Incidence of alkaloids and cyanogenesis in plants in southwestern Western Australia
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The following Post is based on analyses conducted by the late Bill Foulds, author of https://nph.onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-8137.1993.tb03901.x and https://www.jstor.org/stable/2558261 and https://library.dbca.wa.gov.au/static/Journals/080050/080050-01.004.pdf.
INTRODUCTION
Western Australia is nutrient-poor. The indigenous plants are accordingly adapted, by tending to have evergreen, nutrient-poor foliage.
These plants defend themselves from herbivory/folivory mainly by means of fibrousness (sclerophylly) and tannins (which, like fibre, contain only carbon, hydrogen, and oxygen, https://en.wikipedia.org/wiki/Tannin), rather than by defensive compounds that contain nutrient elements.
However, the flora contains many plants that supplement their supplies of nitrogen, by means of microbial nitrogen-fixation, hemi-parasitism, or 'carnivory'.
This suggests that these plants might tend to contain defensive substances based on amino acids (which contain nitrogen) and cyanide (the molecular formula of which is HCN, viz. a combination of equal parts of hydrogen, carbon, and nitrogen).
RESULTS
The following is information on the incidence of alkaloid-containing and cyanogenic plants (including non-indigenous spp.) in southwestern Western Australia, in relation to the nutrient status of soils. (In the case of Styphelia and Cakile, the analysed values range from negative to strongly positive.)
The strongly alkaloid genera were
- Exocarpus (Santalaceae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=157281&view=species)
- Acacia (Mimosoideae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=47452&view=species)
- Templetonia (Faboideae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=138669&view=species)
- Cassytha (Lauraceae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=119899&view=species)
- Styphelia (Ericaceae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=155967&view=species)
- Solanum (Solanaceae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=50641&view=species)
- Anthocercis (Solanaceae, https://www.publish.csiro.au/CH/CH9690221 and https://www.inaturalist.org/observations?place_id=6827&taxon_id=153185&view=species)
The strongly cyanogenic genera were
- Acacia (2 sampled spp.), and
- Cakile (Brassicaceae, https://www.inaturalist.org/observations?place_id=6827&taxon_id=52163&view=species).
In the case of legumes other than Acacia, Lotus (https://www.inaturalist.org/observations?place_id=6827&taxon_id=47436&view=species) and Trifolium (https://www.inaturalist.org/observations?place_id=6827&taxon_id=51876&view=species) were found to be cyanogenic, and the former also contained alkaloids.
However, cyanogenic spp. also occurred in
- Proteaceae, e.g. Hakea francisiana (https://www.inaturalist.org/taxa/499455-Hakea-francisiana) and Grevillea thelemanniana (https://www.inaturalist.org/taxa/981486-Grevillea-thelemanniana), and
- Myoporaceae (e.g. Eremophila maculata, https://www.inaturalist.org/taxa/135356-Eremophila-maculata).
A wide variety of plants on coastal dunes was cyanogenic. This included Banksia woodland on the Swan Coastal Plain (https://en.wikipedia.org/wiki/Swan_Coastal_Plain).
The flora on sand dunes at Hillarys (https://en.wikipedia.org/wiki/Hillarys,_Western_Australia) was notably rich (27% of spp. sampled) in alkaloid-containing plants. including a succulent introduced from South Africa, viz. Tetragonia decumbens (https://www.inaturalist.org/taxa/560154-Tetragonia-decumbens). However, Exocarpos in this environment lacked alkaloids.
No alkaloid-containing plants were recorded in woodland (sampled at Hepburn Heights, https://en.wikipedia.org/wiki/Hepburn_Heights_bushland) of Banksia (https://www.inaturalist.org/observations?place_id=19379&taxon_id=64518&view=species) and Eucalyptus gomphocephala (https://www.inaturalist.org/taxa/162753-Eucalyptus-gomphocephala) on coastal sand.
In each plant community sampled near Perth (https://en.wikipedia.org/wiki/Perth#:~:text=The%20metropolitan%20region%20is%20defined,of%20Serpentine%2DJarrahdale%20to%20the), only one species (of the hemi-parasitic genus Cassytha, https://www.inaturalist.org/taxa/851590-Cassytha-flava) was found to contain alkaloids.
In the case of the hemi-parasitic genus Cassytha,
- the analysed values ranged from weakly to strongly positive,
- the sample on sandplain near Geraldton (Burma Road Nature Reserve, https://en.wikipedia.org/wiki/Burma_Road_Nature_Reserve) was particularly rich in alkaloids, despite the fact that none of the host spp. contained alkaloids, and
- the samples in forest of Eucalyptus marginata (https://en.wikipedia.org/wiki/Jarrah_Forest) on laterite were remarkably variable in their concentrations of alkaloids.
Woodland of Eucalyptus salmonophloia (https://www.inaturalist.org/taxa/499438-Eucalyptus-salmonophloia), under a semi-arid climate (https://en.wikipedia.org/wiki/Great_Western_Woodlands), was particularly rich in hemi-parasitic plants (particularly as sampled near Yellowdine, https://en.wikipedia.org/wiki/Yellowdine,_Western_Australia).
The hemi-parasite Exocarpos (https://www.inaturalist.org/taxa/525357-Exocarpos-aphyllus) contained alkaloids, but mistletoes did not. The mistletoes referred to are
- Amyema (https://www.inaturalist.org/taxa/481934-Amyema-miquelii, which occurs on 30% of individual trees of E. salmonophloia), and
- Lysiana (https://www.inaturalist.org/taxa/849053-Lysiana-casuarinae, which parasitises Acacia coolgardiensis, https://www.inaturalist.org/taxa/775152-Acacia-coolgardiensis, in broombush vegetation on sand).
The incidence of cyanogenetic plants in this woodland was less than in Banksia woodland under a mesic climate.
DISCUSSION
Many Acacia spp. are known to be cyanogenic.
The overall pattern seems to be that
- toxicity in the form of alkaloids is fairly weakly expressed in southwestern Australia, and
- in the same region, cyanogenic plants occur in environments with relatively mild, mesic climates and nutrient-rich soils.
There seems to be a lack of cyanogenic plants in mediterranean-type climates in which frost occurs. There may also be a lack under arid climates, but the pattern is unclear.
Why do grasses feature cyanogenesis but not alkaloids?
ACKNOWLEDGEMENT
I am grateful to the late Bill Foulds for sharing his data with me, and for discussing the patterns.
RAW DATA: PHOSPHORUS IN SOILS
The following refers to concentrations of phosphorus (parts per million) in topsoils.
- 1-3 ppm P: broombush, mallee woodland, kwongan, banksia woodland, and jarrah forest
- 4-8 ppm P: salmon gum woodland, granite outcrop, wandoo forest, limestone heathland, sand dunes
- 26 ppm P: salina, ironstone outcrop
RAW DATA: CYANOGENIC PLANTS
The following numbers of spp. were analysed for cyanogenesis in the various plant communities:
Semi-arid climate adjacent to mediterranean-type climate:
- Broombush 25, mallee woodland 11, salina 23, kwongan 7, salmon gum woodland 17, granite outcrop 5, ironstone outcrop 7
Dry form of mediterranean-type climate:
- Kwongan 28
Mesic form of mediterranean-type climate:
- Banksia woodland 24, limestone heathland 42, sand dunes 19
Incidence of cyanogenic spp.:
Zero in all cases, except for banksia woodland (8.3% of analysed spp.) and sand dunes (31.6% of analysed spp.)
RAW DATA: ALKALOIDS
In each case, the first value is the no. of spp. analysed, and the second is the percentage of these found to contain alkaloids.
Semi-arid climate adjacent to mediterranean-type climate:
- Broombush 36, of which 5.6%
- Mallee woodland 23, of which 17.4%
- Salina 43, of which 20.9%
- Kwongan 21, of which 23.8%
- Salmon gum woodland 30, of which 40%
- Granite outcrop 26, of which 42.3%
- Ironstone outcrop 4, of which 50%
Dry form of mediterranean-type climate:
- Kwongan 90, of which 13.3%
Mesic form of mediterranean-type climate:
- Banksia woodland 19, of which 0%
- Jarrah forest 41, of which 2.4%
- Granite outcrop 35, of which 2.8%
- Wandoo forest 31, of which 6.4%
- Limestone heathland 42, of which 7.1%
- Sand dunes 12, of which 8.3%
