Distribution Of Calcium During The Process Of Sexual Reproduction In Nicotiana Tabacum L.

Calcium is a significant element in organisms and participates in numerous" physiological functions and its role has been revealed more and more in plant development. Calcium has also an essential signaling, physiological,and regulatory role during the whole process of sexual reproduction in flowering plants, including microsporogenesis, megasporogenesis, development of male and female gametophyte, pollination and double fertilization. Howevere, seldom work has been carried out in this field. Tobacco is a kind of modal plant in the research on sexual plant reproduction and suitable for my study.Calcium distribution during anther development of tobacco was investigated using antimonite technique. It is obvious that calcium contributes to microsporogenesis and the maturity of pollen grains greatly, especially during the stages when extremely intense changes on morphology and structure take place in microspore, such as the construction of cell wall, the fusion of the large vacuole and the appearance of cytoplasmic polarity in microspore. Calcium is also related to the programmed cell death (PCD) of callose wall, tapetal cells and middle wall layer during the specific stages of anther development. The transportation of calcium from tapetal cells to locule of the anther is performed by Ubisch bodies secreted by tapetal cells.In the mature pollen grain, there is no calcium precipitates distribution in it except the surface of its aperture. And it is the same with the pollen loading and germinating on the compatible stigma.When pollen grain arrives at the stigma, becomes hydrated, and as the pollen swells, Cacium precipitates accumulate at the aperture. Calcium precipitates accumulate in pollen cytoplasm initially are concentrated within small vacuoles but as germination proceeds these appear to fuse, forming prominent densely labeled vesicles that preferetially accumulate near the apical region of the growing tube. Although the stigma has abundant particles, few calcium precipitates are observed in the transmitting tissue from anthesis to 11h after pollination. However, at 22h after pollination, accumulation of calcium increases distally from the stigmatic interface with the transmitting tissue through the length of the style to the ovary. And at this stage, in tobacco styles appears a significant calcium gradient which will be the pathway followed by pollen tubes.Potassium pyroatimonate was also used to localize loosely-bound calcium in young ovules of tobacco during megasporogenesis to investigate the relationship between ionically available calcium and megaspore degeneration. Following meiosis in the megasporocyte, a "S" shaped tetrad of four megaspores is formed, with three of the four megaspores degenerating from the micropylar end inward. Only the chalazal-most megaspore continues to develop, becoming the functional megaspore. The dynamic distribution of calcium precipitates in the megaspore precedes .the breakdown of the micropylar megaspores, which subsequently undergo structual disintegration and loss of recognizable cellular features. But the function of calcium in affecting and controlling this critical phase of embryo sac establishment and functional megaspore determination have not been fully elucidated, and also needs to be testified by more proof.The distribution and function of calcium during the process of double fertilization, especially in the sexual cells in embryo sac of tobacco, have also been carefully observed and investigated.The study on calcium dynamic distribution during sexual reproduction in tobacco may facilitate future research aimed at elucidating the mechanisms of the process of sexual reprodction in flowering plants.