Anatomy of Watermelon Embryo Sacs Following Pollination, Non-pollination or Parthenocarpic Induction of Fruit Development

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Abstract
There is little information on the fate of embryo sacs in plant ovules if pollination is prevented. In this study embryo sacs from watermelon were observed over a 13 day period following flowering with (a) normal pollination, (b) non-pollination and (c) induction of parthenocarpic fruit development with naphthalene acetic acid. Following pollination, and prior to fertilization approximately 2 days later, the embryo sacs completed development and consisted of two synergids with prominent filiform apparatus, an egg cell, a central cell with two polar nuclei and three antipodal cells. Sperm nuclei were observed within the embryo sac at 2 days and by 4 days the endosperm was proliferating. In the non-pollination treatment the embryo sac was still intact after 4 days although the antipodal nuclei were becoming hard to distinguish. By 7 days only the two synergids and the egg cell were still well defined, the polar nuclei appeared in some preparations to be fused, and the antipodals had degenerated. By 10 days the embryo sac was a structure-less watery mass. In parthenocarpic fruit the fate of the embryo sac was similar to that in non-pollinated fruit except that final breakdown was delayed past 10 days. Maturity of the majority of embryo sacs in an ovary appeared to be contemporaneous with penetration of the pollen tube, and on the basis of the anatomical results it seems possible that embryo sacs could be fertilized up to 2 days beyond the normal time.