Abstract: We used unfertilized ovules of the pumpkin cultivar ‘Experiment 1’ as explants to examine the effect of combinations and concentrations of growth regulators, different developmental periods, high-temperature pretreatment, and AgNO₃ concentrations on embryoid induction. The combination of 2,4-D, NAA, and 6-BA favoured embryoid induction; the medium with the best effect was Murishige and Skoog (MS)+1.0 mg·L^–12,4-D+0.25 mg·L^–1NAA+0.5 mg·L^–1 6-BA, for 31.1% embryoid induction. The flowering ovules for the day showed the highest frequency of embryoid induction (26.7%) and the lowest frequency of callus formation (<5%). The explants treated with darkness and high temperature (35°C) for 5 days showed the best embryoid induction, 32.2%. However, the addition of AgNO₃ remarkably inhibited embryoid induction. Transfer of embryoids to plant-regeneration medium led to the highest regeneration frequency, 64.3%. Plant regeneration expressed a typical pathway as for embryogenesis. Cytologic observations revealed that embryoids probably derived from cells close to the micropyle (i.e., ovums or synergids).

Key words: embryoid, in vitro culture, plant regeneration, pumpkin, unfertilized ovules