Opuntia stricta

Range and segmented stem

POWO recognises Opuntia stricta as a distinct species. Its native sites lie in the southeastern USA and Mexico. Its extensive present-day range outside the Americas includes numerous introduced populations. The observation location, for example Saudi Arabia or South Africa, therefore describes the study site of an introduced population. [2-WWW]

The plant forms flattened, fleshy segments with areoles. In material from Jazan described in the study from 2020, the areoles had small glochids and longer spines. Comparison of their surfaces revealed an important difference. Glochids bore microbarbs and structures directing water, while the surfaces of the longer spines were relatively smooth. [Masrahi 2020]

Droplets on glochids

The conical glochids bear barbs pointing towards the base, where dense, hygroscopic hairs occur. In the mist experiment, droplets settled on this surface, merged and moved towards the areole. The authors’ model links capillary pressure differences with organ geometry and variation in wettability. [Masrahi 2020]

The study title refers to dew collection, while the experiment used a stream of droplets generated by a humidifier. The documented result is the capture and transport of liquid water on glochid surfaces. The amount of water supplied to the whole plant by natural dew and fog was not determined. [Masrahi 2020]

Islands of fertility in the savanna

In Kruger National Park, soils at sites invaded by O. stricta were compared with uninvaded open areas and soil beneath native trees. The invasion was associated with higher moisture, altered nutrient content and enzyme activity. Sequencing of bacterial 16S rRNA also showed a change in microbial community composition. [Novoa et al. 2021]

Seeds germinated faster and in greater numbers in soils collected beneath native trees, especially Vachellia nilotica subsp. kraussiana, than in open-area soil. Soil from sites already occupied by opuntias reduced germination compared with the corresponding uninvaded soils. This pattern links facilitated colonisation beneath trees with different conditions beneath established opuntia clumps. [Novoa et al. 2021]

The authors consider a role for germination-inhibiting substances but did not isolate a compound responsible for the effect. The experiment determines the outcome of the whole collected soil, including its chemical and biological properties. It therefore allows assessment of the actual substrate, while leaving the separation of individual mechanisms to further studies. [Novoa et al. 2021]