Cultivation of Combretum bracteosum (Hochst.) Brandis.

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.