| Kentucky bluegrass (Poa pratensis L.) is one of the widely used cool-season turfgrasses. Rudimentary panicle development is an important foundation for the formation of turfgrass seed yield and quality. Embryonic development is an important period for the process of angiosperm reproductive growth, directly determine the embryo traits and seed-setting rate. In this paper, rudimentary panicle development and embryology on Poa pratensis L.'Nassau' was studied, with the application of smear method, conventional paraffin method, ovary whole-mounted clearing method, overall dye laser scanning confocal microscopy, microscopic photography. The aim is to provide a theoretical basis for Kentucky bluegrass seed production and further improve breeding.The process of rudimentary panicle development on Kentucky bluegrass 'Nassau' involves two aspects:rudimentary panicle differentiation and pistil & stamen development. According to morphology change, rudimentary panicle differentiation was divided into four stages:the differentiation stage of bract primordium, the differentiation stage of branch primordium, the differentiation stage of spikelet and floret primordia, and the stage of stamen and pistil primordia formation.The anther wall is composed of four layers, epidermis, endothecium, middle layer and tapetum respectively. There were some abnormalities lagging chromosomes, early separation of homologous chromosomes, chromosome bridges, unequal division, univalent and multi-bivalent about the progress of polen mother cells meiosis. The non-synchronous phenomena on majority of microspore mother cells was exist among the anthers and within the same anther in the same floret. The microspore mother cell undergoes meiosis before it turns into microspore tetrad. Its cytokinesis is successive type, and its tetrad is bilateral. The mature pollen grains are three-celled. In the process of pollen development, abortion microspore was observed.Kentucky bluegrass 'Nassau' has ovary 1-locular; Bitegrninous, crassinucellate, anatropous ovule and polygonum type embryo sac. Four Megaspore array linear in Sexual embryo sac, only the megaspore at chalazal end formed functional megaspore. Two polar nucleus located in the vicinity of the egg cell before tasselling. The antipodal cells exist in the form of cell mass via proliferation. The antipodal cells have exuberant metabolize, thickly dye, longer life-span and still exist at the stage of pro-embryo.In the period of pistil and stamen formation and development, panicle length increased significantly. The final length of panicle can be up to 120mm. Spikelet length increase rapidly in the period of microspore mother cell meiosis. The final length of spikelet is from 3.5 mm to 3.7mm. Stamen developed more quickly at the early stage of pistil and stamen development. After microspore tetrad period, the progress of pistil and stamen development was synchronous basically.Kentucky bluegrass 'Nassau' has apomictic characteristics and the rate of apomixis is 87.6%, almost obligate apomixis. Apomixis in Kentucky bluegrass 'Nassau' is of aposporous-pseudogamous type. The development of apomeiotic embryo sac is of Hieracium type. Multi-sac is common in Kentucky bluegrass 'Nassau' and two embryo sac co-exist in the ovlue generally. Some anomalies exist in apospory embryo sac, such as the location and polarity of egg cell and synergid have changed, the antipodal cells mass degenerated before flowering, the multi-polar nucleus in the embyro sac. These abnormal embryo sac account for 3.9%.The formation of apospory pro-embryo did not depend on fertilization. The rate of pro-embryo occurrance is 85.8%, close to the rate of apomixis. Double embryo which close to the micropylar end is common in Kentucky bluegrass 'Nassau'. The occurrence of embryo have polyphyletism. The formation of endosperm is of non-spontaneous type. Endosperm development belonged to nuclear type. Some abnormalities existed in the process of embryonic development, such as empty ovary, non-embryo ovary. |
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