Developing Horticulture in Botanic Gardens: the Role of Research for the Benefit of Plant Conservation

D.J. Botha

National Botanical Institute
Kirstenbosch, Private Bag X7, Claremont, South Africa


Abstract

The significance of horticultural research in plant conservation and the positive results recently obtained in applying various techniques such as grafting, rooting of cuttings, tissue culture, seed germination, pollination biology, etc. are highlighted.

However, the ultimate success of any plant conservation programme in botanic gardens is not solely dependent on practical horticulture, but also relies heavily on other non-applied research disciplines such as taxonomy. The relevance of these disciplines to the integrated conservation of rare and endangered plants is explained.

Introduction

The majority of plants grown in most botanical gardens today are selected to create a pleasing display of floral wealth, diversity, colour and growth forms, and also because of their horticultural potential, and scientific, educational or economic importance. Normally these plants require almost no or very few special propagating techniques and plant conservation per se is usually not involved.

Conserving threatened plants in botanical gardens, however, often demands not only a thorough investigation into the reasons why their numbers in the wild are on the decline, but also requires a holistic research approach to factors involved in the reproduction or propagation of these plants.

The various aspects that will be discussed, are based on research that is currently being undertaken in South Africa, of which some of the results have not been published yet. It is not the intention to provide a comprehensive literature survey, but, by referring to examples, to focus the attention on the importance of research as part of the conservation programme in botanical gardens.

Taxonomy

The inclusion of taxonomy in a discussion on the needs of horticultural research in a plant conservation programme might seem irrelevant, but in countries with a diverse flora such as South Africa, it is of vital importance that taxonomy should form an integral part of the research programme. This is applicable not only to South African botanical gardens, but also to botanical gardens elsewhere in the world growing plants from countries with a rich and diverse flora. The following examples will focus on two different aspects.

The sweet thorn (Acacia karroo Hayne) is one of the most common and widespread trees in southern Africa, extending northwards as far as Zambia. Apart from being protected in two areas in South Africa (Coats Palgrave, 1983), the need to conserve this species in botanical gardens therefore seems totally unnecessary. Recent research by Swartz P.P. & Robbertse P.J. (pers. comm.), however, indicated that the southern African representatives of this taxon can be subdivided in six different taxa. One of these new proposed taxa (Acacia spes. nov.) which is confined to one specific small locality of a few hectares, is now regarded as rare and therefore needs to be conserved. Any disturbance of this site could lead to the total extinction of this taxon. Without a proper taxonomic treatment, this fact could have easily been overlooked.

Problems encountered with the integrated conservation of Encephalartos eugenemaraisii Verdoorn (senso lato), serve as the second example to stress the importance of taxonomic research. The distribution of this variable taxon was characterised by several widespread, disjunct, but well defined colonies of plants. As part of a programme to increase the numbers of plants in the wild, seeds were collected at the various localities and germinated by the Transvaal Department of Nature and Environment Conservation (Fourie, S.P., pers. comm.).

A taxonomic study that was subsequently undertaken, however, led to the recognition of two species and two subspecies (Goode 1989), while a fifth undescribed taxon is simply called No. 5.

When this information became available, the reintroduction programme had to be cancelled because the seeds from the various localities representing different taxa, that had been harvested at great cost and effort, were not kept separately during germination, and the well-intentioned integrated conservation programme, could eventually have led to hybridization.

Pollination

Conservation of South African cycads, together with a few other taxa, currently receives the highest priority in the NBI's conservation programme.

It is a wellknown fact that both wind and insects play a role in pollination of cycads. What has not been established beyond any doubt, is whether both these factors are involved in the pollination of every Encephalartos species or whether some are wind- and others are insect-pollinated (Grobbelaar 1992). With regard to insect pollination, it is also not clear whether the insects are species-specific, and whether more than one insect species is involved in the pollination of the same cycad species. This is extremely important because it has implications on both ex situ and in situ conservation.

If a cycad species is pollinated by only one insect species in the wild and that insect species becomes extinct, the existence of this particular cycad species is also in jeopardy. For ex situ conservation in cases where artificial pollination is not carried out, it is of equal importance that the pollinator insects must also be released in the garden, if not present already. Entomological research is therefore currently undertaken to provide the answers to the abovementioned problems (Donaldson, pers. comm.).

Research on controlled pollination of cycads in botanical gardens needs to be done in order to ensure the production of pure seed. Although hand or artificial pollination proved to be successful in most cases, it does not exclude the possibility of hybridization through wind or insect pollination, especially where many different species are grown closely together in the same garden.

Research on the longevity of cycad pollen has received considerable attention in recent years. Osborne et al. (1991) concluded that cycad pollen stored properly will retain a significant level of viability for at least three years and possibly up to five or six years. This fact, together with the technique on how to store pollen, has broadened the scope for cycad conservation.

Autecological research done by De Lange (1992) on Audouinia capitata (L.) Brongn., further underlines the importance of research for the conservation of plants. In nature, the distribution of a total of 1200 known plants is now, due to, inter alia, an unfavourable fire regime associated with the advent of humans, invasive alien vegetation, urban and agricultural development, limited to 27 isolated populations, 18 of which comprise less than 14 plants each. The scattered population structure and greater interplant distances have resulted in decreasing levels of crosspollination between and within populations. To complicate matters further, De Lange (op.cit.) has shown that selfpollination is usually characterized by a disorientated growth pattern or cessation of growth of pollen tubes at a stage, just before entering the ovary, resulting in unfertilized ovules. The implications of these findings for ex situ conservation of this species in botanical gardens are evident.

Seed germination

The stimulating effect of veld fires on flower initiation and eventually on seed production in bulbous plants is a common phenomenon (Reid & Dyer 1984; Snyman 1984).

The delayed release of mature seed, known as serotiny, is characteristic of many plants from mediterranean areas. Several Cape fynbos families including Proteaceae, Cupressaceae, Ericaceae, Asteraceae and Bruniaceae, have serotinous members. In some taxa, e.g. Leucadendron, the seeds, stored in woody structures or 'cones', are released when the cones open after a fire (Bond 1985).

Research initiated by De Lange & Boucher (1990) and now continued by Brown, however, indicated that as far as seed germination of many fynbos species is concerned, it is not the effect of heat that plays the important role, but rather an unknown water-soluble volatile substance in smoke, derived from burning fynbos plant material. Experiments using fynbos species in the incinerator, also indicated that not all species are equally effective. One species Syncarpha vestita (L.) B.Nord., in particular, is extremely responsive and is now being used as an assay species (Brown, pers. comm.). The practical implication of this method is that it is now possible to germinate the seed of various fynbos species, including many endangered species, for the first time in the nursery. In the Restionaceae for instance, most attempts in the past were unsuccessful, now, by treating the seed with smoke, 75% success is obtained.

Cuttings

The inclusion of this technique in a propagation programme is certainly nothing new, but the rooting of hardwood cuttings proved to be a very difficult task in most species. Although it required a considerable amount of research, it seemed to be the only solution in cases where seed was not available, viable or could not be germinated.

De Kock P. (pers. comm.) did some pioneering research work in South Africa in this field and has recently, by using rooting hormones in various concentrations and different rejuvenation techniques, obtained success in rooting cuttings of several endangered trees in South Africa e.g. Warburgia salutaris (Bertol. f.) Chiov., Ocotea bullata (Burch.) Baill., and Ziziphus rivularis Codd.

All attempts by De Lange J.H. (pers. comm.) to root cuttings of Raspalia trigyna (Schltr.) Duemmer, of which only one plant is known to exist in nature, by using auxin in seven conventional treatments, were unsuccessful. However, when cuttings were treated in a medium containing two auxins and various inorganic and organic nutrients, 95% success was obtained. Today, apart from plants grown in Kirstenbosch, plants have been successfully reintroduced to areas where it was known to occur many years ago.

Tissue Culture

It is a wellknown fact that attempts to propagate plant material by means of tissue culture are not always a guarantee of success or a quick solution in cases where seed is not available or other techniques have failed, and basic research is needed for every new taxon to be propagated. Work done by De Lange et al. (1989) on the propagation of endangered plants was very successful e.g. Siphonochilus aethiopicus (Schweinf.) B.L. Burtt, virtually extinct in the wild due to overexploitation in the traditional medicine trade, has been multiplied by tissue culture. These plants were supplied to several nature conservation bodies as well as representatives from the herbal trade.

Research on Mycorrhizal Associations

Eulophia littoralis Schltr., an exceptionally rare terrestrial orchid from the South Western Cape, has an underground rhizome from which a large inflorescence develops every few years. However, no leaves are ever produced, other than one or two vestigial scales at the base of the peduncle. Although the peduncle is photosynthetic for a few weeks, it appears that the adult plant is dependent on a mycorrhizal association to compensate for the absence of leaves (Hall 1965). Attempts to cultivate this species by conventional methods have failed completely. Until detailed research on mycorrhizal associations have been undertaken, threatened species like E. littoralis and others such as Herschelia lugens (H. Bol.) Kraenzl. will remain beyond the protection of botanical gardens.

Tests that have been done on succulents, using soil from the natural habitat instead of sterilized soil, showed a remarkable increase in growth rate. It was believed that the soil from the natural habitat contains macro and trace elements, hence the positive results. There are, however, indications that the results could be ascribed to the presence of mycorrhiza in the soil, and not only to inorganic nutrients.

Research will have to be conducted to establish the role of mycorrhizal associations with many of the endangered plants.

Observations and Plant Records

The importance of field studies and observations for the conservation of plants is often overlooked and receives not enough attention, especially in cases where seed or plants are supplied to other botanical gardens.

Developing a plant record system to which data could be added or retrieved should be part of the conservation programme, and records need to be kept of rare and endangered plants supplied to other botanical gardens. Records should also include successes or failures in the cultivation of these plants, and references to special propagating techniques is of vital importance.

Conclusion

The poor success rate in conserving threatened plants in many botanical gardens can, in most cases, be traced back to insufficient support from scientists, in their not providing much-needed information about the propagation of plants. Unless this unhealthy situation is rectified, the wellintended conservation programmes in many botanical gardens will be delayed, or be doomed to fail.

References

Preface  |  Contents List  |  Congress Report  |  Workshop Conclusions  |  List of Authors