The ovule ontogenesis and the megasporogenesis events in Onobrychis schahuensis Bornm. were studied with light microscopy. The primordium is tetra-zonate and gives rise to an anatropous ovule. The archesporium may consist of one or more archesporial cells, but only one of them undergoes meiosis, forming a linear shaped tetrad. Normally, only a single megaspore is functional, which is located in the chalazal position, while the others degenerate very soon. The young ovule is hemianatropous but the mature one is anatropous, crassinucellar, and bitegmic; integuments form a zigzag micropyle. A 7-celled embryo-sac is formed corresponding to the Polygonum type. The present study provides the first report on embryological characteristics in O. schahuensis. Based on our results and in comparison with prior studies, several characters may have potential taxonomic significance for the Hymenobrychis section, such as the tetra-zonate ovule primordium, anatropous ovule type, dermal origin of the integuments, asymmetrical initiation of the outer integument, linear shaped tetrad with the presence of one functional megaspore, and having 2 young ovules but degeneration of one of them.
Male and female gametophytes have special characters that show a great variety in different taxa. In this study, gametophytes of four species belonging to three sections of the genus Onobrychis Mill. were studied with light microscopy. Results showed that the ovular primordium is tetra-zonate and gives rise to an anatropous ovule. The archesporium may consist of one or more archeosporial cells, but only one of them undergoes meiosis, forming a linear or T-shaped tetrad. Normally, only a single megaspore is functional which is located in the chalazal position while the others degenerate very soon. The young ovule is hemi-anatropous but the mature is anatropous, crassinucellar and bitegmic; integuments form a zig-zag micropyle. A 7-celled embryo-sac is formed corresponding to the Polygonum type. Based on our results, the ovular variable characters are the form and condition of ovary, presence or absence of ovary peduncle, the number and condition of ovule in ovary, length and width of ovule, length and width of embryo sac, number of layers in outer integument, condition of megaspore, alignment pattern of the integuments, asymmetrical initiation of the outer integument, shape of tetrad with the presence of one functional megaspore and so on. The separator characters in male gametophyte are including tri-cellular pollen grains and the number of tapetum nuclei. According to our study the female gametophyte characters are more variable than male gametophyte. The present study provides the first report on embryological description in the genus Onobrychis and also in section Heliobrychis.
Developmental aspects of anther, pollen grains, ovule, embryo and seed has described in Onobrychis persica Sirj. and Rech.f. (Fabaceae) under bright field, polarizing and fluorescence microscopy. Anther development starts when the flowers are very small. The anther is tetrasporangiate, and its wall development follows the dicotyledonous type and consists of four layers: epidermis, endothecium, middle layer and a secretory tapetum. Cytokinesis is simultaneous and arrangement of microspores is tetrahedral and tetragonal. Fibrous thickenings are developed in the endothecium when shed. Ellipsoidal tricolpate pollen grains are twocelled when anthers dehisce. The young hemianatropous ovule changes to a anatropous, crassinucellar and bitegumic mature one with zigzag micropyle. Meiosis of megasporocytes results in a T-shaped tetrad. The chalazal megaspore develops into an eight-nucleate embryo sac with the pattern of Polygonum type. The polar nuclei remain separated before fertilization. After cellularization of endosperm, peripheral cells show dense lipid content. The axial embryo shows fleshy cotyledons, which accumulate lipid and starch. The inner integument differentiates into an endothelium and largely vanishes during development while the outer one produces several layers and establishes the typical seed coat structure: macrosclereid cells, osteosclereids and parenchyma cells. Different compounds, such as starch and lipid content were demonstrated with special staining in the tissues. The systematic significance of the embryological characters is discussed in O. persica.
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