Succulent Euphorbia in Southern Africa and their Extraordinary Plant Parasite Hydnora

Sebastian Hatt

Hydnora africana flowers underneath its host Euphorbia mauritanica
Hydnora africana flowers underneath its host Euphorbia mauritanica

Euphorbia (Euphorbiaceae) is amongst the largest genera of flowering plants, with recent estimates suggesting around 2,000 species, of which ~900 are succulent. Southern Africa represents a key biodiversity hotspot for succulent Euphorbia, featuring a wide range of species with often highly localised distributions. Over considerable evolutionary time, a number of these succulent Euphorbia have become the target of an extraordinarily bizarre group of parasitic plants: Hydnora (Hydnoraceae), a genus of 10 species native to Africa and the Middle East. As holoparasitic plants (plants that are incapable of photosynthesis), they are reduced to a sprawling underground rhizome that survives by parasitising nutrients from the roots of its host plant through a specialised organ called a haustorium. Hydnora produce amongst the most unusual and spectacular flowers in the entire plant kingdom.

Four species of Hydnora are endemic to Southern Africa, all of which parasitise exclusively succulent Euphorbia. Interestingly, each of these four species parasitise different species of Euphorbia. Indeed, even when multiple Hydnora species co-occur at the same locality, they have still been observed to strictly parasitise their exclusive hosts. Therefore, these Hydnora species have evolved an intimate relationship with specific succulent Euphorbia species. The mechanisms and evolutionary explanations behind these relationships have not been studied and remain completely unknown.

Host plant Euphorbia mauritanica, growing in the Karoo Botanic Garden, South Africa
Host plant Euphorbia mauritanica, growing in the Karoo Botanic Garden, South Africa

The anatomy of the host-parasite connection has been relatively well studied in Orobanche, but the majority other holoparasitic plants have not been investigated at all. Viscum minimum is another parasite of succulent Euphorbia, now increasingly popular in cultivation. However, V. minimum taps directly into the stem tissue, while Hydnora targets the roots. Unlike V. minimum, our understanding of the mechanisms of parasitism and nutrient exchange in Hydnora is extremely poor. Besides a single study in 2007 detailing the tissue anatomy of H. triceps, there have been no published attempts to research either nutrient exchange or the anatomy of Hydnora species.

We aim to investigate the relationship between succulent Euphorbia and their Hydnora parasites by studying their anatomy. Later this year, in collaboration with colleagues from the South African National Biodiversity Institute (SANBI), we will travel to several known populations of Hydnora, each growing on different Euphorbia host species, to make detailed ecological observations and collect root and rhizome material for comparative anatomical studies. We are very grateful to the British Cactus and Succulent Society research grant for their generous financial support, without which this work would not be possible. This work represents an important contribution to our understanding of the evolution, anatomy and ecology of succulent Euphorbia in their natural habitat.

Host plant Euphorbia gregaria, growing near Rosh Pinah, Namibia
Host plant Euphorbia gregaria, growing near Rosh Pinah, Namibia
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