Ascochyta fabae Speg. f.sp. lentis (Gossen et al. 1986) causes lesions on the leaf, stem, and pod of lentil (Lens culinaris Medik.), thereby reducing seed quality and yield. Lesion formation was studied in two cultivars, Laird and Invincible, using light and electron microscopy of intact and excised leaves and stems inoculated with spore suspension. Spores germinated usually within 6 h of inoculation and germ tubes grew for varying distances along the leaf surface before forming an appressorium, sometimes within less than 10 h. A penetration peg then either directly entered the underlying epidermal cell, or grew as a subcuticular hypha for a short distance before entering the cell. The first response of epidermal cells to presence of the fungus was an aggregation of cytoplasm abutting the site of infection. This was followed closely by deposition of a papilla. Some relatively thick papillae were seen at 29 h postinoculation. The fungus then grew into the papilla and formed an infection vesicle. In susceptible host cells, the protoplasm became necrotic before hyphae grew into the lumen of the cell from the infection vesicle. In more resistant cells, the infection vesicle often became surrounded by electron-dense wall material developed by the host. The fungus remained in susceptible epidermal cells for up to 4 days, amongst remnants of the protoplast, before spreading to the adjacent mesophyll. Hyphae grew into intercellular spaces of the mesophyll and remained there for 2 – 3 days before penetrating the cells. The mesophyll reacted in a similar way to infection as did the epidermis, with only host cells close to the fungus becoming affected. Cultivar Laird was found to be less susceptible to infection than cv. Invincible. At the structural level, the infection process was found to be similar except that in cv. Laird the infection vesicle more frequently became surrounded by electron-dense wall material formed by the host. In stem tissue of cv. Laird the middle lamella was also occasionally thickened with electron-dense material deposited on either side of it. After the degeneration of host tissue, pycnidia-bearing spores were formed 10 – 14 days after inoculation of the leaf. Key words: Ascochyta, lentil, ultrastructure, infection process.
Contrary to previous studies, cytokinesis has been observed on formation of the first anticlinal walls during initial cellularization in the endosperm of wheat. Typical phragmoplasts develop between the free nuclei in the peripheral cytoplasm, and the cell plates arising from these soon fuse with the wall of the central cell. The inner free margin of the walls, however, continues to grow towards the central vacuole within an advancing front of cytoplasm. Anticlinal walls arise around nuclei approximately simultaneously in different planes; three dimensionally the nuclei, therefore, become located within open-ended compartments of cytoplasm. The compartments then undergo cell division and develop periclinal walls by normal cytokinesis. These walls delimit the first complete layer of endosperm cells centrifugally and a new layer of open compartments centripetally. The inner compartments usually continue to grow centripetally until those from the opposite side of the grain meet. During this closing phase, along the zone where the compartments fuse, a further row of periclinal walls is formed. Subsequent immediate differentiation of endosperm involves the subdivision of the first-formed large endosperm cells into smaller cells.
Descriptions are provided of cultured isolates of four species of chlorophyte microalgae which would traditionally be assigned to the Chaetophoraceae, Leptosiroideae (sensu Bourrelly, 1990). Light microscope observations are supplemented by electron microscopy, particularly of pyrenoid features. Three new species are described: Coccobotrys mucosus sp. nov., Dilabifilum prostratum sp. nov. and Desmococcus endolithicus sp. nov. Desmococcus olivaceus is a new record for Antarctica. Diagnostic features of the new species include extensive gelatinisation of cell walls in old cultures of C. mucosus, growth of a well-developed pseudoparenchymatous prostrate system in liquid cultures of D. prostratum, and production of quadriflagellate zoospores by D. endolithicus which is a new observation for this genus. Unusual and contrasting ultrastructural features of the pyrenoid are described for D. prostratum and the Desmococcus species. The former is traversed by appressed pairs of thylakoids coated with pyrenoglobuli; the latter is traversed by single thylakoids with pyrenoglobuli surrounding the pyrenoid matrix.
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