Cultivation of Combretum bracteosum (Hochst.) Brandis.
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Date
2001
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Abstract
In maximizing South Africa's floral diversity, plant propagators have begun exploiting the
rich array of indigenous plants, especially those with horticultural potential. Plants
previously unavailable to the professional and amateur gardeners alike, are legally
becoming common-place in nurseries. However, in promoting the trade of indigenous
plants to nursery-owners, rapid, easy and cost effective methods of propagating these plants
need to be established.
Combretum bracteosum is one such indigenous plant, the aesthetic appeal thereof exhibits
great potential for ornamentation, especially when flowering. In facilitating the introduction
of Combretum bracteosum into nurseries, small gardens or even pots, investigations carried
out aimed to determine and analyse quick and easy methods of propagating this plant. Of
the various propagation techniques considered, only one, micropropagation, required
specialized skill and training prior to carrying out the relevant procedures and protocols.
The two other techniques used, which are accessible to most plant propagators, were seed germination and propagation from cuttings.
Propagation by seed germination yielded less than optimal results from a commercial
perspective. Although the hard pericarp surrounding the embryo did not impose any
dormancy inducing mechanisms, such as the restriction of water uptake or the leaching of
an inhibitory compounds, it did act as a mechanical barrier to the emerging radicle and
roots. Recommendations for optimal Combretum bracteosum seed germination would be
to remove the protective pericarp completely, incubate imbibed embryos in complete
darkness at 25°C. After radicle emergence the germinating embryos could be moved into an alternating light: dark cycle.
A more viable and simpler alternative to seed germination, was propagation by stem
cuttings. Treating the cuttings with 10% and 50% or 75% of the commercially available Kelpak concentrate (using the Soak Method and Quick-dip Methods respectively), provided
the most promising results, with the rapid development of roots and subsequent vegetative
growth. Synthetic hormones such as IBA and NAA were also applied to the cuttings both
alone or in combination however, although callus growth was profuse, root development
was slow and unsubstantial, if any at all. Therefore, in recommending a protocol for the
successful rooting of Combretum bracteosum cuttings taken during spring, summer or early
autumn, the application of Kelpak at either 10% (Soak Method) or 50% (Quick-dip
Method) of the full strength solution, is advised. Subsequent to hormone treatment, the
cuttings still required attention with regard to nutrient supplementation as well as
atmospheric moisture and temperature regulation.
Success in generating Combretum bracteosum plantlets was obtained by germinating the
seed in vitro as well as stimulating axillary shoot elongation from nodal explants. Placing
the sterilized Combretum bracteosum embryo onto a nutrient rich basal medium (containing
no hormones) was sufficient to stimulate 100% germination. The frequent poor availability
of the seed may hamper the use of in vitro seed germination for commercial propagation
purposes. The use of nodal explants from in vitro germinated stock plants, is a rapid and
reliable means of generating a large seedling stock. Nodal explants excised from the newly
developed shoot were subsequently placed onto 0.5 mg.ℓ ¯¹ BA which encouraged axillary
bud elongation. After elongation, the lateral shoots were removed and placed onto a rooting medium (1.0 mg.ℓ ¯¹ IBA).
The more mature nodal explants, collected from parent plants growing in vivo, required
either a BA: NAA hormone combination or Kelpak to stimulate axillary shoot elongation,
with the latter being most successful. Root initiation followed the protocol described above.
Once rooted plantlets were hardened off they displayed a strong and vigorous growth,
which is desirable from a commercial perspective.
Upon maturity, the habit of many indigenous trees and shrubs could become too big for confined spaces such as the urban garden. Therefore, determining a means of modifying
the plants' habit in order to maintain its suitability as a smaller garden plant was important.
Treating the Combretum bracteosum plants with a 50 mg.ℓ ¯¹ paclobutrazol soil drench
proved most successful, with the desired effects being visible within a few weeks of initial
application. No negative morphological or developmental effects were noted on plants
treated with the dwarfing agent, conversely however, the treated Combretum bracteosum
plants were compact and bushy, with considerable visual appeal and aesthetic attractiveness.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2001.
Keywords
Combretum bracteosum--Propogation., Theses--Botany.