Host specific variation in photosynthesis of an obligate xylem‐tapping mistletoe Dendrophthoe curvata in a Bornean heath forest

Most mistletoe–host ecophysiological studies have paid attention to the influence of parasites on host performance. This paper explored the impact of varying hosts on the photosynthesis of a single mistletoe species. Here, we studied an obligate xylem‐tapping tropical mistletoe (Dendrophthoe curvata (Blume) Miquel) parasitizing four different hosts (Acacia auriculiformis A. Cunn. Ex Benth, Andira inermis (W. Wright) DC., Mangifera indica L. and Vitex pinnata L.) in a homo geneous tropical heath forest patch in Brunei Darussalam. We compared photosynthetic capacity and photosynthesis‐related characteristics of the mistletoe on four different hosts to evaluate the overall impact of hosts on the parasite. Results showed that the mistletoe–host patterns of CO₂ assimilation rates, transpiration rates and water use efficiency varied significantly based on the host. In the D. curvata–Vitex pinnata association, the mistletoe exhibited significantly lower CO₂ assimilation rates but showed no significant variations in transpiration rates and water use efficiency when compared to the host. In D. curvata–Andira inermis and D. curvata–Mangifera indica associations, the mistletoe showed significantly higher photosynthetic rates than the hosts, whereas in the D. curvata–Acacia auriculiformis association, there was no significant difference in photosynthetic rates between the counterparts. Host specificity also significantly influenced some mistletoe photosynthetic parameters such as light saturated photosynthesis, specific leaf area, leaf chlorophyll content, CO₂ assimilation rates, stomatal conductance, transpiration rates and water use efficiency. Different tree hosts intrinsically offer different resources to their obligate mistletoe parasites based on their physiology and environmental parameters. We argue that host‐specific responses have driven these intra‐specific variations in mistletoe physiology. This study provides background for future investigation on potential host‐regulated mechanisms that drive functional changes in host‐dependent mistletoes.     

Effects of parasitism by a mistletoe on the structure and functioning of branches of its host

Infestation of Acacia acuminata by the xylem-tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and eventual death of host branch parts distal to the mistletoe. Branch sectional areas proximal (P) and distal (D) to mistletoes are used to classify stages of parasitism, with P:D area ratios of 5–6 invariably associated with distal branch senescence. As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining host foliage area, and the specific hydraulic conductivity of distal host wood declines sharply relative to that of proximal wood, mineral composition and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal wood. After the demise of the distal branch parts, the transectional area of the host branch stump increases linearly with increasing mistletoe leaf area, the branch area supporting a unit of mistletoe leaf area always being about 3 times greater than that supporting a unit of host foliage area on unparasitized branches. This differential, compounded with high transpiration rates and selective uptake of host xylem solutes by the haustorium, fosters substantial mineral enrichment of the mistletoe relative to its host. The study provides a background for future investigation of possible cellular mechanisms continuously driving structural and functional changes in favour of the mistletoe.     

Carbon and nitrogen parasitism by a xylem‐tapping mistletoe (Tapinanthus oleifolius) along the Kalahari Transect: a stable isotope study

The present study explores the xylem‐tapping parasitism by mistletoe (Tapinanthus oleifolius) on native tree species along the Kalahari Transect (KT) using the stable isotopes of carbon and nitrogen. Mistletoe‐host pairs were collected at three geographical locations along the KT rainfall gradient in the 2005 and 2006 wet seasons. Foliar total carbon, total nitrogen and their stable isotope compositions (δ¹³C and δ¹⁵N) were measured. Heterotrophy (H) was calculated using foliar δ¹³C values of mistletoes and their hosts as an indicator of proportion of carbon in the mistletoes derived from host photosynthate. Based on the mistletoe H‐value and relationship between the mistletoe foliar δ¹⁵N and their host foliar δ¹⁵N, the results showed that mistletoes along the KT derived both nitrogen and carbon from their hosts. Mistletoes may regulate water use in relation to nitrogen supply. The proportion of carbon in the mistletoes derived from host photosynthate was between 35% and 78%, and the degree of heterotrophy was species‐specific with only limited annual variation. The study emphasizes the importance of incorporating parasitic associations in future studies on studying carbon, water and nutrient cycling along the Kalahari.     

A Host Creates an Enemy‐free Space for Mistletoes by Reducing Seed Predation Caused by a Woodboring Beetle: A Hypothesis

Parasitic plants rely on host plants for nutrition. The number of host species varies largely between groups, from single species or genus to hundreds of species. Relative abundance of the host and evolutionary history are the main requisites for parasitic plants to develop specificity to abundant hosts. In the present study, we suggest a novel mechanism by which a hemiparasitic mistletoe can develop local specificity mediated by its host. First, we describe a novel interaction in which a woodboring beetle (Hypothenemus obscurus) preys on mistletoe seeds (Psittacanthus plagiophyllus) attached to tree branches. This beetle preys more frequently on seeds deposited on branches of non‐host species than on branches of its unique local host species (53 percent on average vs. 26 percent respectively). We hypothesize that local host specificity for this mistletoe could be partly mediated by beetle‐host incompatibility, since the host offers a predation‐free space in which mistletoes have better chances to grow. Furthermore, that the exceptional gum exudates produced by this unique host species minimize beetle attacks on branches, thus reducing predation of mistletoe seeds. This novel tritrophic interaction opens an avenue for research on macroscopic host‐specificity mechanisms that occur at the level of the host bark and that have been poorly studied by plant ecologists.     

Aggregation in field parasitoid populations: foraging time allocation by a population of Diadegma (Hymenoptera, Ichneumonidae)

ABSTRACT.

• 1 A field study was made of foraging time allocation by a population of parasitic wasps, Diadegma spp. (Ichneumonidae), to plants containing different densities of their hosts, the caterpillars of Plutella xylostella (L.).
• 2 The parasitoid population exhibited a clear aggregative response, spending more total time on higher density patches, which probably resulted from wasps making more and longer visits to these densities.
• 3 Despite this aggregation, positive density dependent parasitism was not found. The functional response of the Diadegma population exhibited an upper asymptote at high host densities, probably due to an increase in the proportion of time spent handling hosts, which countered the effect of aggregation.
• 4 While Diadegma may select and forage preferentially on plants with higher host density, they do not exhibit the tendency, predicted by some optional foraging models, to exploit progressively less profitable plants during a foraging bout. Some factors affecting patterns of parasitoid foraging in the field are discussed.

     

Impact of eastern dwarf mistletoe (Arceuthobium pusillum) on host white spruce (Picea glauca) development,growth and performance across multiple scales

Infection by eastern dwarf mistletoe (Arceuthobium pusillum) modifies needle and branch morphology and hastens white spruce (Picea glauca) mortality. We examined potential causal mechanisms and assessed the impacts of infection‐induced alterations to host development and performance across scales ranging from needle hormone contents to bole expansion. Needles on infected branches (IBs) possessed higher total cytokinin (CK) and lower abscisic acid contents than needles on uninfected branches (UBs). IBs exhibited greater xylem growth than same‐aged UBs, which is consistent with the promotive effect of CKs on vascular differentiation and organ sink strength. Elevated CK content may also explain the dense secondary and tertiary branching observed at the site of infection, i.e. the formation of ‘witches’ brooms' with significantly lower light capture efficiencies. Observed hormone perturbations were consistent with higher rates of transpiration, lower water use efficiencies (WUEs) and more negative needle carbon isotope ratios observed for IBs. Observed reductions in needle size allowed IBs to compensate for reduced hydraulic conductivity. Severe infections resulted in dramatically decreased diameter growth of the bole. It seems likely that the modifications to host hormone contents by eastern dwarf mistletoe infection led white spruce trees to dedicate a disproportionate fraction of their photoassimilate and other resources to self‐shaded branches with low WUE. This would have decreased the potential for fixed carbon accumulation, generating a decline in the whole‐tree resource pool. As mistletoe infections grew in size and the number of IBs increased, this burden was manifested as increasingly greater reductions in bole growth.     

Identifying the abiotic and biotic drivers behind the elevational distribution shift of a parasitic plant

• Climate change will alter the biotic and abiotic environment and dissipate ecological barriers, reorganising maps of current distribution of parasites and their hosts. In this study, we analyse the population dynamics of the parasitic plant Viscum album subsp. austriacum and explore key biotic (host availability and seed dispersal) as well as abiotic (temperature) factors influencing elevational distribution.
• The study was conducted along an elevational gradient of a Mediterranean mountain, covering the distribution belts of three potential pine hosts: Pinus halepensis (1300–1500 m), P. nigra (1300–1900 m) and P. sylvestris var. nevadensis (1600–2000 m). Along this gradient, we measured multiple variables of mistletoe population (prevalence, abundance and demographic profile) and different factors that might define the current mistletoe distribution (host suitability and availability, temperature and seed dispersal services).
• We found a decline in mistletoe prevalence and abundance with increasing elevation, detecting larger values of both variables at lower elevations of the most suitable host (Pinus nigra). Pinus sylvestris var. nevadensis was a suboptimal but suitable host for the parasite at high elevations. Mistletoe found suitable temperatures and seed dispersal services all along the gradient, being able to recruit at any site.
• With warming temperatures, the presence of suitable vectors for parasite dispersion, and the presence of a sub‐optimal host (P. sylvestris var. nevadensis) at the mountain top, mistletoe currently has a window of opportunity to expand its present geographic distribution to the summits.

     

Mineral nutrition of the hyperparasitic mistletoe Viscum articulatum Burm. f. (Viscaceae) in tropical Brunei Darussalam

A plant parasite parasitizing another plant parasite is known as a hyperparasite. Information is scarce regarding the ecophysiology of hyperparasites and their hosts despite their potential to illuminate processes of host–parasite solute flux. Here we present mineral profiles and stable isotopic data for two associations of the hyperparasite Viscum articulatum and its primary mistletoe and tree hosts. Acting as the terminal sink, the hyperparasite had consistently higher contents of all major and minor elements evaluated compared to the primary parasite and the proximal portion of the tree host branch. The primary parasite had lower contents of Cu, Mg, Mn, N, and Z relative to the proximal portion of the tree host branch, suggesting nutritional stress applied by the hyperparasite. Interestingly Fe and Cu showed no consistent pattern between host and primary parasite, while the osmotically active elements P and K increased from tree host, to primary mistletoe, and finally the hyperparasitic mistletoe. The δ¹³C partitioning patterns for hyperparasites, primary parasites, and hosts were non‐linear in contrast to linear patterns reported from the literature for autoparasitic mistletoe associations, demonstrating fundamental differences between nutrition in hyperparasites and autoparasites.     

Are xylem-tapping mistletoes partially heterotrophic?

Summary Carbon isotope ratios, photosynthesis, and transpiration were measured on a xylem-tapping mistletoe (Phoradendron juniperinum) and its host (Juniperus osteosperma) in southern Utah, USA. For host tissues, the carbon isotope ratios agreed with theoretical values predicted from gas exchange observations. However, for mistletoe tissues, carbon isotope ratios deviated significantly from values predicted by gas exchange observations. This apparent discrepancy in mistletoe carbon isotope ratios can be resolved if one assumes that organic carbon dissolved in host xylem water was assimilated by the parasite. The mistletoes' high transpiration rates and low photosynthetic rates contributed to their heavy dependence on host xylem carbon. Two lines of evidence suggest that 62±2% of the carbon in the Utah mistletoe is derivated from the host and not from mistletoe autotrophic activities. Whereas xylem-tapping mistletoes have previously been characterized as wholly autotrophic parasites, we suggest that they may instead derive significant amounts of carbon from their hosts.     

The effect of multi‐species host density on superparasitism and sex ratio in a gregarious parasitoid

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Macroscale analysis of mistletoe host ranges in the Andean‐Patagonian forest

• The number of host species infected by a mistletoe (host range) is critical in that it influences prevalence, virulence and overall distribution of the parasite; however, macroecological analyses of this life history feature are lacking for many regions.
• The Andean‐Patagonian forest, found along the southern Andes from 35 °S to Tierra del Fuego at 55 °S, contains 12 mistletoe species in three families (Loranthaceae, Misodendraceae and Santalaceae). By tabulating herbarium records, the host ranges and geographical distributions of these mistletoes were explored.
• Our results show that these parasites occur on 43 plant species in 24 families but with varying degrees of specificity. All Misodendrum species and Desmaria mutabilis (Loranthaceae) are specialists that use Nothofagus as their primary hosts. Tristerix and Notanthera (Loranthaceae) and Antidaphne and Lepidoceras (Santalaceae) are generalists parasitizing more than six host species from several genera and families. Although many of the mistletoe species are sympatric, there is low overlap in host use.
• Our data show that in the southern South American bioregion, generalist mistletoes have smaller geographic ranges than specialists. This contrast with a previous hypothesis that predicted mistletoes with large geographic ranges would also have large host ranges, and conversely, less diverse regions would have more specialised mistletoes.

     

NUTRIENT AND WATER RELATIONS OF THE MISTLETOE PHORADENDRON LEUCARPUM (VISCACEAE): HOW TIGHTLY ARE THEY INTEGRATED?

Tissue mineral concentrations of calcium, copper, iron, magnesium, manganese, nitrogen, phosphorus, potassium, sodium, and zinc, and long-term water-use efficiency (as estimated by δ¹³C±) were measured in the mistletoe Phoradendron leucarpum and its hosts to investigate the relationship between nutrient concentrations and water economies of the parasite and hosts. The mistletoe had mineral concentrations 0.97 to 2.88 times greater than the hosts. Mean long-term mistletoe water-use efficiency (-27.89±) was comparable to mean long-term host water-use efficiency (-27.69±) and generally greater than the water-use efficiency reported for most mistletoe species. Differences between mistletoe and host water-use efficiency ranged from +0.65 to -0.75± and were more similar to mistletoes found growing on nitrogen-fixing host species as reported in other studies. Mistletoe nutrient concentrations, in particular nitrogen, were not related to changes in mistletoe water-use efficiencies. Nutrient: calcium ratios indicated that mistletoes acquire nutrients in excess of that which can be delivered by the host xylem. These data are discussed relative to the passive vs. active theories of nutrient uptake.     

Density-related foraging behaviour in Closterocerus tricinctus, a parasitoid of the leaf-mining moth, Cameraria hamadryadella

• 1 We examined the foraging behaviour of the parasitoid wasp, Closterocerus tricinctus (Ashmead) (Chalcidoidea: Eulophidae), as it visited larvae of the leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae), in an outbreak population.
• 2 We tracked females of C.tricinctus, recording the time spent searching for mines and handling host larvae. The density of leaf-mines (host larvae) and their condition were recorded for each leaf visited. A subset of leaves visited by C.tricinctus was enclosed in fine mesh bags so that foraging success could be determined by rearing or dissection. The average density of mines and the average leaf-area mined was estimated for a random sample of leaves from each tree.
• 3 The selection of leaves upon which to forage appears to be density-dependent. C.tricinctus visits leaves with leaf-mine densities twice the average, and when switching leaves lands directly on leaf-mines 5 times more often than expected assuming random landings.
• 4 The total time spent foraging on a leaf, the average time spent handling hosts, and the total search time within leaves tend to decline on leaves with many hosts, but the observed declines are not statistically significant.
• 5 The proportion of leaf-mines visited within a leaf is strongly inversely density-dependent. 30% of visits to leaf-mines are re-visits and 29% of handling time is spent re-handling previously visited hosts. Furthermore, only 21% of visits to mines lead to successful parasitism. We suggest that self-interference and the avoidance behaviour of the host may reduce the number of visits of leaf-mines by C.tricinctus within a leaf.
• 6 The effect of the strongly inversely density-dependent foraging investment within leaves is to offset the observed density-dependent pattern of leaf visitation making the overall spatial pattern of visitation by C.tricinctus to mines of C.hamadryadella inversely density-dependent.
• 7 We suggest that the uncertainty of C.tricinctus surviving on multiply mined leaves because of density-dependent host mortality due to intraspecific competition in high-density host populations, the rarity of high-density host populations, and the rarity of multiply-mined leaves in low-density host populations combine to select against an aggregative response within leaves by C.tricinctus.

     

Spread of an introduced parasite across the Hawaiian archipelago independent of its introduced host

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Indirect positive effects of a parasitic plant on host pollination and seed dispersal

Parasitic plants often have a strong fitness‐impact on their plant hosts through increased host mortality and reduced or complete suppression of reproduction. Tristerix corymbosus (Loranthaceae) is a hemiparasitic mistletoe that infects a wide range of host species along its distribution range. Among such species, Rhaphithamnus spinosus (Verbenaceae) is a frequent host with a flowering and fruiting season partially synchronized with mistletoe reproductive phenology. As parasitized hosts have, in principle, a larger flower display and fruit crop size than non‐parasitized hosts, we examined whether host and parasite reproductive synchrony make infected hosts more attractive for pollinators and seed dispersers than uninfected hosts. Our results showed that pollinator visit rates did not differ between parasitized and non‐parasitized hosts. Conversely, seed rain was higher in parasitized than non‐parasitized individuals. The number of seeds fallen under non‐parasitized plants was spatially associated with crop size, while parasitized plants did not show such association. Finally, the number of seedlings of R. spinosus was significantly larger near parasitized than non‐parasitized hosts. Our results suggest that the presence of the mistletoe might be responsible of the higher reproductive success showed by the parasitized fraction of R. spinosus. This effect, however, seems to be related to seed dispersal processes rather than pollination effects.     

Mesophyll cell‐sucking herbivores (Cicadellidae: Typhlocybinae) on rainforest trees in Papua New Guinea: local and regional diversity of a taxonomically unexplored guild

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Thermal dissipation probe measurements of sap flow in the xylem of trees documenting dynamic relations to variable transpiration given by instantaneous weather changes and the activities of a mistletoe xylem parasite

The thermal dissipation probe was described in the early 1930s for the demonstration of a volume and mass flow of sap in the conductive elements of the xylem in trees. It was subsequently developed further and is now widely used in physiological ecology including measurements in the field. Thermal dissipation demonstrates the occurrence of sap flow and allows determination of its velocity. Here we report simultaneous continuous measurements of sap flow using the thermal dissipation technique and of transpiration by infrared gas analysis for diurnal and annual cycles in a deciduous and an evergreen oak tree, Quercus robur L. and Quercus turneri Willd., respectively, in a deciduous and an evergreen conifer, Larix decidua Mill. and Pinus griffithii McClell., respectively, and the host/mistletoe consortium of the deciduous linden Tilia mandschurica Rupr. & Max. and the evergreen Viscum album L. We show (1) that in diurnal cycles sap flow closely follows dynamic changes of the rate of transpiration elicited by daily fluctuations of weather parameters (sunshine, cloudiness, air temperature and humidity), (2) that in annual cycles sap flow reflects autumnal yellowing and shedding of leaves of the deciduous trees. We report for the first time comparative measurements of sap flow towards mistletoe shoots and host branches in a parasite/host consortium. This demonstrates (3) that mistletoes maintain considerably larger sap flow rates in their xylem conduits than the adjacent host branches dragging the transpiration stream of their host towards their own shoots. We also show (4) that even after the deciduous host has shed its leaves and itself does not transpire any more the evergreen mistletoe towards its shoots can maintain the persistence of a continuous sap flow via the stem and branches of the host as long as frost does not prevent that. The work presented underlines the contention that transpiration is the driving force for sap flow with continuous files of water in the xylem. It shows for the first time that mistletoes direct the flow of water through host roots and stems towards its own shoots by not only performing stronger transpiration as it is known from the literature but also by maintaining larger sap flow rates in the xylem conduits of its stems.     

Integrated nitrogen,carbon, and water relations of a xylem-tapping mistletoe following nitrogen fertilization of the host

Xylem-tapping mistletoes transpire large volumes of water (E) while conducting photosynthesis (A) at low rates, thus maintaining low instantaneous wateruse efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitrogen dissolved at low concentration in host xylem water; however, low A/E also results in substantial heterotrophic carbon gain. In this study, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to determine whether mistletoe A/E would approach that of the host if mistletoes were supplied with abundant nitrogen. Fertilization significantly increased foliar N concentrations (N), net assimilation rates, and A/E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the other hand, when instantaneous water-use efficiency and A/N were calculated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budgets was constructed to analyze the relative benefits of nitrogen- and carbonparasitism. The model assumes constant E and includes feedbacks of tissue nitrogen concentration on photosyn-thesis. These results, combined with our earlier observation that net assimilation rates of mistletoes and their hosts are approximately matched (Marshall et al. 1994), support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain. However, the low ratio of A/E is interpreted not as a means of acquiring nitrogen, but as an inevitable consequence of an imbalance in C and N assimilation.This research was supported by the National Science Foundation (grants BSR-8706772 and 8847942).     

Asymmetrical Dependence Between a Neotropical Mistletoe and its Avian Seed Disperser

The degree of interdependence among interacting species has major implications for our understanding of the coevolutionary process and biodiversity maintenance. However, the mutualism strength among fruiting plants and their seed dispersers remains poorly understood in tropical ecosystems. We evaluated simultaneously the effectiveness of the avian seed dispersers of the mistletoe Struthanthus flexicaulis (Loranthaceae) and the contribution of its fruits to their diets in a highland rocky savanna in southeastern Brazil. The mistletoe fruits are small lipid‐rich pseudoberries available throughout the year. Four passerine birds fed on fruits, but Elaenia cristata (Tyrannidae) was the most effective disperser, responsible for more than 96 percent of the dispersed seeds. This bird swallowed fruits whole, expelling and depositing undamaged seeds by regurgitation and bill wiping on perches. From 646 dispersed seeds, 56 percent were deposited on safe sites, thin live twigs of 38 susceptible host species. Elaenia cristata were predominantly frugivorous, feeding on typically ornithocoric fruits of at least 12 species, but also on arthropods. Although fruits represented 75 percent of the feeding bouts along the year, S. flexicaulis fruits represented only 34 percent of the E. cristata diet. Our results highlight the asymmetrical nature of this mutualistic interaction, with the mistletoe life cycle locally linked to one highly effective seed disperser that is more weakly dependent on mistletoes fruits as a food source. We suggest that merging the seed dispersal effectiveness framework with diet assessment of seed dispersers is needed to clarify the asymmetries in mutualistic pairwise interactions involving plants and their animal partners.     

The restricted seed rain of a mistletoe specialist

Specialist frugivores are the dominant consumers of mistletoe fruit in many regions and have been shown to intensify infections of host plants as a result of their rapid gut passage rates and dependence on existing infections. The role of specialist frugivores in long distance dispersal of mistletoe and establishment of new infections is unclear, and has not been explicitly evaluated previously. Here we critically examine the premise that specialists are the dominant dispersers by examining the role of an Australian mistletoe specialist (mistletoebird Dicaeum hirundinaceum Dicaeidae) in dispersing mistletoe (Amyema preissii Santalales: Loranthaceae) seeds beyond infected host stands. We use two primary lines of evidence – presence of birds using remote call recorders, and presence of dispersed seeds via surveys for defecated seeds on host branches. The observed and inferred movements of the mistletoebird were wholly restricted to habitat patches containing mistletoe, and this bird was not observed to transport seeds to nearby uninfected host stands within the study system. While mistletoe specialists may provide much of the within‐stand dispersal service for mistletoes, this serves only to aggregate and intensify existing infections. We suggest that long distance dispersal of mistletoe seeds beyond existing hosts and infection centres is not performed by these dietary specialists, these services more likely to be provided by generalist frugivores and other occasional mistletoe fruit consumers.