Seed Development and Abortion in Vitis vinifera L., cv. Chardonnay

Seed development was investigated by light microscopy in early- and late-flowering bunches of the grapevine cv Chardonnay on days 4, 14, 28, and 42 after the first flowers opened and at berry maturity (98 d). Only ca. 20% of ovules were penetrated by pollen tubes and were fertilized, as evidenced by a zygote and free nuclear endosperm. Normally, the first division of the zygote occurred between days 14 and 28 and coincided with cell wall formation in the endosperm. Embryo development was slow from day 14 to 42 but accelerated thereafter. Ovules without an embryo sac did not develop and nucelli degenerated. Those with a normal but unfertilized embryo sac aborted after a small amount of testa and nucellus growth, evidenced by a degenerate nucellus. Abortion occurred also during later stages of seed development, with the testa developing normally but the nucellus and the endosperm degenerating, giving a hollow ("floater") seed. Some floaters were completely empty while others contained degenerating embryos, endosperms, or nucelli. The incidence of floaters was 31% in the bunches where flowering commenced early, and 9% in the bunches flowering 2 wk later, in both cases among more than 500 seeds. Empty seeds were mostly the consequence of embryo degeneration followed usually by endosperm degeneration. In cv. Chardonnay, three consecutive, possibly interconnected processes are seen as the causes of no or defective transition from ovule to seed: (a) aberrant embryo sac development before flowering, (b) lack of fertilization at flowering, and (c) incomplete development of zygote/embryo after flowering. All impair development of berries in which none of the four ovules develop into normal seeds. Preanthesis abortion of all ovules per flower leads to flower abscission. Berries containing only arrested ovules and/or seed traces remain small but nevertheless ripen ("chickens"). Berries with floater seeds but without normal ("sinker") seeds develop and ripen normally but remain somewhat smaller than berries containing sinker seeds.