Partial mycoheterotrophy is common among chlorophyllous plants with Paris-type arbuscular mycorrhiza

Background and AimsAn arbuscular mycorrhiza is a mutualistic symbiosis with plants as carbon providers for fungi. However, achlorophyllous arbuscular mycorrhizal species are known to obtain carbon from fungi, i.e. they are mycoheterotrophic. These species all have the Paris type of arbuscular mycorrhiza. Recently, two chlorophyllous Paris-type species proved to be partially mycoheterotrophic. In this study, we explore the frequency of this condition and its association with Paris-type arbuscular mycorrhiza.MethodsWe searched for evidence of mycoheterotrophy in all currently published ¹³C, ²H and ¹⁵N stable isotope abundance patterns suited for calculations of enrichment factors, i.e. isotopic differences between neighbouring Paris- and Arum-type species. We found suitable data for 135 plant species classified into the two arbuscular mycorrhizal morphotypes.Key ResultsAbout half of the chlorophyllous Paris-type species tested were significantly enriched in ¹³C and often also enriched in ²H and ¹⁵N, compared with co-occurring Arum-type species. Based on a two-source linear mixing model, the carbon gain from the fungal source ranged between 7 and 93 % with ferns > horsetails > seed plants. The seed plants represented 13 families, many without a previous record of mycoheterotrophy. The ¹³C-enriched chlorophyllous Paris-type species were exclusively herbaceous perennials, with a majority of them thriving on shady forest ground.ConclusionsSignificant carbon acquisition from fungi appears quite common and widespread among Paris-type species, this arbuscular mycorrhizal morphotype probably being a pre-condition for developing varying degrees of mycoheterotrophy.     

Ant species identity mediates reproductive traits and allocation in an ant-garden bromeliad

Background and Aims

Determining the sources of variation in floral morphology is crucial to understanding the mechanisms underlying Angiosperm evolution. The selection of floral and reproductive traits is influenced by the plant''s abiotic environment, florivores and pollinators. However, evidence that variations in floral traits result from mutualistic interactions with insects other than pollinators is lacking in the published literature and has rarely been investigated. We aimed to determine whether the association with either Camponotus femoratus or Pachycondyla goeldii (both involved in seed dispersal and plant protection) mediates the reproductive traits and allocation of Aechmea mertensii, an obligatory ant-garden tank-bromeliad, differently.

Methods

Floral and reproductive traits were compared between the two A. mertensii ant-gardens. The nitrogen flux from the ants to the bromeliads was investigated through experimental enrichments with stable isotopes (¹⁵N).

Key Results

Camponotus femoratus-associated bromeliads produced inflorescences up to four times longer than did P. goeldii-associated bromeliads. Also, the numbers of flowers and fruits were close to four times higher, and the number of seeds and their mass per fruit were close to 1·5 times higher in C. femoratus than in P. goeldii-associated bromeliads. Furthermore, the ¹⁵N-enrichment experiment showed that C. femoratus-associated bromeliads received more nitrogen from ants than did P. goeldii-associated bromeliads, with subsequent positive repercussions on floral development. Greater benefits were conferred to A. mertensii by the association with C. femoratus compared with P. goeldii ants.

Conclusions

We show for the first time that mutualistic associations with ants can result in an enhanced reproductive allocation for the bromeliad A. mertensii. Nevertheless, the strength and direction of the selection of floral and fruit traits change based on the ant species and were not related to light exposure. The different activities and ecological preferences of the ants may play a contrasting role in shaping plant evolution and speciation.     

Herbivory-induced mortality increases with radial growth in an invasive riparian phreatophyte

Background and Aims

Under equal conditions, plants that allocate a larger proportion of resources to growth must do so at the expense of investing fewer resources to storage. The critical balance between growth and storage leads to the hypothesis that in high-resource environments, plants that express high growth rates are more susceptible to episodic disturbance than plants that express lower growth rates.

Methods

This hypothesis was tested by measuring the radial growth, basal area increment (BAI) and carbon isotope ratios (δ¹³C) in tree-ring α-cellulose of 62 mature tamarisk trees (Tamarix spp.) occurring at three sites in the western USA (n = 31 live and 31 killed trees across all sites, respectively). All of the trees had been subjected to periods of complete foliage loss by episodic herbivory over three or more consecutive growing seasons by the tamarisk leaf beetle (Diorhabda carinulata), resulting in approx. 50 % mortality at each site.

Key Results

Mean annual BAI (measured from annual ring widths) in the 10 years prior to the onset of herbivory was on average 45 % higher in killed trees compared with live trees (P < 0·0001). Killed trees that had higher growth rates also expressed higher (less negative) δ¹³C ratios compared with live trees. In fact, at one site near Moab, UT, the mean annual BAI was 100 % higher in killed trees despite having about a 0·5 ‰ higher δ¹³C relative to live trees (P = 0·0008). Patterns of δ¹³C suggest that the intrinsic water-use efficiency was higher in killed than surviving trees, possibly as a consequence of lower whole-canopy stomatal conductance relative to live trees.

Conclusions

The results show that a likely trade-off occurs between radial growth and survival from foliage herbivory in Tamarix spp. that currently dominates riparian areas throughout the western USA and northern Mexico. Thus, herbivory by D. carinulata may reduce the overall net primary productivity of surviving Tamarix trees and may result in a reduction in genetic variability in this dominant invasive tree species if these allocation patterns are adaptive.     

How yield relates to ash content, ��13C and ��18O in maize grown under different water regimes

Background and Aims

Stable isotopes have proved a valuable phenotyping tool when breeding for yield potential and drought adaptation; however, the cost and technical skills involved in isotope analysis limit its large-scale application in breeding programmes. This is particularly so for Δ¹⁸O despite the potential relevance of this trait in C₄ crops. The accumulation of minerals (measured as ash content) has been proposed as an inexpensive way to evaluate drought adaptation and yield in C₃ cereals, but little is known of the usefulness of this measure in C₄ cereals such as maize (Zea mays). The present study investigates how yield relates to ash content, Δ¹³C and Δ¹⁸O, and evaluates the use of ash content as an alternative or complementary criterion to stable isotopes in assessing yield potential and drought resistance in maize.

Methods

A set of tropical maize hybrids developed by CIMMYT were subjected to different water availabilities, in order to induce water stress during the reproductive stages under field conditions. Ash content and Δ¹³C were determined in leaves and kernels. In addition, Δ¹⁸O was measured in kernels.

Key Results

Water regime significantly affected yield, ash content and stable isotopes. The results revealed a close relationship between ash content in leaves and the traits informing about plant water status. Ash content in kernels appeared to reflect differences in sink–source balance. Genotypic variation in grain yield was mainly explained by the combination of ash content and Δ¹⁸O, whilst Δ¹³C did not explain a significant percentage of such variation.

Conclusions

Ash content in leaves and kernels proved a useful alternative or complementary criterion to Δ¹⁸O in kernels for assessing yield performance in maize grown under drought conditions.     

Shifts in mycorrhizal fungi during the evolution of autotrophy to mycoheterotrophy in Cymbidium (Orchidaceae)

? Premise of the study: Mycoheterotrophic plants, which completely depend upon mycorrhizal fungi for their nutrient supply, have unusual associations with fungal partners. The processes involved in shifts in fungal associations during cladogenesis of plant partners from autotrophy to mycoheterotrophy have not been demonstrated using a robust phylogenetic framework. ? Methods: Consequences of a mycorrhizal shift were examined in Cymbidium (Orchidaceae) using achlorophyllous and sister chlorophyllous species. Fungal associates of the two achlorophyllous mycoheterotrophs (C. macrorhizon and C. aberrans), their close relatives, the chlorophyllous mixotrophs (C. goeringii and C. lancifolium) and an outgroup, the chlorophyllous autotroph C. dayanum, were identified by internal transcribed spacers of the nuclear ribosomal DNA sequences. ? Key results: Molecular identification of mycorrhizal fungi revealed: (1) the outgroup autotroph is predominantly dependent on saprobic Tulasnellaceae, (2) the mixotrophs associate with the Tulasnellaceae and ectomycorrhizal groups including the Sebacinales, Russulaceae, Thelephoraceae and Clavulinaceae, and (3) the two mycoheterotrophs are mostly specialized with ectomycorrhizal Sebacinales. ? Conclusion: Fungal partners in Cymbidium have shifted from saprobic to ectomycorrhizal fungi via a phase of coexistence of both nutritional types of fungi. These three phases correspond to the evolution from autotrophy to mycoheterotrophy via mixotrophy in Cymbidium. We demonstrate that shifts in mycorrhizal fungi correlate with the evolution of nutritional modes in plants. Furthermore, gradual shifts in fungal partners through a phase of coexistence of different types of mycobionts may play a crucial role in the evolution of mycoheterotrophic plants.     

Pesticidal and pest repellency activities of a plant derived triterpenoid 2α,3β,21β,23,28-penta hydroxyl 12-oleanene against Tribolium castaneum

Background

Tribolium castaneum (Herbst) is a major pest of stored grain-based products, and cause severe damage to cereal grains throughout the world. The present investigation was aimed to determine the pesticidal and pest repellent activities of 2α,3β,21β,23,28-penta hydroxyl 12-oleanene against T. castaneum. The compound 2α,3β,21β,23,28-penta hydroxyl 12-oleanene is a triterpenoid which was isolated from the roots of Laportea crenulata Gaud. Surface film technique was used for pesticidal screening, whereas, pest repellency property of the triterpenoid was determined by filter paper disc method.

Results

At 24 hours of exposure duration, significant mortality records (80% and 86%) were observed at doses 0.88 and 1.77 mg/cm². No significant change in mortality records was observed when duration of exposure was increased up to 48 hours. The triterpenoid showed significant repellency activity at doses 0.47 and 0.94 mg/cm².

Conclusion

These data suggest that the triterpenoid 2α,3β,21β,23,28-penta hydroxyl 12-oleanene possess both pesticidal and pest repellency activities against T. castaneum and can be used in controlling the pest of grain-based products.

Electronic supplementary material

The online version of this article (doi:10.1186/0717-6287-47-68) contains supplementary material, which is available to authorized users.     

Targeting β-tubulin:CCT-β complexes incurs Hsp90- and VCP-related protein degradation and induces ER stress-associated apoptosis by triggering capacitative Ca2+ entry,mitochondrial perturbation and caspase overactivation

We have previously demonstrated that interrupting the protein–protein interaction (PPI) of β-tubulin:chaperonin-containing TCP-1β (CCT-β) induces the selective killing of multidrug-resistant cancer cells due to CCT-β overexpression. However, the molecular mechanism has not yet been identified. In this study, we found that CCT-β interacts with a myriad of intracellular proteins involved in the cellular functions of the endoplasmic reticulum (ER), mitochondria, cytoskeleton, proteasome and apoptosome. Our data show that the targeted cells activate both the heat-shock protein 90 (Hsp90)-associated protein ubiquitination/degradation pathway to eliminate misfolded proteins in the cytoplasm and the valosin-containing protein (VCP)-centered ER-associated protein degradation pathway to reduce the excessive levels of unfolded polypeptides from the ER, thereby mitigating ER stress, at the onset of β-tubulin:CCT-β complex disruption. Once ER stress is expanded, ER stress-associated apoptotic signaling is enforced, as exhibited by cellular vacuolization and intracellular Ca²⁺ release. Furthermore, the elevated intracellular Ca²⁺ levels resulting from capacitative Ca²⁺ entry augments apoptotic signaling by provoking mitochondrial perturbation and caspase overactivation in the targeted cells. These findings not only provide a detailed picture of the apoptotic signaling cascades evoked by targeting the β-tubulin:CCT-β complex but also demonstrate a strategy to combat malignancies with chemoresistance to Hsp90- and VCP-related anticancer agents.     

The influence of cone age on the relative longevity of Banksia seeds

Background and Aims

Serotiny is common in the genus Banksia, so any seed collection is likely to be comprised of seeds that were produced in many different years. This study aimed to determine the impact of cone age and degree of serotiny on longevity in ex situ storage.

Methods

Cones of identifiable age classes were collected from three species of Banksia. Seeds were extracted from cones and the degree of serotiny calculated. An estimate of initial viability (K_i), the time for viability to fall by one probit (σ) and the relative longevity of seeds (p₅₀) for each species and cone age class was determined using a comparative longevity test (50 °C, 63 % relative humidity).

Key Results

The degree of serotiny ranged from moderate (7·9) for Banksia attenuata to strong (40·4) for B. hookeriana. Survival curves for all seed age classes within each species could be described by regressions with a common slope (1/σ), but with different values for K_i. The time taken for viability to fall by one probit (σ) could be described by a common value (29·1 d) for all three species.

Conclusions

Differences in seed longevity between cone age classes and species was related to variation in initial viability (K_i) rather than to differences in σ. While targeting the youngest mature seed cohort on a plant will maximize the viability of seeds collected, a wide range of age classes should be collected (but stored as separate cohorts if possible) for quality conservation/restoration seed collections where genetic diversity is important.     

AMP-activated protein kinase-��1 as an activating kinase of TGF-��-activated kinase 1 has a key role in inflammatory signals

Although previous studies have proposed plausible mechanisms of the activation of transforming growth factor-β-activated kinase 1 (TAK1) in inflammatory signals, including Toll-like receptors (TLRs), its activating kinase still remains to be unclear. In the present study, we have provided evidences that AMP-activated protein kinase (AMPK)-α1 has a pivotal role for activating TAK1, and thereby regulate NF-κB-dependent gene expressions in inflammatory signaling mediated by TLR4 and TNF-α stimulation. AMPK-α1 specifically interacts with TAK1 and reciprocally regulates their kinase activities. Upon the stimulation of lipopolysaccharide, AMPK-α1-knockdown (AMPK-α1^KD) or TAK1-knockdown human monocytic THP-1 cells exhibit a dramatic reduction in the TAK1 or AMPK-α1 kinase activity, respectively, and subsequent suppressions of its downstream signaling cascades, which further leads to inhibitions of NF-κB and thereby productions of proinflammatory cytokines, such as TNF-α, IL-1β, and IL-6. Importantly, the microarray analysis of AMPK-α1^KD cells revealed a dramatic reduction in the NF-κB-dependent genes induced by TLR4 and TNF-α stimulation, and the observation was in significant correlation with the results of quantitative real-time PCR. Moreover, AMPK-α1^KD cells are highly sensitive to the TNF-α-induced apoptosis, which is accompanied with dramatic reductions in the NF-κB-dependent and anti-apoptotic genes. As a result, our data demonstrate that AMPK-α1 as an activating kinase of TAK1 has a key role in mediating inflammatory signals triggered by TLR4 and TNF-α.     

Water relations and photosynthetic capacity of two species of Calotropis in a tropical semi-arid ecosystem

Background and Aims

Calotropis procera and Calotropis gigantea, originally from warm parts of Africa and Asia, are now pan-tropical and in ecological terms considered an indicator of overgrazed, disturbed lands; they grow successfully in dry areas. Variations in water relations, morphology and photosynthesis of the two species growing in the same habitat were studied to assess possible mechanisms of tolerance to drought and how these relate to their ecophysiological success. Also the hypothesis that their photosynthetic rate (A) under drought would be affected by stomatal and non-stomatal limitations was tested.

Methods

Water relations, gas exchange, water use efficiency (WUE), fluorescence parameters, pubescence and specific leaf area (SLA) of Calotropis procera and C. gigantea plants growing in the field were evaluated during the wet (WS) and dry (DS) seasons.

Results

The xylem water potential (ψ) was similar in both species during the WS and DS; drought caused a 28 % decrease of ψ. In C. procera, A, stomatal conductance (g_s) and carboxylation efficiency (CE) were higher in the WS with half the values of those during the DS, this species being more affected by drought than C. gigantea. A high δ¹³C of C. gigantea (–26·2 ‰) in the WS indicated a higher integrated WUE, in agreement with its lower g_s. Leaves of C. gigantea were more pubescent than C. procera. Relative stomatal and non-stomatal limitation of A increased with drought in both species; no changes in maximum quantum yield of photosystem II (PSII; F_v/F_m) were observed. The decrease in the relative quantum yield of PSII (φ_PSII) and in the photochemical quenching coefficient (q_P) was more pronounced in C. procera than in C. gigantea.

Conclusions

The photosynthetic capacity of C. procera was higher than that of C. gigantea. During the DS, A was regulated by stomatal and non-stomatal factors in a coordinated manner and drought did not cause chronic photoinhibition. A higher density of trichomes and leaf angle in C. gigantea may contribute to the maintenance of A and confer more efficient protection of photochemical activity in the DS. Ecophysiological traits such as high photosynthetic rate throughout the year even during the DS, and high WUE, highly pubescent leaves and low SLA observed in both species contribute to the establishment and growth of Calotropis in dry conditions.     

Clinical, hematologic and molecular variability of sickle cell-β thalassemia in western India

BACKGROUND:

Sickle cell-β thalassemia (HbS-β thalassemia) is a sickling disorder of varying severity, which results from compound heterozygosity for sickle cell trait and β thalassemia trait. The present study was undertaken to determine the genetic factors responsible for the clinical variability of HbS-β thalassemia patients from western India.

MATERIALS AND METHODS:

Twenty-one HbS-β thalassemia cases with variable clinical manifestations were investigated. The α and β globin gene clusters were studied by molecular analysis.

RESULTS:

Thirteen patients showed milder clinical presentation as against eight patients who had severe clinical manifestations. Four β thalassemia mutations were identified: IVS 1-5 (G→C), codon 15 (G→A), codon 30 (G→C) and codon 8/9 (+G). α thalassemia and XmnI polymorphism in homozygous condition (+/+) were found to be common among the milder cases. The β^S chromosomes were linked to the typical Arab-Indian haplotype (#31). Framework (FW) linkage studies showed that four β thalassemia mutations were associated with different β globin gene frameworks. Linkage of codon 15 (G→A) mutation to FW2 is being observed for the first time.

CONCLUSION:

The phenotypic expression of HbS-β thalassemia is not uniformly mild and α thalassemia and XmnI polymorphism in homozygous condition (+/+) are additional genetic factors modulating the severity of the disease in the Indian subcontinent.     

Highly diversified fungi are associated with the achlorophyllous orchid Gastrodia flavilabella

Background

Mycoheterotrophic orchids are achlorophyllous plants that obtain carbon and nutrients from their mycorrhizal fungi. They often show strong preferential association with certain fungi and may obtain nutrients from surrounding photosynthetic plants through ectomycorrhizal fungi. Gastrodia is a large genus of mycoheterotrophic orchids in Asia, but Gastrodia species’ association with fungi has not been well studied. We asked two questions: (1) whether certain fungi were preferentially associated with G. flavilabella, which is an orchid in Taiwan and (2) whether fungal associations of G. flavilabella were affected by the composition of fungi in the environment.

Results

Using next-generation sequencing, we studied the fungal communities in the tubers of Gastrodia flavilabella and the surrounding soil. We found (1) highly diversified fungi in the G. flavilabella tubers, (2) that Mycena species were the predominant fungi in the tubers but minor in the surrounding soil, and (3) the fungal communities in the G. flavilabella tubers were clearly distinct from those in the surrounding soil. We also found that the fungal composition in soil can change quickly with distance.

Conclusions

G. flavilabella was associated with many more fungi than previously thought. Among the fungi in the tuber of G. flavilabella, Mycena species were predominant, different from the previous finding that adult G. elata depends on Armillaria species for nutritional supply. Moreover, the preferential fungus association of G. flavilabella was not significantly influenced by the composition of fungi in the environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1422-7) contains supplementary material, which is available to authorized users.     

Quantifying the impact of soil compaction on root system architecture in tomato (Solanum lycopersicum) by X-ray micro-computed tomography

Background and Aims

We sought to explore the interactions between roots and soil without disturbance and in four dimensions (i.e. 3-D plus time) using X-ray micro-computed tomography.

Methods

The roots of tomato Solanum lycopersicum ‘Ailsa Craig’ plants were visualized in undisturbed soil columns for 10 consecutive days to measure the effect of soil compaction on selected root traits including elongation rate. Treatments included bulk density (1·2 vs. 1·6 g cm⁻³) and soil type (loamy sand vs. clay loam).

Key Results

Plants grown at the higher soil bulk density exploited smaller soil volumes (P < 0·05) and exhibited reductions in root surface area (P < 0·001), total root volume (P < 0·001) and total root length (P < 0·05), but had a greater mean root diameter (P < 0·05) than at low soil bulk density. Swelling of the root tip area was observed in compacted soil (P < 0·05) and the tortuosity of the root path was also greater (P < 0·01). Root elongation rates varied greatly during the 10-d observation period (P < 0·001), increasing to a maximum at day 2 before decreasing to a minimum at day 4. The emergence of lateral roots occurred later in plants grown in compacted soil (P < 0·01). Novel rooting characteristics (convex hull volume, centroid and maximum width), measured by image analysis, were successfully employed to discriminate treatment effects. The root systems of plants grown in compacted soil had smaller convex hull volumes (P < 0·05), a higher centre of mass (P < 0·05) and a smaller maximum width than roots grown in uncompacted soil.

Conclusions

Soil compaction adversely affects root system architecture, influencing resource capture by limiting the volume of soil explored. Lateral roots formed later in plants grown in compacted soil and total root length and surface area were reduced. Root diameter was increased and swelling of the root tip occurred in compacted soil.     

Germination responses to temperature and water potential in Jatropha curcas seeds: a hydrotime model explains the difference between dormancy expression and dormancy induction at different incubation temperatures

Background and Aims

Jatropha curcas is a drought-resistant tree whose seeds are a good source of oil that can be used for producing biodiesel. A successful crop establishment depends on a rapid and uniform germination of the seed. In this work we aimed to characterize the responses of J. curcas seeds to temperature and water availability, using thermal time and hydrotime analysis,

Methods

Thermal and hydrotime analysis was performed on germination data obtained from the incubation of seeds at different temperatures and at different water potentials.

Key Results

Base and optimum temperatures were 14·4 and 30 °C, respectively. Approximately 20 % of the seed population displayed absolute dormancy and part of it displayed relative dormancy which was progressively expressed in further fractions when incubation temperatures departed from 25 °C. The thermal time model, but not the hydrotime model, failed to describe adequately final germination percentages at temperatures other than 25 °C. The hydrotime constant, θ_H, was reduced when the incubation temperature was increased up to 30 °C, the base water potential for 50 % germination,Ψ_b(50), was less negative at 20 and 30 °C than at 25 °C, indicating either expression or induction of dormancy. At 20 °C this less negative Ψ_b(50) explained satisfactorily the germination curves obtained at all water potentials, while at 30 °C it had to be corrected towards even less negative values to match observed curves at water potentials below 0. Hence, Ψ_b(50) appeared to have been further displaced to less negative values as exposure to 30 °C was prolonged by osmoticum. These results suggest expression of dormancy at 20 °C and induction of secondary dormancy above 25 °C. This was confirmed by an experiment showing that inhibition of germination imposed by temperatures higher than 30 °C, but not that imposed at 20 °C, is a permanent effect.

Conclusions

This study revealed (a) the extremely narrow thermal range within which dormancy problems (either through expression or induction of dormancy) may not be encountered; and (b) the high sensitivity displayed by these seeds to water shortage. In addition, this work is the first one in which temperature effects on dormancy expression could be discriminated from those on dormancy induction using a hydrotime analysis.     

Spider-fed bromeliads: seasonal and interspecific variation in plant performance

Background and Aims

Several animals that live on bromeliads can contribute to plant nutrition through nitrogen provisioning (digestive mutualism). The bromeliad-living spider Psecas chapoda (Salticidae) inhabits and breeds on Bromelia balansae in regions of South America, but in specific regions can also appear on Ananas comosus (pineapple) plantations and Aechmea distichantha.

Methods

Using isotopic and physiological methods in greenhouse experiments, the role of labelled (¹⁵N) spider faeces and Drosophila melanogaster flies in the nutrition and growth of each host plant was evaluated, as well as seasonal variation in the importance of this digestive mutualism.

Key Results

Spiders contributed 0·6 ± 0·2 % (mean ± s.e.; dry season) to 2·7 ± 1 % (wet season) to the total nitrogen in B. balansae, 2·4 ± 0·4 % (dry) to 4·1 ± 0·3 % (wet) in An. comosus and 3·8 ± 0·4 % (dry) to 5 ± 1 % (wet) in Ae. distichantha. In contrast, flies did not contribute to the nutrition of these bromeliads. Chlorophylls and carotenoid concentrations did not differ among treatments. Plants that received faeces had higher soluble protein concentrations and leaf growth (RGR) only during the wet season.

Conclusions

These results indicate that the mutualism between spiders and bromeliads is seasonally restricted, generating a conditional outcome. There was interspecific variation in nutrient uptake, probably related to each species'' performance and photosynthetic pathways. Whereas B. balansae seems to use nitrogen for growth, Ae. distichantha apparently stores nitrogen for stressful nutritional conditions. Bromeliads absorbed more nitrogen coming from spider faeces than from flies, reinforcing the beneficial role played by predators in these digestive mutualisms.     

Among-individual variation in pollen limitation and inbreeding depression in a mixed-mating shrub

Background and Aims

Variation in inbreeding depression (δ) among individual plants is considered to play a central role in mating system evolution and population genetics. Moreover, such variation could be linked to individual susceptibility to pollen limitation (PL) because those individuals strongly affected by δ for seed production will require more outcross pollen for setting a given number of fruits or seeds. However, no study has tested explicitly for associations between PL and δ at the individual plant level. This study assesses the extent of among-individual variation in PL and δ, the consistency of δ across life stages, and the relationships between individual PL and δ in the mixed-mating shrub Myrtus communis.

Methods

Controlled hand-pollinations were performed in a natural M. communis population. Marked flowers were monitored until fruit production and a greenhouse experiment was conducted with the seeds produced.

Key Results

Compared with selfing, outcross-pollination enhanced seed number per fruit, germination rate and seedling growth, but did not enhance fruit-set. Only seed number per fruit was pollen limited and, thus, cumulative pollen limitation depended more on pollen quality (outcross pollen) than on quantity. The effects of δ varied considerably across life stages and individual plants. Cumulative δ was high across individuals (mean δ = 0·65), although there were no positive correlations between δ values at different life stages. Interestingly, maternal plants showing stronger δ for seed production were more pollen limited, but they were also less affected by δ for seedling growth because of a seed size/number trade-off.

Conclusions

Results show a general inconsistency in δ across life stages and individuals, suggesting that different deleterious loci are acting at different stages. The association between δ and PL at the individual level corroborates the idea that pollen limitation may be ‘genotype-dependent’ regardless of other factors.     

No evidence of carbon limitation with tree age and height in Nothofagus pumilio under Mediterranean and temperate climate conditions

Background and Aims

Trees universally decrease their growth with age. Most explanations for this trend so far support the hypothesis that carbon (C) gain becomes limited with age; though very few studies have directly assessed the relative reductions of C gain and C demand with tree age. It has also been suggested that drought enhances the effect of C gain limitation in trees. Here tests were carried out to determine whether C gain limitation is causing the growth decay with tree age, and whether drought accentuates its effect.

Methods

The balance between C gain and C demand across tree age and height ranges was estimated. For this, the concentration of non-structural carbohydrates (NSCs) in stems and roots of trees of different ages and heights was measured in the deciduous temperate species Nothofagus pumilio. An ontogenetic decrease in NSCs indicates support for C limitation. Furthermore, the importance of drought in altering the C balance with ontogeny was assessed by sampling the same species in Mediterranean and humid climate locations in the southern Andes of Chile. Wood density (WD) and stable carbon isotope ratios (δ¹³C) were also determined to examine drought constraints on C gain.

Key Results

At both locations, it was effectively found that tree growth ultimately decreased with tree age and height. It was found, however, that NSC concentrations did not decrease with tree age or height when WD was considered, suggesting that C limitation is not the ultimate mechanism causing the age/height-related declining tree growth. δ¹³C decreased with tree age/height at the Mediterranean site only; drought effect increased with tree age/height, but this pattern was not mirrored by the levels of NSCs.

Conclusions

The results indicate that concentrations of C storage in N. pumilio trees do not decrease with tree age or height, and that reduced C assimilation due to summer drought does not alter this pattern.