Tapetum development in Cucurbita pepo (Cucurbitaceae)

The tapetal cell walls are of uniform thickness before prophase; from leptotene onwards, those of the inner tangential and the proximal half of the radial wall swell. At late tetrad stage the swollen walls disappear, only the outer (distal) tangential and the distal part of the contiguous radial walls persist until degeneration. In prophase, plastids are undifferentiated, resembling those of microspore mother cells, but their development pattern changes from the first meiotic division: they divide actively in the early tetrad stage and differentiate into elaioplasts in the late microspore stage. Elaioplast and spherosome lipids contribute to pollenkitt formation. Degeneration of the tapetal cells results in a single lipid sphere for each cell; these spheres are pollenkitt precursors and cover the pollen grain in the late bicellular stage. Sporophytic proteins are deposited under the operculum during late microspore stage, face to face with gametophytic proteins localized in the pectocellulosic part of the pore. The tapetal cells degenerate during early bicellular pollen stage, in contrast to other species in which the degeneration takes place in the late microspore stage.