Seed development in Ipomoea lacunosa (Convolvulaceae), with particular reference to anatomy of the water gap

Disruption of one or both of the bulges (water gap) in the seed coat adjacent to the micropyle is responsible for breaking physical dormancy (PY) in seeds of Ipomoea lacunosa and other taxa of Convolvulaceae. Hitherto, neither ontogeny of these bulges nor onset of PY together with anatomical development and maturation drying of the seed had been studied in this family. The aims of this study were to monitor physiological and anatomical changes that occur during seed development in I. lacunosa, with particular reference to ontogeny of the water gap.

Developmental anatomy (ontogeny) of seed coat and dry mass, length, moisture content, germinability and onset of seed coat impermeability to water were monitored from pollination to seed maturity. Blocking/drying and dye-tracking experiments were done to identify site of moisture loss during the final stages of seed drying.

Physiological maturity of seeds occurred 22 d after pollination (DAP), and 100 % of seeds germinated 24 DAP. Impermeability of the seed coat developed 27-30 DAP, when seed moisture content was 13 %. The hilar fissure was identified as the site of moisture loss during the final stages of seed drying. The entire seed coat developed from the two outermost layers of the integument. A transition zone, i.e. a weak margin where seed coat ruptures during dormancy break, formed between the bulge and hilar ring and seed coat away from the bulge. Sclereid cells in the transition zone were square, whereas they were elongated under the bulge.

Although the bulge and other areas of the seed coat have the same origin, these two cell layers underwent a different series of periclinal and anticlinal divisions during bulge development (beginning a few hours after pollination) than they did during development of the seed coat away from the bulge. Further, the boundary between the square sclereids in the transition zone and the elongated ones of the bulge delineate the edge of the water gap.