Characterisation and propagation of hibiscus coddii subsp. barnardii, an indigenous plant with ornamental potential from Sekhukhuneland, Limpopo Province, South Africa

Abstract

South Africa is known globally for its rich floral biodiversity. A number of the country’s indigenous plants are commercialised and grown worldwide. An increased interest in growing drought-tolerant plants due to limited water resources, created a need for introducing additional novel wild species onto the horticultural market. One such plant occurring in the drought-prone areas of the Sekhukhuneland Centre of Plant Endemism is Hibiscus coddii Exell subsp. barnardii (Exell) Leistner & P.J.D. Winter. In nature, the plant is an upright, branched perennial herb that produces bright red flowers for about four months during the summer season. Its characteristics makes it suitable for growing as a small shrub, especially in sunny rockeries, and as a pot plant. Effective propagation is, however, imperative for any commercial production of plants. This study, therefore, addresses the lack of reliable propagation protocols for ex situ conservation purposes and for large-scale production of this valuable ornamental plant with commercial potential. The study showed that plants could be propagated by both conventional (in vivo) methods, using seeds and stem cuttings, and in vitro culture methods under controlled conditions. The effect of seed scarification with various concentrations of sulfuric acid (25%, 50%, and 98%) for various durations (5–40 minutes) and of temperature (15°C, 20°C, 25°C, 30°C, and 35°C) on seed germination were studied. The best response for both seed germination and seedling development was respectively obtained with 98% sulfuric acid for 30 minutes at 25°C in moist vermiculite. This protocol for seed germination can be used for the production of seedlings and of plants that could serve as stock for further propagation. Seedlings and mature plants grown in pots require adequate nutrients supplied once per week to ensure optimum performance. Mature plants grown under a controlled environment exhibited strong apical dominance. Removal of the apex from the main stem of four to five-month-old plants promoted outgrowth of axillary shoots suitable for use as cuttings for vegetative propagation. The effect of cutting type, exogenous application of commercial rooting hormone powder (DynarootTM No.1 and No.2) containing 0.1% and 0.3% Indole-3butyric acid (IBA) respectively, and culture media on rooting of stem cuttings was v investigated. Both types (apical and basal) of axillary shoot cuttings showed high rooting percentages (89–91%) and formation of numerous roots per cutting when treated with both strengths of rooting hormone, which was more pronounced with Dynaroot No.2. The best rooting responses were observed in vermiculite medium alone, and in combination with coco peat, followed by a coco peat and sand mixture. High rooting percentages (>80%) were observed in these media. For establishment of aseptic in vitro cultures, seeds and nodal explants from wild and in vivo grown plants were surface disinfected with 70% ethanol and 25% and 50% commercial bleach (Jik®) solution. Nodal explants from both sources did not survive the treatment and proved unsuitable for in vitro shoot culture. Disinfection with 70% ethanol (90 seconds) and 50% commercial bleach solution (25 minutes) proved the best for establishment of aseptic seed cultures. Germination of scarified seeds and seedling development under controlled environmental conditions (24±2°C with a 16-hour photoperiod at 55–60 µmol m-2 s-1) was the best on full strength Murashige and Skoog medium (MS). In vitro grown seedlings proved suitable as an aseptic explant source for in vitro shoot culture. The aseptic excision of the apex from five-week-old seedlings directly in the culture vessel, without the need for transplanting to a new culture medium, resulted in the proliferation of axillary shoots (up to 8) over a short period. Nodal explants obtained from these shoots, as well as directly from in vitro grown seedlings, were used for in vitro shoot multiplication on MS medium with and without plant growth regulators (PGRs). Basal shoot explants (3–4 nodes) obtained directly from in vitro grown seedlings and cultured on PGR-free MS medium in a vertical position was the most suitable for generating axillary shoots and for plant regeneration. Low shoot regeneration efficiency in all types of explants was observed in the presence of various concentrations of 6-Benzylaminopurine (BAP) (0.25–4 mg L-1) alone and in combination with 0.5 mg L-1 IBA and 1Naphtalene-acetic acid (NAA). Microcuttings derived from axillary shoots of seedlings with a removed apex were used for in vitro and ex vitro rooting. Poor in vitro rooting responses were observed in both PGR-free (15% rooting) and MS media supplemented with 0.5 and 1 mg L-1 IBA and NAA (5–20% rooting). In contrast, high rooting percentages (80–100%) were attained when microcuttings treated with DynarootTM No.1 commercial rooting powder (0.1% IBA) were rooted ex vitro in moist vermiculite. In vitro seedlings and plantlets were successfully vi acclimatised (>90%) for 2–3 weeks under controlled environmental conditions (24°C±2°C and a 16-hour photoperiod at 150–200 µmol m-2 s-1). Further hardeningoff in a greenhouse (uncontrolled environment) resulted in well-established mature plants, which proved suitable for transplanting to an open environment. This study revealed that H. coddii subsp. barnardii can be propagated both by in vivo and in vitro cultures using seeds and cuttings. However, in vitro culture could be more suitable for large-scale plant production under controlled environmental conditions, since it is unaffected by seasonal variations and requires limited space and low maintenance of the plant material. This approach also resulted in a shorter time for seedling establishment (7–8 weeks) in an artificial nutrient medium as compared to conventional in vivo propagation in soil (12 weeks), where cultures require constant care (water and nutrients) for optimum growth. Plant regeneration by microcuttings derived from in vitro grown seedlings was also shorter (up to 4 months) than the regeneration by stem cuttings (6 months) derived from in vivo grown plants. This study could form the basis for domestication of H. coddii subsp. barnardii and its introduction as ornamental xeriscaping plant, especially in dry, rocky places, or for cultivation as pot plants. This would contribute to the popularisation of indigenous plants for gardening purposes. The study also supports ex situ conservation of the species threatened by the mining industry, expansion of human settlements and destruction of its natural habitat.