Autecological studies on Audouinia capitata (Bruniaceae). 2. Insects as pollen vectors
M.G.Wright, D.Visser & J.H.de Lange 1991 South African Journal of Botany 57 260-263 https://doi.org/10.1016/S0254-6299(16)30927-9
The role of insects as pollen vectors of Audouinia capitata was studied. The frequency of insect visits to flowers and pollen load counts showed that small beetles (Nitidulidae) should be the best insect pollen vectors. Pollen dispersal by these beetles, however, was limited to short distances, which reduced their beneficial impact on the plants. It is suggested that poor pollen transfer may be one of the reasons for the scarcity of A. capitata.
Visitors: (* = significant pollen loads, # n of visotors)
Coleoptera
- Nitidulidae Carpophilus sp * 25
- Nitidulidae Pria sp * 109
- Chrysomelidae: Alticinae 10
- Helodidae Helodes sp * 3
- Buprestidae Sphenoptera sp 1
- Curculionidae (only ate flower parts) 19
Lepidoptera - Psychidae 1
Diptera - Various & Lauxaniidae 5
Hymenoptera - Formicidae 7
Thysanoptera 2
Autecological studies on Audouinia capitata (Bruniaceae). 8. Role of fire in regeneration
J.H.de Lange & C.Boucher 1993 South African Journal of Botany 59 188-202 https://doi.org/10.1016/S0254-6299(16)30752-9
In A. capitata the protracted juvenile phase, the longevity of plants and their effective vegetative fire regeneration, are all possibly adaptively linked to poor sexual reproduction. After a fire-free period of more than ten years, averages of only 4.7 and 0.4 seedlings per plant were recruited following summer and winter burns, respectively. The positive correlation between the prevalence of fynbos fires from January to March and the germination response of soil-borne seeds when treated with smoke at different times of the year, suggest that the species has evolved in a fire-prone vegetation in a climate with a seasonal rainfall distribution similar to the present one. The vegetative vigour of shoots, the size and numbers of inflorescences, and seed set were increased by fire. The continuation of a policy of late autumn and winter burns may be fatal for survival of the species. Plants responded favourably to fire at a post-fire age of 11 years. Some populations, however, were still vigorous and sexually productive at a post-fire age of 20 years.
&
"A. capitata seedlings are recruited in the same or the following season and only after fire. A total of 110 seedlings were found in three populations (Sirkelsvlei North, Bonteberg East and Klaasjagers Plateau) at the commencement of the present A. capitata project in July 1987, 17 months following an extensive fire in the Cape of Good Hope Nature Reserve on 1 February 1986. The environment at the Teeberg population is strewn with loose rocks and the quick post-fire regrowth of the vegetation precluded a reliable survey for seedlings here. In populations burnt in the same fire, but discovered later, an additional 38 seedlings were recorded. A number of dead seedlings were observed.
The controlled bum in the Sirkelsvlei West population (21 February 1989) .. for the first four months following emergence showed no signs of predation despite the small size of the burnt area. Prior to the bum, 45 flowering plants were marked and mapped. At the end of September, seven months after the fire, a total of 334 seedlings had been recorded, all of them in close proximity to the parent plants"
more? see the paper.
Autecological studies on Audouinia capitata (Bruniaceae). 7. Nutritional aspects in the mature seed
J.H.de Lange G.R.Thompson C.R.Swart 1993 South African Journal of Botany 59: 178-187 https://doi.org/10.1016/S0254-6299(16)30751-7
Lipids and proteins are the major storage compounds present in the starchless endosperm of dormant A. capitata seed. Calcium oxalate crystals are present in protein bodies of the epidermal endosperm, and in the pericarp cells. The globoid inclusions of protein bodies are a major source of P, K, Cl and Mg and the epidermal endosperm crystals, a major source for Ca. No crystalloids are found in the protein bodies. The seeds store N and P. They also contain substantial concentrations of Cu, Zn and Mn. Lower amounts of these elements are present in the pericarp. Nutrients in the pericarp of the indehiscent fruit are probably utilized by the developing seedling. The high N, P and lipid content in the seed, and also the distribution pattern of calcium oxalate crystals in the seed and pericarp, seem to be beneficial evolutionary adaptations. Lipids in seeds from the soil seed bank differ quantitatively from those in fresh seeds. Low densities of protein bodies are found in many seeds resulting in deficiencies of proteins and mineral elements. This may partially explain high incidence of pre-emergent seedling mortality contributing to the rarity of the species.
&
"The two major nutrient reserves in A. capitata seeds are lipids and proteins. No starch is found in dormant seeds. The 55.4% oil recorded in fresh A. capitata seeds is high when compared to that found in most other plants. The high oil content and the absence of starch in the small A. capitata seeds could be an evolutionary adaptation. The energy released during oxidation of fatty acids is approximately 2.2 times the energy released by the same volume of carbohydrates
...
A. capitata occurs on heathlands which have generally nutrient-poor soils, particularly deficient in Nand P (Kruger 1979; Groves 1981; Low & Bristow 1983). The high concentrations of Nand P in the seeds of A. capitata, as well as in the seeds of other plants in the fynbos and Australian heathlands, appear to ensure seedling establishment in
nutrient-poor environments."
Autecological studies on Audouinia capitata (Bruniaceae). 6. Nutritional aspects of the developing ovule
J.H.de Lange, J.J.A.van der Walt & C.Boucher 1993 South African Journal of Botany 59 168-177 https://doi.org/10.1016/S0254-6299(16)30750-5
The present investigation provides a basis for studies about morphological and nutritional aspects involved in late abortion of embryos and nutrient-linked, pre-emergent mortality of seedlings in this threatened species. The nucellus is digested completely during megagametogenesis and the integument likewise during embryogenesis, except for a diminutive testa. Mass starch deposition is found in the mature embryo sac. Involvement of the persistent synergid in nutrition of the zygote and the 2-celled proembryo until the fifth week, is suggested. This function is taken over by the suspensor which initially channels integumentary reserves to the developing proembryo. From the eighth week, spherical protein bodies are evident in the endosperm. At this stage, the embryo proper is still surrounded by free nuclear endosperm, and the suspensor of the linear proembryo mobilizes, in addition to integumentary reserves, also adjacent cellular endosperm as well as the micropylar layer of cellular endosperm. The embryo proper of the more advanced stages mobilizes nutrients directly from the adjacent cellular endosperm. The seed enters dormancy at the heart-shape stage of embryo development.
Autecological studies on Audouinia capitata (Bruniaceae). 5. Seed development, abortion and pre-emergent reproductive success
J.H. de Lange, J.J.A. van der Walt and C. Boucher 1993 S.Afr.1.Bot., , 59 156 - 167
Ovule development, megasporogenesis, megagametogenesis, embryogenesis, endosperm development and seed germination of Audouinia capitata are described. In sharp contrast to the relatively high incidence of abortion and non-viability of microspores demonstrated previously, no pre-anthesis aberrations occur in the megagametophyte. In many fertilized ovules the primary endosperm nuclei remain undivided, enlarge considerably for up to five weeks and eventually degenerate, accompanied by abortion of the zygote. A low incidence of abortion is evident after the fifth week. Abortions after 11 weeks of development are identifiable in mature fruits. A substantial number of mature seeds are devoid of, or contain low densities of, protein bodies. After germination, the radicles of these seeds stop growing soon after emergence from the fruits. It is suggested that abortion is regulated by inter-ovular competition for available nutrients, or by a strongly fixed, genetically controlled system. This strategy could have been selected to allow for the economization of reserves in the 6-ovuled, indehiscent ovaries that act as dispersal units. Possible explanations are suggested for abortion rates in excess of those implied by a fixed general system of one seed per indehiscent fruit. When accepting a seed:ovule ratio of 1:6 as full realization of the seed-setting potential, and ignoring all nonsexual factors, the pre-emergent reproductive success is only 9%. This deficiency is a contributing factor in the rarity of the species.
&
"A. capitata seedlings are extremely rare. In an earlier study, Parisi (1985) recorded that no seedlings were found. During the present series of investigations, very low numbers of seedlings were recorded after bums (de Lange &Boucher 1993b). The seedlings have a very prolonged vegetative stage. The exact duration of this non-flowering stage is unknown but is probably more than 15 years (de Lange & Boucher 1993b). A selective strategy with more emphasis on efficient resprouting following fire and longevity than on reproductive capability, seems to be possible. It is likely that large percentages of seedlings may not even have reached the flowering stage at the end of their first inter-fire period. This implies that seedlings would be subjected to selection for resprouting efficiency (and longevity) after the next fire before they even have had the opportunity to produce seeds."
Autecological studies on Audouinia capitata (Bruniaceae). 4. Seed production, seed banks and seedling recruitment. de Lange, J.H.; Boucher, C. (1993). South African Journal of Botany, 59, 145–155. doi:10.1016/s0254-6299(16)30748-7
Audouinia capitata (Lt.) Brongn. has an average annual production of 98 seeds per plant, determined from a study on approximately 800 of the 1200 plants in 25 known populations. Most of the buried seeds occurred within a radius of 2 m from the parent, in the top 20 mm of soil. Following exposure to plant-derived smoke, 13% of the fresh seeds and more than 80% of the oldest seeds germinated. Thirty-nine percent of the germinated seeds were unable to emerge from a mere 5 mm soil depth under optimal laboratory conditions. Seventeen to 31 months after a burn, 148 seedlings were recorded in seven populations containing 332 plants, from an estimated total annual production of approximately 40 000 seeds. The natural occurrence of small numbers of seedlings is only partially explained by factors investigated in this study. Seed set in at least 14 of the populations, having less than approximately 10 mature plants each, is probably too low to sustain future population expansions or even to maintain present levels.
&
"Conclusions
Facets about the life cycle of A. capitata discussed in this study, are probably of crucial importance in the population dynamics of the species. Seed set in at least 14 of the 25 populations, having less than approximately 10 plants each, is probably too low to sustain future population expansions or even to maintain present levels. Seed production and the seed store in the soil are considered to be most important criteria which determine the successful maintenance of population levels (Boucher 1981).
The question may be raised whether eventual extinction faces all the smaller populations and whether some of the larger populations might eventually be degraded to the 'small population' category facing similar prospects as the present smaller populations. Demographic changes are likely to be gradual because A. capitata is an effective resprouter from a large lignotuber after fire. "
&
"According to Bond and Slingsby (1983), A. capitata seeds are dispersed by ants. This observation was not confirmed in the present study. No elaiosome is present. The fruits are indehiscent and at the time of dispersal from the plant, the fruits are dry and do not present any attraction to ants. Observation of ant activity near A. capitata plants for continuing periods of up to two hours on several occassions over four years did not reveal any interest by foraging ants in the fruits. "
Populations:
Table 1 Results of a survey of natural seed production in all 25 known populations.
Presented here is just the number of plants per population. (1987-1990) for other details see the paper
500~ Karbonkelberg
162 Bonteberg South
98 Sirkelsvlei North
90 Olifantsbos
73 Bonteberg East
72 Sirkelsvlei West
71 Teeberg
27 Sirkelsvlei East
22 Bonteberg West
15 Rooihoogte West;
10 Anvil Rock East
8 EJsespiek
8 Klaasjagers Plateau
7 Diaz Cross
7 Kobrakamp
6 Lean Rock North
6 Rooihoogte East
6 Sirkelsvlei South
5 Rooikrans
4 Restersdam
1 Anvil Rock South
1 Babilonstoring
1 Brightwater
1 Kanonkop
1 Muizenberg Caves
Total 1202
Autecological studies on Audouinia capitata (Bruniaceae). 3. Pollination biology
J.H. de Lange C. Boucher J.J.A. van der Walt
1993 South African Journal of Botany 59: 135-144
DOI:10.1016/S0254-6299(16)30747-5
Flower production in Audouinia capitata is highest during the first post-fire season. Self-pollinated flowers produced a mean of only 0.8% fruits bearing seeds. This seed set was increased 17-fold by natural insect-mediated cross-pollination. Hand cross-pollination induced an approximately 5-fold increase in seed set when compared to natural cross-pollination. Germination and initial pollen tube growth are normal following self- as well as cross-pollination. Selfing, however, is characterized by abnormal pollen tube growth at the stylar base, resulting in cessation of growth except in the case of occasional pollen tubes. By one week after cross-pollination, pollen tubes have penetrated the egg apparatus. Average in vitro germination of pollen and frequencies of small underdeveloped microspores in 20 populations varied from 2.5 to 28.4% and from 2.0 to 23.7%, respectively, with large interplant variations within populations in both germination (0 – 79.6%) and underdeveloped microspores (0 – 61.0%). The prevalence of non-viable pollen and insufficient insect-mediated transfer of pollen between the plants which have an insignificant ability to produce seed following self-pollination, are limiting factors in reproduction.
&
"Fires induce markedly increased flower production which gradually decreases with time after fire. The flowering season is protracted, extending from May to the end of September in the Cape of Good Hope Nature Reserve. Receptive flowers in the larger populations were present for approximately 3 months and on individual plants from approximately 3 weeks to 2 months."
"A. capitata has a limited ability to set seed in the absence of cross-pollination. A 17-fold increase in seed production was evident following natural insect-mediated cross-pollination. The use of pollen with high in vitro germination capabilities in hand cross-pollination, resulted in an approximately S-fold increase in seed set compared to natural cross-pollination. Even in the absence of insect activity, considerable self-pollination was evident, the extent depending on the difference in height between stigma and anthers, with averages ranging from 11 to more than 100 pollen grains per stigma for the different flower types investigated. "
read more in the paper ...
Autecological studies on Audouinia capitata (Bruniaceae). I. Plant-derived smoke as a seed germination cue
J.H.de Lange & C.Boucher 1990. S.AfrJ.Bot., 56
https://doi.org/10.1016/S0254-6299(16)31009-2
Seeds of Audouinia capitata, a threatened fynbos species, are known to germinate under natural conditions only after fires. Experimental results are presented which demonstrate that seed germination is initiated by chemical factor(s) found in smoke, derived from burning fynbos plant material.
Hannes's preamble:
Audouinia capitata (L.f.) Brongn. (Bruniaceae) is a threatened monotypic fynbos plant, categorized by Hall & Veldhuis (1985) as 'vulnerable'. It was decided to study the autecology of A. capitata because:
(i) young plants are rarely found in nature;
(ii) the plant has a decided horticultural potential;
(iii) to our knowledge nobody has succeeded in cultivating the plant;
(iv) the Bruniaceae is a family containing many threatened species and is endemic to the southwestern, southern and south-eastern Cape with the single exception of an outlier (Raspalia trigyna which occurs in southern Natal); &
(v) it could serve as a model to investigate the processes of rarification and extinction of plants in the western Cape.
