Mistletoe effects on acacia species in western Saudi Arabia

Abstract

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. In Wadi Alshafa, I examined the influence of different levels of infection on the nutrient status of the host. Compared with non-host trees and slightly infected hosts, the Ca and Mg concentrations were higher in hosts heavily infected with V. schimperi, while N, P, Na and Ca concentrations were higher in hosts heavily infected with P. austroarabica, and P and Na concentrations were higher in hosts heavily infected with P. curviflorus. Regardless of mistletoe species, my results suggest that high mistletoe load on the host reflects a response of the parasite to high nutrient availability in larger rather than smaller hosts, as the host characteristics are significantly different in the wadi system. I examined whether three species of mistletoe coexisting on the same host and their respective infected branches differ in nutrient status. There was a significant difference between mistletoe species and their effects on distal branches of the host. Overall, K concentration was higher in mistletoe species than in the host branches. P. austroarabica showed higher levels of N and Na compared with V. schimperi and P. curviflorus; additionally, P. austroarabica accumulated significantly more N and P than its respective host branches. This can be interpreted as a result of functional variation of morphological traits of the mistletoes, such as of the photosynthetic organ, reflecting their different requirements in nutrient acquisition. In Taif National Park, I investigated the effect of P. curviflorus on water status and photosynthetic performance of four acacia species, S. asak, V. flava, V. gerrardii, and V. tortilis. The light-use efficiency of leaves and water status of twigs were assessed. Predawn and midday quantum yields, and water potential were assessed during the dry and wet periods. I compared mistletoe performance regarding their effect on distal branches of host species during dry and wet periods; in addition, I compared physiological performance of infected and uninfected branches of hosts and non-hosts. Mistletoes performed differently in the two periods on shrub-like S. asak, V. flava, and V. tortilis, which had lower predawn quantum yields and greater negative predawn water potentials in the dry season compared to the wet period; but mistletoe performance did not vary between periods on V. gerrardii. This is possibly because V. gerrardii grows in lower areas where more water may be available and is likely to have deeper roots to access reliable water in dry periods. Mistletoe infection increased the predawn water stress of infected branches of S. asak, V. flava and V. tortilis compared with uninfected branches or non-hosts during the dry period. Interestingly, mistletoes had a greater Fv/Fm ratio in the wet season and showed a negative impact on the infected branches of S. asak, which were water stressed, resulting in chronic photoinhibition even in the wet period. This could be due to the constant water demand of P. curviflorus to avoid host exclusion in the drier slope environment. Three different species of mistletoe infect a single host, V. gerrardii, in Wadi Alshafa. I examined water status and light utilisation at predawn and midday, and whether they were affected by different levels of infection (low, medium, heavy). Heavy mistletoe infection increased predawn water stress, and thus chronic photoinhibition, compared with moderately and slightly infected hosts. The branches infected by P. austroarabica had a higher level of water stress than did the respective uninfected branch, but those branches infected by the other two species did not differ with heavy infection. On the mistletoe species level, loranthaceous mistletoes P. austroarabica and P. curviflorus had more negative predawn water potentials and a higher photosynthetic performance at daylight than V. schimperi. The disparity in mistletoe effects and their performance were most likely due to different levels of physiological stress in response to differences in temperature and moisture conditions in the Wadi. This research reveals the crucial role of abiotic factors which drive the effects of mistletoes on hosts in two different ecosystems. My ecological and physiological data contribute new knowledge about potential biotic interactions between V. schimperi, P. austroarabica, and P. curviflorus in Wadi Alshafa. Results from this study can improve the scientific basis for management and control of mistletoe infection in natural drylands. Individual trees infected by high abundances of mistletoes, in particular, those trees infected by P. austroarabica, were suffering the most from physiological stress; therefore, the appropriate practice may be to remove mistletoe by pruning the infected branches. This practise can slow the spread of infection and maximise the growth rates of the trees as a short-term solution. For large-scale management, further investigations are essential to address the underlying biotic and abiotic causes in order to establish a longer-term solution.