Reproductive Anatomy in Cultivated Grapes - A Review

The anatomy of reproductive structures of cultivated grapes is summarized from selected literature. The inflorescence is initiated in the year prior to flowering. It is a much-branched cluster, each branch ending in a terminal flower. Hermaphroditic flowers have 5 partly fused sepals, 5 petals united at the top, 5 stamens, and a 2-loculed pistil with a short style and a stigma. Some cultivars and species are characterized by flowers functionally male or female, intermediate or sterile. Meiosis in pollen mother cells produces 4 reduced nuclei which become separated by simultaneous formation of walls. Pollen is shed from the anthers in a 2-nucleate condition. Nonfunctional pollen can result from failure of meiosis or of the first mitotic division, respectively as in certain hybrids or in female cultivars. The ovule is anatropous and has 2 integuments and a massive nucellus. The single megaspore mother cell undergoes meiosis to produce a linear tetrad of 4 megaspores. From the chalazal spore an embryo sac of the Polygonum type is formed. The development of an embryo sac may be arrested in some or all of the ovules either before or after meiosis, as occurs in varying degrees in seeded or parthenocarpic cultivars, male flowers, or nonfunctional female flowers. After fertilization the pattern of cell division follows that of the Geum variation of the Asterad type. Endosperm forms according to the Helobial type and becomes ruminate. The inner layer of the outer integument of the seed is sclerenchymatous and becomes hard in the mature seed. Certain cultivars are characterized by so-called seedless berries. In some of these cultivars development of embryo and endosperm is arrested at various stages, resulting in stenospermic (soft) or empty (hard) seeds. In other seedless cultivars fertilization does not occur; their berries are parthenocarpic. Seeded berries generally show three periods of growth. Most of the cell division in the berry and most of the development of the seed occur in the first period of rapid growth. This is followed by a period of slow growth, which varies in duration. Cell enlargment characterizes the last period of rapid enlargement of the berry. The mature berry is parenchymatous with complicated vascularization. "Seedless" (stenospermocarpic or parthenocarpic berries generally show less distinctive growth periods than do seeded berries.