Calcicole behaviour of Callisthene fasciculata Mart., an Al‐accumulating species from the Brazilian Cerrado

• Most aluminium (Al)‐accumulating species are found on soils with high Al saturation and low Ca availability (Ca poor). Callisthene fasciculata Mart. (Vochysiaceae), however, is an Al‐accumulating tree restricted to Ca‐rich soils with low Al saturation in the Brazilian Cerrado savanna. Here we tested its calcicole behaviour, and the possible role of organic acids in detoxification of Al during the early stages of plant development.
• We assessed growth, dry mass, nutrients, Al and organic acids in seedlings grown for 50 days on two contrasting Cerrado soils; one with high Ca concentrations and low Al saturation and the other with low Ca availability and high Al saturation.
• Relative to plants on Ca‐rich soil, plants on Ca‐poor soil had necrotic spots and bronzing of leaves. Roots and shoots contained reduced concentrations of P and Cu, but higher concentrations of Fe, Al and citrate. Despite lower concentrations in the soil, Ca and Mg increased in shoots. Shoot concentrations of oxalate were also higher.
• We confirmed C. fasciculata as an Al‐accumulating species with calcicole behaviour. The increased concentrations of organic acids in plants with higher Al accumulation suggest that high availability of soluble Al does not prevent occurrence of this species on soils with high Al saturation. Instead, the absence of C. fasciculata from Ca‐poor soils is probably due to imbalances in tissue Fe, Cu and Zn imposed by this soil type.

     

Silicon reduces herbivore performance via different mechanisms,depending on host–plant species

There is mounting evidence silicon (Si) can alter plant nutrient dynamics and is an important functional trait in plant defence and plant–insect ecology. Despite this, there remains a paucity in our understanding of how Si‐driven changes in nutritional quality can impact herbivore performance across different plant species. We investigated how Si alters plant nutritional quality and the concomitant effects on the performance of the Australian native generalist herbivore Helicoverpa punctigera feeding on three economically significant plant species of varying Si‐uptake ability: Brassica napus (non‐Si accumulator), Cucumis sativus (intermediate Si accumulator) and Sorghum bicolor (high Si accumulator). Si supplementation reduced the nutritional quality of B. napus but increased phosphorus concentrations in S. bicolor. Si reduced herbivore performance in all host–plant species, which correlated directly with Si concentrations in Si‐accumulating host plants C. sativus and S. bicolor. However, on B. napus, Si affected herbivore performance indirectly by reducing nutritional quality (foliar carbon:nitrogen ratio and phosphorus concentration). This suggests Si availability can affect herbivore performance directly via Si concentration on Si‐accumulating hosts, and indirectly via nutritional quality in a non‐Si accumulator. The resistance‐enhancing effects of Si on multiple species offer opportunity for agriculture to utilise this abundant element in sustainable management practices.     

Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.     

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.     

Olfactory cues used in host‐habitat location and host location by the parasitoid Cotesia urabae

There is a growing body of evidence that many hymenopteran parasitoids make use of olfaction as the primary mechanism to detect and locate hosts. In this study, a series of bioassays was conducted to investigate the orientation behaviour of the gum leaf skeletonizer larval parasitoid Cotesia urabae Austin & Allen (Hymenoptera: Braconidae) in both Y‐tube and four‐arm olfactometers. In a Y‐tube olfactometer, male C. urabae were attracted only to virgin conspecific females. Host‐plant leaves, damaged leaves, host larvae, and host larvae feeding on leaves were highly attractive to female C. urabae, whereas host frass and conspecific males were not. The multiple‐comparison bioassay conducted in a four‐arm olfactometer clearly indicates that C. urabae females were significantly more attracted to the host Uraba lugens Walker (Lepidoptera: Nolidae) larvae feeding on Eucalyptus fastigata H Deane & Maiden (Myrtaceae) leaves than to any other of the odour sources tested. The results of this study show that C. urabae individuals responded to chemical cues specific to the host plant and target host insect, and support hypotheses that unreliable cues are not utilized for host location by specific natural enemies.     

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.

     

Population structure of a vector‐borne plant parasite

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector‐borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host‐associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within‐host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.     

Diurnal courses of leaf conductance and transpiration of mistletoes and their hosts in Central Australia

Summary In Australia, diurnal courses of leaf conductance and transpiration of hemiparasitic mistletoes (Loranthaceae) and their hosts were measured using steady-state porometers under conditions of partial drought and high evaporative demand. The sites spanned a diversity of climatic regions ranging from the subtropical arid zone with winter rainfall, through the subtropical arid zone with summer rainfall to the tropical summer rainfall zone. With one exception (Acacia farnesiana with deciduous leaves), the hosts were trees or shrubs with evergreen, sclerophyllous leaves or phyllodes.The measurements confirm previous observations that mistletoes transpire at higher rates than their hosts. For adult leaves from all of the 18 different host/mistletoe pairs investigated, the daily average leaf conductances were higher in the parasites than in their hosts. The ratios ranged from 1.5 to 7.9. In the most extreme case,Amyema maidenii had a daily rate of water loss 8.9 times higher than its hostAcacia cowleana. Hoever, the parasites did not exhibit unlimited transpiration. Despite high water loss rates, leaf conductance showed large and consistent changes during the course of the day, indicating definite stomatal regulation. The typical diurnal pattern of conductance in both mistletoes and hosts consisted of an early morning peak followed by a continuous decrease throughout the remainder of the day. There was no abrupt decrease in leaf conductance of the parasites that might be interpreted as a threshold response with respect to internal water potential. In most cases, the continuous stomatal closure occurred without substantial changes in leaf water potential over a time span of several hours. The decrease in leaf conductance was correlated with an increase in leaf-to-air water vapor difference, which was associated with increasing leaf temperatures. It seems probable that external humidity plays a major role in the stomatal response. Diurnal courses of leaf conductance of the host/parasite pairs usually showed similar general patterns, even when the absolute rates were quite different. Thus, mistletoes not only control their water loss by stomatal action but this regulation seems to occur in coordination with the stomatal response of their hosts.The integrated mistletoe/host system must also endure severe drought conditions. Controlled water use is necessary for long-term survival of the host. Assuming stomatal behavior in the host is well adapted to ensure its existence, then similar performance in the mistletoe would promote survival of both host and parasite.     

Mistletoes Play Different Roles in a Modular Host–Parasite Network

Antagonistic interactions between host plants and mistletoes often form complex networks of interacting species. Adequate characterization of network organization requires a combination of qualitative and quantitative data. Therefore, we assessed the distribution of interactions between mistletoes and hosts in the Brazilian Pantanal and characterized the network structure in relation to nestedness and modularity. Interactions were highly asymmetric, with mistletoes presenting low host specificity (i.e., weak dependence) and with hosts being highly susceptible to mistletoe‐specific infections. We found a non‐nested and modular pattern of interactions, wherein each mistletoe species interacted with a particular set of host species. Psittacanthus spp. infected more species and individuals and also caused a high number of infections per individual, whereas the other mistletoes showed a more specialized pattern of infection. For this reason, Psittacanthus spp. were regarded as module hubs while the other mistletoe species showed a peripheral role. We hypothesize that this pattern is primarily the result of different seed dispersal systems. Although all mistletoe species in our study are bird dispersed, the frugivorous assemblage of Psittacanthus spp. is composed of a larger suite of birds, whereas Phoradendron are mainly dispersed by Euphonia species. The larger assemblage of bird species dispersing Psittacanthus seeds may also increase the number of hosts colonized and, consequently, its dominance in the study area. Nevertheless, other restrictions on the interactions among species, such as the differential capacity of mistletoe infections, defense strategies of hosts and habitat types, can also generate or enhance the observed pattern. Abstract in Portuguese is available at http://www.blackwell‐synergy.com/loi/btp .     

Photosynthesis of mistletoes in relation to their hosts at various sites in tropical Brazil

 Chlorophyll a fluorescence parameters showing the instantaneous performance and carbon-isotope ratios reflecting long-term behaviour of leaves were determined for a large number of mistletoe/host-pairs in the cerrado belt of Brazil. Study sites were a very exposed rupestrian field, a semi-exposed savanna and a highly shaded gallery forest. The major question asked was if photosynthetic capacity of mistletoe leaves differed from that of the leaves of their respective hosts. It is shown that except for the very exposed rupestrian field site, photosynthetic capacity appeared to be similar in mistletoes and host leaves. The superior behaviour of host leaves in the rupestrian field was due to particularly expressed sun-plant characteristics of the host. However, mistletoes always had higher average stomatal conductances, lower leaf temperatures at similar or even higher irradiance and higher intercellular CO₂-partial pressures than hosts. Photosynthetic performance of mistletoe leaves was independent of whether a given mistletoe species parasitized aluminium-accumulating or non-accumulating host species in the cerrados with their aluminium-rich soils. Received: 7 April 1997 / Accepted: 20 August 1997     

To recycle or steal? Nutrient resorption in Australian and Brazilian mistletoes from three low‐phosphorus sites

Resorption is the process by which nutrients are withdrawn from leaves prior to leaf fall. Mistletoes are generally thought not to rely on nutrient resorption; being xylem‐tapping parasites, they instead derive the nutrients required for new growth from their host plant, at little or no cost. We measured nutrient (N, P, K, Ca, Mg) resorption in 18 parasitic mistletoe–host species pairs distributed across three sites with notably low‐P soil, also quantifying relationships with leaf lifespan (LL) and specific leaf area (SLA). There was little or no evidence of N, Ca or Mg resorption. By contrast, on average ～30% of P and ～20% of K were resorbed prior to leaf fall. Longer LL in mistletoes was associated with lower N and P concentrations in mistletoes and in host leaves. We provide evidence that, even though mistletoes are relatively inefficient in terms of nutrient resorption compared to non‐parasite species, on low‐P soils their ecological and evolutionary strategies for conserving phosphorous involve modulation of both leaf lifespan and P concentration in senesced leaves.     

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.     

The contribution of mistletoes to nutrient returns: Evidence for a critical role in nutrient cycling

Both nutrient cycling and nutrient relationships between mistletoe and host have been widely studied; yet it is unclear whether high nutrient concentrations commonly found in mistletoes affect rates of nutrient cycling. To address this question, we assessed 13 elements in the leaf litter of a temperate eucalypt forest in southern New South Wales, comparing concentrations from trees (Eucalyptus blakelyi, E. dwyeri, and E. dealbata) with and without the hemiparasitic mistletoe Amyema miquelii. Results were in accord with previous research on fresh leaves showing that concentrations of many elements were higher in the mistletoe than the host. This was not the case for all elements; most notably for N, where concentrations were significantly lower in the mistletoe. However, the return of all elements increased with mistletoe infection because of the combined effect of enrichment in mistletoe tissues and high rates of mistletoe litterfall. Annual returns of N and P in leaf litter increased by a factor of 1.65 and 3 respectively, with the greatest increase being for K by a factor of 43 in spring. These increased element returns were not significantly influenced by any changes in host leaf litter quality, as mistletoe infection was not found to affect host element concentrations. Mistletoe infection also altered the spatial and temporal distribution of element returns because of the patchy occurrence of mistletoes and extended period of mistletoe litterfall compared with the host. These findings provide a mechanistic explanation for the role of mistletoes as a keystone resource and, together with comparable results from root‐parasitic plants in boreal tundra and cool‐temperate grasslands, suggest that enhancing nutrient return rates may be a generalized property of parasitic plants.     

Is the pollen‐limited mistletoe Peraxilla tetrapetala (Loranthaceae) also seed limited?

Abstract Seed production in the endemic New Zealand mistletoe Peraxilla tetrapetala (Loranthaceae) has been shown to be consistently pollen limited, but to date there has been no test of whether the species is also seed limited. If it is not seed limited, then pollen limitation may have no effect on population size. We tested for seed limitation by sowing seeds onto host trees at high and low densities at two sites, and following survival for up to 69 months. Our rationale was that unless there is strong density‐dependent seedling mortality sufficient to negate increases in seed supply, the species would be seed limited. We simultaneously measured pollen limitation at both sites using pollen augmentation. Peraxilla tetrapetala was strongly pollen limited at both sites over 7 years at Ohau (mean Pollen Limitation Index 0.62), and 12 years at Craigieburn (mean PLI 0.44), before and during the seed‐limitation study. There was no significant overall negative effect of density on survival in sown P. tetrapetala seeds over 42–69 months. There was a significant positive effect: seeds were more likely to adhere to branches when sown at high densities. This initial advantage to high‐density seeds was gradually eroded by slightly (but non‐significantly) lower survival rates of adhered seeds at high density. By the end of the study there was no significant difference in overall survival in high and low density plantings. This means that P. tetrapetala was apparently both pollen limited and seed limited at both our sites. Hence, reduced densities of native bird pollinators caused by introduced mammalian carnivores are likely to reduce the density of adult mistletoes in the next generation. However, the generality of this result may be affected by the fact that mistletoes do not have a dense ‘seedling shadow’ under the parent mistletoe.     

Plant growth-promoting effect and genomic analysis of the beneficial endophyte <Emphasis Type="Italic">Streptomyces</Emphasis> sp. KLBMP 5084 isolated from halophyte <Emphasis Type="Italic">Limonium sinense</Emphasis>

Background and aims

Xylem-tapping mistletoes may experience relaxed selective pressure to use water efficiently during photosynthesis because of lower per-unit costs for water acquisition than experienced by host plants. As a result, we hypothesised that mistletoes would exhibit parallel but dampened leaf-level adaptations and responses to aridity, compared to those seen in hosts.

Methods

Photosynthetic traits, leaf dark respiration, nutrient concentrations and specific leaf area (SLA) were measured on 42 mistletoe-host species-pairs sampled from five sites in Australia and Brazil that vary widely in aridity.

Results

Mistletoes exhibited similar trait-shifts to hosts in relation to site aridity. In both groups, arid-site species showed stronger control over stomatal water loss, larger drawdown of CO₂ during photosynthesis (lower c_i: c_a), higher leaf N and P concentrations per unit leaf area, and lower SLA. Nevertheless, mistletoes were profligate water users compared to their hosts and showed substantially less efficient use of water during photosynthesis. On average, mistletoes showed twice higher leaf dark respiration rates at a given photosynthetic capacity, suggesting relatively higher leaf maintenance costs for these parasitic plants.

Conclusions

Despite fundamental differences in lifestyle and in photosynthetic traits, mistletoes exhibit trait responses and adaptations to site aridity in parallel and to approximately the same extent as their hosts.

     

No effect of host–parasite co‐evolution on host range expansion

Antagonistic co‐evolution between hosts and parasites (reciprocal selection for resistance and infectivity) is hypothesized to play an important role in host range expansion by selecting for novel infectivity alleles, but tests are lacking. Here, we determine whether experimental co‐evolution between a bacterium (Pseudomonas fluorescens SBW25) and a phage (SBW25Φ2) affects interstrain host range: the ability to infect different strains of P. fluorescens other than SBW25. We identified and tested a genetically and phenotypically diverse suite of co‐evolved phage variants of SBW25Φ2 against both sympatric and allopatric co‐evolving hosts (P. fluorescens SBW25) and a large set of other P. fluorescens strains. Although all co‐evolved phage had a greater host range than the ancestral phage and could differentially infect co‐evolved variants of P. fluorescens SBW25, none could infect any of the alternative P. fluorescens strains. Thus, parasite generalism at one genetic scale does not appear to affect generalism at other scales, suggesting fundamental genetic constraints on parasite adaptation for this virus.     

Silicon‐mediated resistance against specialist insects in sap‐sucking and leaf‐chewing guilds in the Si non‐accumulator collard

Soil amendment with Silicon (Si) can increase plant resistance against insect herbivores, but the underlying mechanisms remain unclear. The mechanical resistance hypothesis (MRH) states that Si accumulated in epidermal cells directly and passively protects against herbivores by creating a mechanical barrier. The physiological resistance hypothesis (PRH) states that Si enhances resistance by activating plant biochemical and physiological processes. We tested both hypotheses by manipulating Si fertilization of the Si non‐accumulator collard, Brassica oleracea L. cv. acephala (Brassicaceae). Then, we assessed functional and ultrastructural plant responses and the developmental and reproductive performance of the leaf‐chewing larvae of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), and the sap‐sucking cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae). There was a 20% increase in leaf Si content. Silicon deposition in epidermal cells was identified by confocal microscopy and directly coincided with lower performance of P. xylostella, but did not affect B. brassicae. On the other hand, we found no unequivocal evidence that Si‐mediated changes in primary and secondary metabolism improved plant resistance against the insects. Negative mechanical effects of Si on the insects may have masked beneficial effects of increased water, nitrogen, and mineral contents in Si‐treated collards. Silicon did not change leaf contents of hemicellulose, cellulose, and lignin. Although Si‐mediated increases in leaf glucosinolates (GLS) correlated with lower larval performance and higher oviposition preference of P. xylostella, both P. xylostella and B. brassicae are highly specialized in overcoming such secondary metabolites. Thus, mechanical resistance may have impaired P. xylostella, rather than the Si‐mediated increase in GLS. We suggest that the PRH may depend on the degree of insect feeding specialization, so that toxic Si‐mediated defenses may be more efficient against unadapted polyphagous herbivores. For them, a toxic barrier may be added to the mechanical resistance.     

Detection of Puccinia pelargonii‐zonalis‐infected Geranium Tissues and Urediniospores

The occurrence of geranium rust (caused by Puccinia pelargonii‐zonalis) in commercial greenhouses can result in unmarketable plants and significant economic losses. Currently, detection of geranium rust relies solely on scouting for symptoms and signs of the disease. The purpose of this research was to develop a rapid detection assay for P. pelargonii‐zonalis‐infected tissues or urediniospores on greenhouse‐grown geraniums. Two oligonucleotide primers were designed based on internal transcribed spacer sequence data from three isolates of P. pelargonii‐zonalis. The primers amplified a 131‐bp product from genomic DNA from each isolate of P. pelargonii‐zonalis but did not amplify a product from genomic DNA from twelve other rust fungi or four other plant pathogenic fungi. A PCR product was amplified consistently from solutions that contained 1 ng or 100 pg/ml of purified P. pelargonii‐zonalis DNA in conventional PCR and at 1 pg/ml using real‐time PCR. The detection threshold was 10² urediniospores/ml for real‐time PCR and 10⁴ urediniospores/ml for conventional PCR using urediniospores collected by vacuum from sporulating lesions. Puccinia pelargonii‐zonalis DNA was amplified by real‐time PCR from urediniospores washed from a single inoculated leaf, but recovered urediniospores were below detection threshold from one inoculated leaf with 5, 10, 25 and 50 non‐inoculated leaves. Conventional and real‐time PCR did not detect P. pelargonii‐zonalis in infected leaf tissues, presumably due to PCR inhibitors in the geranium leaf tissue. The inhibition of both conventional and real‐time PCR by geranium tissues suggests that a detection assay focusing on urediniospore recovery and microscopic examination with subsequent species verification by PCR may be the most efficient method for assessing the presence of geranium rust in greenhouses.     

Culturable bacteria from Zn‐ and Cd‐accumulating Salix caprea with differential effects on plant growth and heavy metal availability

Aims: To characterize bacteria associated with Zn/Cd‐accumulating Salix caprea regarding their potential to support heavy metal phytoextraction. Methods and Results: Three different media allowed the isolation of 44 rhizosphere strains and 44 endophytes, resistant to Zn/Cd and mostly affiliated with Proteobacteria, Actinobacteria and Bacteroidetes/Chlorobi. 1‐Aminocyclopropane‐1‐carboxylic acid deaminase (ACCD), indole acetic acid and siderophore production were detected in 41, 23 and 50% of the rhizosphere isolates and in 9, 55 and 2% of the endophytes, respectively. Fifteen rhizosphere bacteria and five endophytes were further tested for the production of metal‐mobilizing metabolites by extracting contaminated soil with filtrates from liquid cultures. Four Actinobacteria mobilized Zn and/or Cd. The other strains immobilized Cd or both metals. An ACCD‐ and siderophore‐producing, Zn/Cd‐immobilizing rhizosphere isolate (Burkholderia sp.) and a Zn/Cd‐mobilizing Actinobacterium endophyte were inoculated onto S. caprea. The rhizosphere isolate reduced metal uptake in roots, whereas the endophyte enhanced metal accumulation in leaves. Plant growth was not promoted. Conclusions: Metal mobilization experiments predicted bacterial effects on S. caprea more reliably than standard tests for plant growth‐promoting activities. Significance and Impact of the Study: Bacteria, particularly Actinobacteria, associated with heavy metal‐accumulating Salix have the potential to increase metal uptake, which can be predicted by mobilization experiments and may be applicable in phytoremediation.     

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.