| The ovule tapetum provides nutrients for the female gametophyte development, and it is important assurance for normal ovule development. The structure’s variation of the tapetum can affect the development of female gametophyte. Nowadays, compared with integumentary tapetum in angiosperms, there are relatively less studies on ovule tapetum in gymnosperms. Especially the understanding of the anatomical features and function is far less clear than angiosperms. The research on ovule abortive mechanisms of No.28 sterile lines has found that compared with the fertile lines in Pinus tabulaeformis Carr., the nucellar tissues appeared variation, and suggested that the ovule abortion may relate to the tapetum development. On basis of previous research, after breaking the first dormancy, the ovules of fertile lines in P. tabulaeformis were selected as the materials, and from the anatomical standpoint the formation and change features of ovule tapetum have been observed. At the key ovule development stage, the study on differences between the sterile and fertile lines was carried for revealing the reasons of ovule abortion and the functions of tapetum on ovule development. The main results are as follows:1. Microscopic observation on the formation of ovule tapetum in P. tabulaeformis showed that after the female cones broke the first dormancy, there were some young cells with similar morphology in the depth of nucellus. The initial tapetum which had 1-2 layers of cells dyeing deeper in irregular shape was differentiated from the nucellar tissues around the megaspore mother cell. As the ovule developed, the tapetal cells which increased to 5-6 layers surrounded the female gametophyte in concentric circles. Since the female cones entered the second time dormancy, the layer number of the tapetum no longer increased. Because the ovule tapetum in P. tabulaeformis developed from the nucellar cells, instead of integument, so it should be called "nucellar tapetum".2. The free nuclear mitosis of female gametophyte in P. tabulaeformis is one of the critical stages of ovule development. Observation on the ovule development in fertile lines has showed that during the mitosis of free nuclear, the tapetal cells close to the female gametophytes almost completely degraded. In part of the tapetal cells far away from the female gametophyte (near the nucellar tissues) the inclusions were consumed in large quantities, and only the nucleus was left. Even some of them were vacuolated completely. The main reason is the nutrients in ovule tapetum flowed to the female gametophyte. However, the ovule tapetum in sterile lines did not obviously degrade, a lot of starches accumulated in tapetum, and formed a starch circle. The circle here always existed with gradually thickening. It has showed that the nutrients did not flow to the female gametophytes. These results have indicated that the ovule tapetum in P. tabulaeformis is the important supplier providing nutrients for the female gametophyte development. From the anatomical point of view, the variation development of ovule tapetum may cause abortion in No.28 female sterile lines. In addition, it has also observed that the callose did not exist in the ovule tapetum, and only accumulated in the nucellar tissues closed to the micropylar end. This has proved that the deposition of callose did not directly hinder the transposition of nutrients from ovule tapetum to the female gametophytes. |
