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University of Adelaide (2019)

Mistletoe effects on acacia species in western Saudi Arabia

Albakre, Dhafer

Titre : Mistletoe effects on acacia species in western Saudi Arabia

Auteur : Albakre, Dhafer

Université de soutenance : University of Adelaide

Grade : Doctor of Philosophy (PhD) 2019

Résumé partiel
Mistletoes are rootless stem-parasitic plants that have various effects on their hosts. Although overabundance of mistletoes has become an ecological concern in the western Arabian Peninsula, the reasons for this as well as the consequences are unknown. In addition, the mechanisms underlying different effects of mistletoes on nutrient and water status of the host remain under-examined. Therefore, it is essential to determine whether abiotic factors such as drought, topographic variation, and host characteristics are increasing mistletoe prevalence, and what factors control the effects they have on their hosts. This study undertook a series of ecological and physiological assessments to contribute to answering those questions. I assumed that the incidence of Plicosepalus curviflorus in Taif National Park, Saudi Arabia, is influenced by the characteristics of the four host species growing in various topographic positions. However, I found no differences in the host characteristics between infected and uninfected plants in three host species (Vachellia flava, Vachellia gerrardii and Vachellia tortilis). Only in the case of Senegalia asak, did the infected hosts have wider canopy areas than the uninfected plants. I suggest that P. curviflorus persists only on larger, more vigorous S. asak hosts. In Wadi Alshafa, I assessed the influence of altitude and host canopy volume on the prevalence (incidence/abundance) of three species of mistletoe coexisting on a single host species (V. gerrardii). The incidence and abundance of Phragmanthera austroarabica increased both with altitude and host canopy volume, while those of Viscum schimperi increased with altitude only, and those of P. curviflorus decreased with altitude and increased with host canopy volume. Moreover, the mistletoe species seemed to affect each other’s presence. P. curviflorus and V. schimperi appeared to compete, whereas V. schimperi appeared to facilitate the incidence and abundance of P. austroarabica. This suggests that biotic interactions among mistletoes could also affect their occurrence. Generally, the pattern of mistletoe species infection may be largely explained by different physiological responses to thermal stress in the summer season and water availability due to altitudinal effects. I investigated whether the presence of P. curviflorus affected the mineral nutrition status of host species growing on different terrains by analysing leaf samples of infected and uninfected branches of parasitised and unparasitised hosts. There was a reduction in N and Mn concentrations in parasitised S. asak, and of P and Zn in parasitised V. flava, while no differences in nutrients were found in V. tortilis. Differences in nutrient status are more likely influenced by the host distribution in different terrains, and availability of nutrients in soil

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