| The cereal caryopsis transport tissues are essential channals for nutrients transported into the caryopsis, and play an important role for the development of the embryo and the endosperm. Caryopsis nutrient transport tissues in sorghum and maize mainly included vascular bundles, the placentochalaza, endosperm transfer cells, aleurone cells, endosperm conducting cells and the embryo surrounding region. Caryopsis nutrient transport tissues in wheat mainly included the main vascular bundle, chalaza, nucellar projection transfer cells, endosperm transfer cells, aleurone cells, endosperm conducting cells and the embryo surrounding region. In this paper, maize, sorghum and wheat caryopses were experimental materials, and the structure development of caryopsis nutrient transport tissues was observed under stereomicroscope, fluorescence microscope, light microscope, scanning electron microscope and transmission electron microscope. And the pathway of nutrient getting into the embryo and the endosperm was analyzed. The results as follows:1. Development of caryopsis nutrient transport tissues in maize(1) Vascular bundles were at the base of the caryopsis. During the caryopsis development, vascular bundles extended and then formed the obvious cup-like structure, and their sieve tubes increased gradually; during the late development stage of the caryopsis, vascular bundles degenerated.(2) The placentochalaza was next to vascular bundles and its longitudinal section was concave, and covered the endosperm base. During the caryopsis development, the placentochalaza degenerated and crimpled gradually, and the integumental placentochalaza degenerated slower than the nucellar placentochalaza.(3) Bndosperm transfer cells lay at the endosperm base and opposite to vascular bundles. During the caryopsis development, endosperm transfer cells increased to about three layers and their wall ingrowths increased to the maximum; during the late development stage of the caryopsis, endosperm transfer cells degenerated and wall ingrowths crimpled.(4) Enospenn conducting cells were between endosperm transfer cells and the center starch endosperm. During the caryopsis development, endospermconducting cells prolonged and enlarged, and took up a large part of the endospenn; during the late development stage of the caryopsis, endosperm conducting cells decreased gradually and the part next to the scutellum degenerated fast.(5) Embryo surrounding region cells lay between endosperm transfer cells and the embryo, and degenerated continually during the caryopsis development.(6) Aleurone cells lay in the endosperm surface layer except for endosperm transfer cells, and had more and more spherosomes and aleurone granules and thicker and thicker walls gradually during the caryopsis development.(7) The nutrients were unloaded by vascular bundles, got through the placentochalaza, got into the apoplastic space, and then were absorbed by endosperm transfer cells and aleurone cells. Nutrients absorbed by endosperm transfer cells got into endosperm conducing cells and embryo surrounding region cells separately; part of nutrients in endosperm conducting cells got into the center starch endosperm and the other part got into the embryo by the scutellum, and nutrients in embryo surrounding region cells also got into the embryo. Nutrients absorbed by aleurone cells also got into embryo and starch endosperm.2. Development of caryopsis nutrient transport tissues in sorghum(1) Vascular bundles lay at the caryopsis base and opposite to the embryo. During the caryopsis development, the vascular bundle region enlarged and sieve tubes increased; during the late development stage of the caryopsis, the vascular bundle region decreased continually.(2) The placentochalza was next to vascular bundles. During the caryopsis development, the placentochalza prolonged and narrowed, and nuclei disintegrated and cytoplasm deceased in its cells.(3) During the caryopsis development, nucellus next to the placentochalza degenerated fast and the apoplastic cavity enlarged gradually to the largest; during the late development stage of the caryopsis, the apoplastic cavity shinked.(4) Enosperm transfer cells lay at the endosperm base and along one side of the apoplastic cavity. During the caryopsis development, endosperm transfer cell zone extended to the longest and wall ingrowths increased to the maximum; during the late development stage of the caryopsis, endosperm transfer cells degenerated and wall ingrowths crimpled.(5) Endosperm conducting cells lay between endosperm transfer cells and the center starch endosperm. During the caryopsis development, the endosperm percentage that endosperm conducting cells took up increased to the maximum, and then endosperm conducting cells degenerated and decreased.(6) Embryo surrounding region cells firstly enveloped the little embryo, but then degenerated and decreased during the caryopsis development.(7) Aleurone cells lay in the endosperm surface layer except for endosperm transfer cells, and had more and more spherosomes and aleurone granules and thicker and thicker walls gradually during the caryopsis development.(8) Nutrients unloaded by vascular bundles got through the placentochalaza, and got into the apoplastic cavity and other apoplastic space. Nutrients of apoplastic cavity were absorbed by endosperm transfer cells and got into endosperm conducting cells and embryo surrounding region cells; part of nutrients in endosperm conducting cells got into the center starch endosperm and the other part got into the embryo by the scutellum, and nutrients in embryo surrounding region cells also got into the embryo. Nutrients of other apoplastic space were absorbed by aleurone cells and then got into embryo and starch endosperm. 3. Development of caryopsis nutrient transport tissues in wheat(1) There was only one main vascular bundle distributing longitudinally along the caryopsis venter. Its sieve tubes increased to the maximum during the caryopsis development; during the late development stage of the caryopsis, the main vascular bundle degenerated and sieve tubes crimpled.(2) The chalaza was next to the main vascular bundle, and nuclei disintegrated, cytoplasm deceased and vacuoles accumulated tannin in its cells during the caryopsis development.(3) Nucellar projection transfer cells were differentiated from nucellar projection that was next to the chalaza. During the caryopsis development, there were more and more layers of nucellar projection transfer cells and their walls thickened continually; during the late development stage of the caryopsis, nucellar projection transfer cells crimpled gradually.(4) Nucellus near the nucellar projection degenerated fast and the apoplastic cavity enlarged during the caryopsis development.(5) During the caryopsis development, endosperm transfer cells increased gradually and their walls thickened continually; the outmost layer of them accumulated aleurone granules and inner layers accumulated starch granules.(6) During the caryopsis development, endosperm conducting cells near endosperm transfer cells transformed into the center starch endosperm continually, and the part next to the scutellum degenerated and decreased.(7) The embryo surrounding region firstly enveloped the little embryo, but then degenerated and disappeared during the caryopsis development.(8) Aleurone cells lay in the endosperm surface layer except for endosperm transfer cells, and had more and more spherosomes and aleurone granules and thicker and thicker walls gradually during the caryopsis development.(9) Nutrients unloaded by the main vascular bundle got through the chalaza and nucellar projection transfer cells and got into the apoplastic cavity and other apoplastic space. Endosperm transfer cells absorbed and stored nutrients from the apoplastic cavity, and also transported nutrients into the inner endosperm. Part of nutrients from endosperm transfer cells got into the center starch endosperm directly, and the other part got through endosperm conducting cells and the embryo surrounding region and got into the embryo. Nutrients of other apoplastic space were absorbed by aleurone cells and then got into embryo and starch endosperm.4. Contrast of caryopsis nutrient transport tissues in maize, sorghum and wheat(1) The maize caryopsis vascular bundle was more advanced than sorghum and wheat, and had stronger ability of nutrient transport.(2) Maize and sorghum caryopses had placentochalaza, while wheat caryopsis had chalaza and nucellar projection transfer cells; wheat and sorghum caryopses had obvious apoplastic cavity.(3) Wall ingrowths of endosperm transfer cells were many in maize, fewer in sorghum and much fewer in wheat; maize and sorghum endosperm transfer cells almost accumulated no nutrients, while the outmost layer of wheat endosperm transfer cells accumulated aleurone granules and the inner layers accumulated starch granules.(4) Endosperm conducting cells were long and many in maize, shorter and fewer in sorghum and much shorter and fewer in wheat; endosperm conducting cells degenerated slower in maize and sorghum than in wheat.(5) Embryo surrounding region cells degenerated slower in maize and sorghum than in wheat.According to above, it is concluded:(1) The growth of the vascular bundle and increase of sieve tubes made more nutrients unloaded; degenerating of the chalaza or placentochalaza cells gave a relief of a physical restraint for nutrient transport; nutrients increased and induced the differentiation of endosperm transport tissues, which promoted nutrient transport much further. Therefore, caryopsis maternal transport tissues had a tight relationship with endosperm transport tissues, and they together played an important role for the development of the embryo and the starch endosperm.(2) The growth of the vascular bundle and endosperm transfer cells promoted the ability of nutrient transport and satisfied the nutrient demands of filial sink tissues. Therefore, caryopsis transport tissues were coordinated with filial sink tissues, which made the whole caryopsis develop all right.(3) Maize caryopsis vascular bundle, endosperm transfer cells and endosperm conducting cells were more advanced than wheat and sorghum and had a stronger ability of nutrient transport. However, wheat had nucellar projection transfer cells and the obvious apoplastic cavity, and sorghum also had the obvious apoplastic cavity. Nucellar projection transfer cells could promote nutrients to be translocated into the apoplastic cavity and the enlargement of the apoplastic cavity could promote the vascular bundle to unload nutrients, which maybe made up the deficiency of nutrient transport ability a little. Therefore, maize, sorghum and wheat caryopses each had their own effective transport system. |
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