Study On The Development Of Surface Cells In Sorghum Endosperm

In this experiment, using sorghum (Sorghum bicolor L.) as plant material, employing the staining methods of Evan's blue, I2-KI and TTC to examine the material accumulation and the activity of endosperm in the caryopsis, applying the method of resin slice to observe morphology and structure of the endosperm surface cells, results as follows were achieved:1. Sorghum caryopsis development. Up to 10 days after flowering, the principal development of caryopsis is to increase the length; after that period, the growth in width plays the dominant role. Eighteen days after flowering, caryopsis reaches its maximum length as well as width, followed then by the reduction of pericarp chloroplasts leading to a gradually change into a white color of the caryopsis. The sorghum endosperm in the early development shows a dehydrogenase activity, while in the late stage due to the accumulation of amyloid in the endosperm its physiological activity gradually decreases and eventually disappears. Endosperm transfer cells carry on a process of apoptosis in late development period. Aleurone cells are the only cells to remain active in endosperm of mature caryopsis, and during seed germination, they can release hydrolases to hydrolysis the inner endosperm and provides nutrients for the embryo development. Embryo will all along retain its physiological activity during the endosperm development of caryopsis.2. Development of endosperm surface cells. According to the characteristics of the development of endosperm surface cells, sorghum endosperm surface cell development can be divided into four distinct periods: the endosperm cellularization stage (0 ~ 2d after flowering), the growth and differentiation stage (3 ~ 18d after flowering), the functioning stage(19 ~ 25d after flowering) and the declining stage (from 26d after flowering on). During the period of 0 ~ 2d after flowering, the free endosperm nuclei in endosperm sac takes the process of cellularization, first forming a cell layer with the cell structure at the surface of the endosperm sac. During the period of 3~5d after flowering, endosperm surface cells carry on periclinal divisions to increase radial cell layer numbers till the whole sac is filled with cells. Around 6d after flowering, the cytoplasm starts to turn denser. On 8d after flowering, spherosome accumulating lipid appears in the cytosol. On 9d after flowering, the original large vacuole divides into several small vacuoles, in which calcium and magnesium phytate begin to accumulate forming aleurone grains. After that period aleurone cells increase their inclusion material, and thicken their cell walls. In mature sorghum caryopsis the aleurone layer is composed of a layer of cells with a squared structure, and large numbers of aleurone grains and spherosomes in the cytoplasm. Compared with the aleurone grains, spherosomes display smaller volumes but appear in a larger quantity.On 3d after flowering, near the vascular bundle of the caryopsis, in contrast to others, certain cells perform a more close arrangement with more slender shapes, which will develop into endosperm transfer cells in the future. Up to 6d after flowering, a small amount of intramural projections take shape in endosperm transfer cells. Till 7d after anthesis , intramural projections begin to differentiate, the nuclei appear more obvious, vacuoles enlarge, and quantities of mitochondria, endoplasmic reticulum and dictyosome are distributed in the cytoplasm. Then intramural projections increase in both quantity and volume. Around 13d after flowering, a transfer cell zone made up of 3 to 4 layers of cells can be observed at the base of endosperm, and numerous mitochondria disperse in the slits of intramural projections. From 15d after flowering, there will be no longer significant growth in the number of endosperm transfer cells, while intramural projections continue to increase, and endosperm transfer cells step into the functioning stage. Till 20 ~ 23d after flowering, the nuclei can no longer be seen in the outermost layer of endosperm transfer cells due to the highly developed intramural projections, and we can only find them in the cells inward from the second layer. Around 25d after flowering, in endosperm transfer cells, intramural projections shrink and undergo deformation, nuclei begin disintegration, and these cells enter the declining stage. Till 33d after flowering, intramural projections of transfer cells contract and the cells die away.During the development of endosperm surface cells, the dynamic of the structure is adapted to its function in delivering the filling material, and the activity of cytoplasmic organelles is compatible with the level of material transportation, suggesting that the endosperm surface cells play an important role in the transport of maternal nutrient to endosperm and embryo. 3. The effects of leaf-removal on the sorghum endosperm surface cells development. By removing all leaves to restrict the organic traffic to the caryopsis, the effects of filling material on the endosperm surface cells development were observed. With leaves removed, the caryopsis of 18d after flowering has no endosperm sac formed near the dorsal large vascular bundle, the nucellus cells adjacent to the aleurone layer haven't undergone obvious degeneration either, calcium and magnesium phytate rarely accumulates in the small vacuoles of aleurone cells, protein bodies just barely take formation in the endosperm cells adjacent to aleurone layer, starch is not fully filled in the starch grains in endosperm cells, and in the transfer cell zone less layers of cells are characterized of opaque cellular structure. All above characteristics are clearly different from those of caryopsis of 18d after flowering performing normal development, indicating that changes in normal sorghum endosperm surface cells are and relevant to the number of filling material.