Calcium Distribution And Functions In Lettuce(Lactuca Sativa L.) During Sexual Reproduction

Calcium is one of indispensable elements with many important physiological functions during plant growth and development. Sexual reproduction of higher plants is a complex process including a series of delicate developmental processes of sexual organs and cells, as well as some special time and space interactions between male and female tissues. Calcium is of importance in the process of sexual reproduction of higher plants. Past studies focused on the relation between calcium in the ovules and directional growth of pollen tube before and after anthesis, and calcium functions in the fertilization. Nevertheless, studies about distribution and functions of calcium during anther development and early development of female gametophyte are very few. Potassium antimonite was used to locate calcium in the anther and gynoecium of lettuce (Lactuca sativa L.) in order to study distribution and function of calcium during sexual reproduction comprehensively. The results were as following.1. During anther development,microspore mother cell and tetrad showed very few calcium granules. Few calcium granules were precipitated in the microspores just released from the tetrad, but many ones on the surface of exine. Then, calcium increased in the nucleus and cytoplasm of early microspores. When some small vacuoles appeared in the cytoplasm, many calcium granules were accumulated in the cytoplasm and nucleus of microspores, as well as many ones in the exine. As a big vacuole appeared in the late microspores, calcium decreased and few calcium granules in the cytoplasm and nucleus, but some still in the vacuole. Calcium distribution in early 2-cellular pollens with a big vacuole resembled that of late microspores. When vacuoles disappeared, few calcium precipitates were located in the cytoplast of bicellular and mature pollens, but abundant calcium granules were accumulated on the surface of pollens. It indicated that calcium was related to the formation of microspore vacuoles. Moreover, calcium in the vacuoles might play a penetrating role. Abundant calcium granules were tactically deposited on the pollen wall but few ones were accumulated in the cytoplast of pollens during anther development, and the former prepared for pollen germinating in the future.Few calcium granules were accumulated in the tapatum cells at the stage of microspore mother cell. At the stage of tetrad, tapatum cells3began to degenerate and calcium evidently increased in the cells, as well as many small calcium granules were accumulated in the locule of anther. Degeneration of tapetum cells was related to calcium. Moreover, tapatum cells during degeneration transported nutrition and calcium into anther locule for pollen development.2. During megasporogenesising, megasporocyte meiosis formed a tetrad, of which three megaspores at the micropyle end was no function and degenerated one by one from micropyle direction. Many small calcium granules were accumulated in the undegraded megaspores. When megaspores were about to degenerate, however, less calcium precipitates were located in the cells. During megaspore degeneration, calcium precipitates increased in the cytoplasm of degenerative megaspores perhaps because of membrane collapsing and calcium influxing again. It suggested that degeneration of megaspores was caused by low calcium concentration in the cells, on the contrary, higher calcium lever in the cells would keep megaspores intact.At the stage of dissociative nucleus embryo sac, calcium in the cytoplasm and nucleus evidently decreased, and abundant calcium in the vacuoles with the volume of vacuoles increased. Therefore, it was presumed that calcium was related to the formation and maintaining of large vacuole during dissociative nucleus embryo sac, moreover, calcium in the vacuole might play a penetrating role.During embryo sac cells development, calcium in the synergids increased with development, and the filiform apparatus of synergids formed at 1 d before anthesis where abundant calcium granules were accumulated to prepare for pollen tubes ent...