Tabernanthe iboga (family Apocynaceae) is native to Western Africa; Angola, Cabinda, Cameroon, Central African Republic, Congo, Gabon and Zaïre. Two species are currently recognised, Tabernanthe elliptica and Tabernanthe iboga ^1. Generally two forms are recognised of the T. iboga species, what the Indigenous people in West Africa call the male and female form, and which Westerners call the long and the round fruited forms. The male or long fruit is called “iboga nome” in the Myene language in Gabon, whilst the female or round fruit is called “iboga ny’ anto” in the same dialect ³. Previously, the female or round form was separated as its own species, Tabernanthe mannii. Modern research has lumped it together with the male or long form, both under Tabernanthe iboga. Apart from the fruit morphology, there are slight differences in leaf shape and especially flower morphology, that obviously couldn’t justify the separation. The fruit colour ranges from a lemon yellow to a dark orange at maturity, and both the long and round forms have a large number of variations in size and shape, as evidenced by a google search of ‘Tabernanthe iboga fruit’.

Apocynaceae consists of at least 5350 recognised species in 378 genera. Species are distributed from tropical to temperate environments in every major biome except arctic tundra, and the family is particularly species rich in the dry and wet tropics ². Flowers within the family show different levels of floral syn-organisation and fusion of androecium and gynoecium, which has allowed the appearance of specialised pollination mechanisms. Next to no information regarding Tabernanthe pollination or pollinators is available online or in published papers. However, Tabernaemontaneae pollinators were recorded as butterflies or settling moths ². Considering its medical potential, it is surprising that Tabernanthe iboga is so understudied, but also not surprising, considering it is a scheduled plant in the USA, which would no doubt hinder more widespread research. Tabernanthe iboga is not a scheduled plant in Australia, making it legal to possess and cultivate. Ibogaine and its metabolite noribogaine are schedule 4 (S4, prescription only) medicines in Australia and in New Zealand are classified for prescription without restrictions or controls. The rescheduling of ibogaine in Australia (from S9 to S4) and New Zealand occurred in 2010 ⁵. Dried herbal material such as dried root would presumably come under the S4 scheduling, as they contain a scheduled substance.

The article ‘The State of Knowledge on Tabernanthe iboga‘ ³ is showing its age now (March 2000), but contains a lot of interesting information. It says “the pollination and breeding system of Tabernanthe iboga are not well documented“. The phenology of Iboga is another poorly documented topic, but I concur with the paper which says that Iboga flowers and fruits throughout the year (in the tropics). In the subtropics where I live, Iboga becomes dormant over the colder months and in most circumstances, deciduous. Flowering in late spring at the earliest, fruit are carried through the summer and ripen in late autumn and winter, with the fruits generally hanging on the shrub while it is without leaves.

Pollination in most Apocynaceae is entomorphilous (normally pollinated by insects). Pollination by wind and water is unknown and obligate selfing extremely rare ². The flowers of Apocynaceae are bisexual and protandrous. Besides the complex floral morphology, the existence of secondary pollen presentation co-occurring with herkogamy and the recent report of functional dioecy in members of the family, also indicate a specialised pollination mechanism in Apocynaceae, preventing self pollination ⁴. The style head shows three functional regions: (1) an apical, sterile region where pollen is deposited (secondary pollen presentation), (2) a median region which produces a sticky substance, and (3) a lower receptive region beneath a basal membranous ring. Upon contact with H₂O₂ this lower region reacted by liberating oxygen ⁴. Pollen is shed shortly before anthesis and may be secondarily presented on the style head. Albers & Van der Maesen (1994) have demonstrated that the tip of the style head is always unreceptive to pollen in Rauvolfioideae and Apocynoideae ⁴. 

This confirms my observations and experiments with Tabernanthe iboga. Insects I have seen visiting have been minimal, but black ants (a member of the genus Ochetellus in the subfamily Dolichoderinae) are the most common visitors in my garden. I believe the ants are attracted to left over nectar after the corolla has been abscised, as they are simply too big to enter the tube itself. I have also seen live thrips in dissected flowers. Are they capable of cross pollination, I am not sure, they are small enough however to access the pollen correctly. The images below show the transversely dissected flower of a round fruit individual. Notice the slits between the anthers which allow access to small insects and/or the proboscis of winged pollinators. Upon the flower opening, the median section of the stigmatic head exudes a sticky substance, which effectively glues the stigma to the anthers, blocking downward pollen transfer to the receptive region.

Several round fruit forms I grow will produce next to no fruit, year on year, despite having a couple of individuals nearby. On the other hand, one other round fruit plant in particular will produce a large number of (assumed) self pollinated fruit, always with viable seeds. This plant is geographically isolated from the other T. iboga in my garden. There is a possibility that a shared pollinator from Tabernaemontana pandacaqui (Banana Bush) is assisting with pollination, as I have several T. pandacaqui in the vicinity, the closest within 2 metres. It is worthwhile investigating further. Another possibility is geitonogamy. Geitonogamy is when pollen is exported by a pollination vector, out of one flower but only to another flower on the same plant. It is a form of self-fertilisation.

We could assume that the limited fruit set we generally see on plants grown in Australia, is a direct result of the incorrect pollination vector, and hence, limited cross pollination. So if we need cross pollination to produce fruit, what are our options for plants in cultivation? We can propagate individuals like the (assumed) self pollinating plant in my garden, hoping that this trait will be passed on to future generations. We can look for pollinators that may be present naturally, such as a shared one in related species such as Tabernaemontana pandacaqui. NB. I had luck several years ago grafting T. iboga to T. pandacaqui, the grafted plant survived several years, showing their compatibility. Another option is assisted cross pollination. This is a skilled and time consuming operation, but it is feasible for a small number of fruit.

Another factor I am investigating is the role of macro and micronutrients and their effects on fruit set. It is entirely feasible that the deficiency of a (more commonly) micronutrient can be associated with poor fruit set in fruit trees and hence, leads to poor fruit set in Tabernanthe iboga. Micronutrients are essential elements that plants require in minimal quantities for optimal growth, development, and reproduction. Although these quantities are relatively small, they are indispensable for the various physiological and metabolic processes ⁶. Micronutrients have a profound influence on the reproductive phases of plants. Their presence at optimal levels during the flowering period can significantly influence both the quantity and quality of seeds produced. For instance, Boron is of particular importance in pollen germination and pollen tube growth, which are crucial for successful fertilisation ⁶. In grapes, a deficiency of Molybdenum can increase the occurrence of fruit set disorders ⁷. While a thorough examination of each micronutrients role in fruit set and development of Tabernanthe iboga is beyond the scope of the author at present, trials involving foliar application of liquid fertiliser and seaweed solutions appears to have had a positive accent on fruit set of the T. iboga hybrid plants, and is put forth here as possible method to increase fruit set.

Hybridisation of the male and female (long and round) forms of T. iboga had eluded me for a number of years. In 2022, I attempted (for maybe the 3rd or 4th time) to hybridise the round and long fruit forms of Tabernanthe iboga. Using a long fruited form grown from Cameroon seed as the pistillate (female) parent, and a round fruit form as the staminate (male) parent. Several pollen donors were tried in case there were issues with any of the round fruit plants. I think my mistakes in the past were not understanding the pollination system of this species and incorrectly placing the pollen on the sterile apical region of the style head. Even at the time of my last successful pollination, I believe I was lucky to achieve a result. My future experiments this season (January 2024) will be guided by all of this new knowledge regarding Apocynaceae pollination.

Following successful cross pollination, the long fruit developed for 6 months and produced 13 seeds, all of which were viable. I have now grown these out and they are approximately 1m tall and flowering, just over one year later. I have had several fruit off two of the F1 plants, and have now germinated a small number of F2 plants. I will be making more cross pollination experiments between siblings and also the parents, as this summer is a true wet season, with almost jungle like conditions here on the Sunshine Coast, Qld. This will be perfect conditions to get fruit set and to hold them until they are ripe.

My technique for cross pollination involves making a cut around the corolla tube just above the ovary, then pulling gently to lift the top half of the flower, including anthers, off the style and stigma. Apocynaceae have milky sap and this is carefully mopped up using a tissue if needed, to stop it interfering with the pollination. Pollen can then be isolated from the staminate donor and collected using a brush or tweezers, preferably under a microscope, to gauge if the pollen has dehisced. This is then transferred to the lower section of the style head/stigma. This can then be protected from the elements for a day or two, to prevent desiccation, by placing the removed corolla back on the flower, or making sure the plant is in a nice shady position.

I believe the future is bright for both Tabernanthe iboga and Ibogaine, as addiction rates soar to prescription painkillers and people are finding themselves disconnected from nature and in turn, their own true nature. Iboga is powerful plant teacher, capable of offering great guidance and relief, but can be dangerous if used incorrectly, so it deserves a great degree of respect, both in the garden and in the herbalists bag. I will be adding further Iboga stories to the blog as the hybridisation project continues.

References

1. Plants of the World Online. Kew Gardens. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:983-1
2. The diversity and evolution of pollination systems in large plant clades: Apocynaceae as a case study. Ollerton et al. 2019 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6344220/
3. The State of Knowledge on Tabernanthe iboga. A report for the Central African Regional Program for the Environment. Marcelin et al. 2000. https://pdf.usaid.gov/pdf_docs/Pnads957.pdf
4. Floral biology and a pollinator effectiveness test of the diurnal floral visitors of Tabernaemontana undulata Vahl. (Apocynaceae) in the understory of Amazon Rainforest, De Moura et al, Brazil. 2011. https://doi.org/10.1590/S0102-33062011000200014
5. Critique of the Royal Australian and New Zealand College of Psychiatrists psychedelic therapy clinical memorandum, dated May 2020. Chiruta et al. 2021. https://mindmedicineaustralia.org.au/wp-content/uploads/C_Critique-RANZCP-psychedelic-therapy-clinical-memorandum_50221.pdf
6. Ahmed N., et al. Micronutrients and their effects on Horticultural crop quality, productivity and sustainability, Scientia Horticulturae, Volume 323, 2024. https://doi.org/10.1016/j.scienta.2023.112512.
7. Williams C. 2007. SARDI Report. Molybdenum Foliar Sprays and Other Nutrient Strategies to Improve Fruit Set and Reduce Berry Asynchrony (‘hen and chickens’). https://www.wineaustralia.com/getmedia/bdc8ea1b-5557-453b-81dc-5a1be021da4a/SAR-02-09b