were abnormalities in meiosis and cytoplasmic division of pollen mother cells,leading to a large number of pollen abortion.In conclusion,pollen quality limitation and pollen abortion are the main causes that affect the seed setting of natural populations of S.officinale.