| Summary: | Leaf discs of Saintpaulia ionantha x confusa hybrids (African Violets) were cultured and transferred between hormonal free medium (MS basal medium) and shoot-inducing medium (SIM) to determine whether there is a window of competence in shoot regeneration. The results showed that cultured cells were not responsive to shoot-inducing signals (i.e. not competent) until 3-5 days after explant isolation but the ability to regenerate shoots was not lost in surviving cells/tissues cultured on basal medium. Light microscopic observations found that first periclinal divisions of epidermal cells occurred at 3-5 days on SIM. Meristemoids were then formed from the derivatives of the original epidermal cells. It is proposed that cellular competence for shoot regeneration is acquired in culture. The pre-competent period consists of a resting phase and a "dedifferentiation" phase in which epidermal cells divide periclinally to form "dedifferentiated" cells which are the true target cells for shoot induction. Mutagenic treatments and propagation of a chimeral African Violet cultivar were carried out to study cell origin of adventitious shoots in African Violet leaf culture. The results suggest that adventitious shoots may originate from either multiple cells or single cells. The multiple cell origin is in contrast to the hypothesis of exclusively single cell origin for adventitious shoots proposed in several studies. Several factors were studied to identify the source of variations in shoot regeneration among individual explants and to define conditions favourable for shoot regeneration in African Violet in vitro culture. These factors include donor plant growth temperature, the presence of light and the quality of light in culture, the role of chlorophyll in cultured tissues, position of explants in leaves, age of explant materials, source and type of explants, wounding and leaf disc orientation on media. It was found that the presence of light in culture, age of leaf explants, source (in vitro cultured vs pot plant) and type (leaf vs petiole) of explants, wounding and leaf disc orientation on media all had a statistically significant effect on shoot production.
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