| Résumé: | This study describes efficient protocols for coloured callus formation, in vitro shoot
multiplication, complete plant regeneration, and in vitro flowering from leaf and shoot
tip explants of Ocimum basilicum ‘Sweet Thai’, chemotype methyl chavicol. In
addition, it reveals information on pollen and seed germinability, non-glandular and
glandular trichome ultrastructures, and essential oil content and composition of the plant
at different plant developmental stages. Leaf and shoot tip explants from two-month-old
aseptic seedlings were induced to form coloured callus, multiple shoots, complete
plants, and in vitro flowers on Murashige and Skoog (MS) medium supplemented with
different plant growth regulators (PGRs) [6-benzyl-aminopurine (BAP) and gibberellic
acid (GA3) either alone or in combination with α-naphthaleneacetic acid (NAA) or 2,4-
dichlorophenoxyacetic acid (2,4-D)]. The highest percentage of purple callus formation
(100%) from both leaf and shoot tip explants was recorded on MS medium
supplemented with 0.5 mg l-1 BAP and 0.5 mg l-1 NAA and also on MS medium
supplemented with 1.0 mg l-1 BAP and 0.5 mg l-1 NAA. Whilst, the highest percentage
of light green callus formation (100%) from both leaf and shoot tip explants was
recorded on MS medium supplemented with 0.5 mg l-1 BAP and 1.0 mg l-1 2,4-D and
also on MS medium supplemented with 1.0 mg l-1 BAP and 1.0 mg l-1 2,4-D. The
highest number of multiple shoots (mean 5 shoots per explant) was obtained from shoot
tip explants cultured on MS medium supplemented with 1.0 mg l-1 BAP. Multiple
shoots showed 100% rooting on half-strength MS basal medium. Shoot tip explants
exhibited the highest percentage of in vitro flowering (40%) on MS medium
supplemented with 1.0 mg l-1 GA3. All (100%) of the multiple shoots flowered in vitro
on MS medium supplemented with 1.0 mg l-1 GA3. The rooted plantlets were
successfully acclimatized in vermiculite and established in a soil mixture (3 garden soil: 2 potting mix) in a field with an 80% survival rate, which further developed flowers ex
vitro after 24 weeks. Relative to the mother plant, in vitro grown plants flowered at a
younger stage of plant development but produced no seeds, and showed a lack of pollen
germination, fewer fully-filled peltate glandular trichomes, lower essential oil content,
and higher methyl chavicol content. Ex vitro grown plants flowered at an intermediate
stage of plant development with seed formation, and showed nearly the same pollen and
seed germinability, essential oil content, and methyl chavicol content as in the mother
plant.
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