乌桕种子成熟过程中种皮和胚乳超微结构观察

Ultrastructure of seed coat and endosperm in the development of Sapium sebiferum seed mature

为探讨乌桕种子成熟过程中种皮结构和胚乳内部营养物质的变化,进一步了解该种子休眠形成的机制,以不同采收期的乌桕种子为材料,分别对种皮和胚乳进行了扫描电镜(SEM)和透射电镜(TEM)观察。结果表明:随着种子的不断成熟,种皮的厚度不断增加;同一种子不同部位种皮的厚度不相同,胚根处、子叶端的种皮较其他部位厚,这可能是种子萌发时胚根难以突破种皮的主要原因。乌桕种皮坚硬革质化,由外到内依次为角质层、栅栏层、骨状石细胞层及薄壁组织层,栅栏层细胞排列紧密,这不仅增加了种皮的坚硬度,还可对种子与外界水、气交换造成影响;乌桕种皮表面和内部均含有蜡质,其含量随着种子的成熟而增多,提高了种子对外界不良环境的抵御能力。透射电镜观察发现,在种子成熟前期,胚乳细胞中存在较多大液泡,各种细胞器结构完整,细胞中散布着一些较小的高电子致密物质。种子成熟过程中,水分含量不断降低,部分细胞器消融,细胞核因失水而缩小变形,脂滴逐渐消融,变成透明状,高电子致密物质大量累积、聚集,脂蛋白含量增加,同时淀粉转化成糖类物质,表明乌桕种子已完全成熟,进入休眠期。

In order to know the mechanism of the dormancy of Sapium sebiferum seeds,we observed the changes of seed coat structure and endosperm nutrients of S. sebiferum seed during the process of maturity by using scanning electron microscope（ SEM） and transmission electron microscope（ TEM）. The study showed that the hard and leathery seed coat composed four different parts： the cuticle,the palisade,the osteosclereids and the parenchymatous layer,arranged according to its position from the exterior to the interior. Compared to the other parts,the seed coat near radiculodium and the micropyle was thicker and the thickness increased as the seed matured. In addition,there was tallow both on the exterior and interior of the seed coat which was thought to be a barrier for exchanging water and gas with the environment.By the TEM,we found that at the early stage,the endosperm cells were characterized by the presence of amyloplasts,completeness of cell nucleus and other organelles,less black granules,and numerous vacuoles and lipid droplets. With the maturity of seed,the cell nucleus dwindled in size due to the dehydration,the starch grains transformed into sugar gradually. Maturation even led to the disappearance of vacuoles and lipid drops,and dissolution of organelles. While the black granules gathered in large quantities,indicating that the lipoprotein content increased,which would further intensify the seed dormancy.