Determining strategies of Acanthosicyos horridus (!nara) to exploit alternative atmospheric moisture sources in the hyper-arid Namib Desert

Abstract

The enigmatic melon species Acanthosicyos horridus Welw. ex Hook. f., locally known as !nara, is endemic to the hyper-arid Namib Desert where it occurs in sandy dune areas and dry river banks. The Namib Desert is a result of the cold Benguela current off the coast of Namibia. This results in extreme environmental conditions including high temperatures, rare pulse rainfall events and desiccating air. In this water-restricted environment, non-rainfall water inputs (NRWIs), including fog, dew and water vapour, may play an important role in ecosystem function and can influence organisms’ behaviour to exploit alternative sources of moisture. Fog is considered to be the most important Non-Rainfall water inputs (NRWI) for most of the coastal Namib Desert, where A. horridus plants are common. It has been suggested that A. horridus is adapted to exploit fog as a moisture source. Acanthosicyos horridus shares many comparable adaptive features with other organisms that are known to exploit fog as a source of moisture. This study focused on A. horridus-fog interaction to determine whether A. horridus exploits fog, as it would illustrate strategies to benefit from NRWIs. The direct water uptake capacity of A. horridus shoots was investigated through absorption tests. Furthermore, the movement and behaviour of fluorescent water droplets on a A. horridus stem were investigated through time-lapse macrophotography. The shoot water potential was measured to investigate the effect of a fog on the water status of A. horridus stems. Chlorophyll a fluorescence was used to compare the photosynthetic potential of A. horridus plants on days with fog events to that on non-foggy days. Other environmental stressors were identified by comparing meteorological data with the photosynthetic potential of A. horridus stems. These tests advised on whether A. horridus has specific strategies to exploit NRWIs, i.e. through behaviour, habitat selection and habitat modification. Acanthosicyos horridus did exhibit the capacity for direct aerial absorption of fog water into the stems. Moreover, A. horridus did not exhibit visible signs of drought stress and this, together with the high shoot water potential, indicates that plants are reliant on permanent underground water sources as they are unlikely to survive on NRWIs alone, even within the zone of abundant fog in the Namib Desert. Measurements of the photosynthetic potential indicated that temperature stress and wind were some of the main abiotic factors influencing the plant’s overall vitality. Furthermore, the plants were able to recover their photosynthetic potential after exposure to air temperatures above 40°C.