关键词: 西瓜, 小孢子, 二细胞花粉, 质体, 线粒体

Abstract: To understand morphological changes of plastids and mitochondria before and after microspore division as well as the distribution of these organelles to vegetative and generative cells during microspore division，microspores at different developmental stages and newly born 2-celled pollen grains were investigated by means of transmission electron microscopy. Plastids in microspores at all developmental stages do not have internal membranous structures，and do not accumulate starch or any other forms of storage reserves，indicating an undifferentiated status. In contrast，plastids in the newly born 2-celled pollen grain accumulate a huge amount of starch，indicating that the microspore division stimulates differentiation of proplastids into amyloplasts. In the newly born 2-celled pollen grain，the generative cell does not contain plastids，while the newly born vegetative cell contains a large number of plastids，indicating a complete exclusion of plastids from the generative cell during microspore mitotic division，i.e.，watermelon possesses the mode of purely maternal plastid inheritance. Based on cytological observations，watermelon should be added to the list of Lycopersicon type. Mitochondria in microspores reveal few cristae，whereas cristae dramatically increase in number in 2-celled pollen grains.

Key words: watermelon, microspore, 2-celled pollen grain, plastid, mitochondrion