The ecological value of Eryngium horridum in maintaining biodiversity in subtropical grasslands

The role of facilitation in the structuring of plant communities has been often demonstrated in environments under high abiotic stress, especially in semi‐arid and arid ecosystems and high elevations. Few studies, however, analysed facilitation in systems that are highly productive and rich in species, which are thought to be theoretically unlikely to demonstrate strong effects of facilitation. Here, we investigate the importance of Eryngium horridum, a rosette species, on the maintenance of plant diversity in subtropical grasslands in southern Brazil. We evaluated facilitation in areas under two different types of management: abandonment and grazing. Plots were established in areas with and without individuals of E. horridum and all species were identified and had their cover estimated. The Relative Neighbour Effect index was calculated in order to verify the presence of competition or facilitation. Our results indicated facilitation in both abandoned and in grazed grasslands, but apparently through different mechanisms. In the first case, the plant's architecture opens the canopy and allows more light to reach small forbs in the grass matrix. In the second case, E. horridum appears to protect more palatable species from herbivores. Otherwise considered an obnoxious species, E. horridum plays an important ecological role in subtropical grasslands in southern Brazil by facilitating other species and consequently, increasing local richness. Areas with this rosette species are important sources of diaspores, which are able to colonize new open sites and thus, maintain biodiversity.     

Analysis of Chlorophyll Fluorescence by Means of Noisy Light

Linearization is an efficient means of detecting individualcomponents in complex fluorescence induction curves. In a previousinvestigation based on sine-waves (Hansen, Kolbowski, and Dau,1987) the components related to the plastoquinone pool, thehigh-energy state of the thylakoid membrane and the state 1-state2 transition controller could be identified. In this paper,binary noise is used as an alternative input signal which canalso allow linearization by mathematical tools. Comparison withexperiments using sine-waves shows that curve-fitting of thenoise experiments yields the same data as the sine-wave analysis.Further, some additional components were revealed as labelledby the related time-constants whose values depended on lightintensity (e.g. 5, 15, 20, 70, 85, 550 s for spinach at 2.5W m^–2). The advantages of noise analysis are a shorter measuring time and the availability ofan on-line criterion which indicates when a sufficient signal-to-noiseratio is reached. Key words: Chlorophyll fluorescence, correlation functions, linearization, noise, spinach, time-constants     

Temperature effects on forest herbs assessed by warming and transplant experiments along a latitudinal gradient

Slow‐colonizing forest understorey plants are probably not able to rapidly adjust their distribution range following large‐scale climate change. Therefore, the acclimation potential to climate change within their actual occupied habitats will likely be key for their short‐ and long‐term persistence. We combined transplant experiments along a latitudinal gradient with open‐top chambers to assess the effects of temperature on phenology, growth and reproductive performance of multiple populations of slow‐colonizing understorey plants, using the spring flowering geophytic forb Anemone nemorosa and the early summer flowering grass Milium effusum as study species. In both species, emergence time and start of flowering clearly advanced with increasing temperatures. Vegetative growth (plant height, aboveground biomass) and reproductive success (seed mass, seed germination and germinable seed output) of A. nemorosa benefited from higher temperatures. Climate warming may thus increase future competitive ability and colonization rates of this species. Apart from the effects on phenology, growth and reproductive performance of M. effusum generally decreased when transplanted southwards (e.g., plant size and number of individuals decreased towards the south) and was probably more limited by light availability in the south. Specific leaf area of both species increased when transplanted southwards, but decreased with open‐top chamber installation in A. nemorosa. In general, individuals of both species transplanted at the home site performed best, suggesting local adaptation. We conclude that contrasting understorey plants may display divergent plasticity in response to changing temperatures which may alter future understorey community dynamics.     

Soil water repellency and its implications for organic matter decomposition – is there a link to extreme climatic events?

Earth system models associate the ongoing global warming with increasing frequency and intensity of extreme events such as droughts and heat waves. The carbon balance of soils may be more sensitive to the impact of such extremes than to homogeneously distributed changes in soil temperature (T_s) or soil water content (θ_s). One parameter influenced by more pronounced drying/rewetting cycles or increases in T_s is the wettability of soils. Results from laboratory and field studies showed that low θ_s, particularly in combination with high T_s can increase soil water repellency (SWR). Recent studies have provided evidence that the stability of soil organic matter (SOM) against microbial decomposition is substantially enhanced in water repellent soils. This review hypothesizes that SWR is an important SOM stabilization mechanism that could become more important because of the increase in extreme events. We discuss wettability‐induced changes in soil moisture distribution and in soil aggregate turnover as the main mechanisms explaining the reduced mineralization of SOM with increasing SWR. The creation of preferential flow paths and subsequent uneven penetration of rainwater may cause a long‐term reduction of soil water availability, affecting both microorganisms and plants. We conclude that climate change‐induced SWR may intensify the effects of climatic drought and thus affects ecosystem processes such as SOM decomposition and plant productivity, as well as changes in vegetation and microbial community structure. Future research on biosphere–climate interactions should consider the effects of increasing SWR on soil moisture and subsequently on both microbial activity and plant productivity, which ultimately determine the overall carbon balance.     

A substrate ambiguous enzyme facilitates genome reduction in an intracellular symbiont

Background

Genome evolution in intracellular microbial symbionts is characterized by gene loss, generating some of the smallest and most gene-poor genomes known. As a result of gene loss these genomes commonly contain metabolic pathways that are fragmented relative to their free-living relatives. The evolutionary retention of fragmented metabolic pathways in the gene-poor genomes of endosymbionts suggests that they are functional. However, it is not always clear how they maintain functionality. To date, the fragmented metabolic pathways of endosymbionts have been shown to maintain functionality through complementation by host genes, complementation by genes of another endosymbiont and complementation by genes in host genomes that have been horizontally acquired from a microbial source that is not the endosymbiont. Here, we demonstrate a fourth mechanism.

Results

We investigate the evolutionary retention of a fragmented pathway for the essential nutrient pantothenate (vitamin B5) in the pea aphid, Acyrthosiphon pisum endosymbiosis with Buchnera aphidicola. Using quantitative analysis of gene expression we present evidence for complementation of the Buchnera pantothenate biosynthesis pathway by host genes. Further, using complementation assays in an Escherichia coli mutant we demonstrate functional replacement of a pantothenate biosynthesis enzyme, 2-dehydropantoate 2-reductase (E.C. 1.1.1.169), by an endosymbiont gene, ilvC, encoding a substrate ambiguous enzyme.

Conclusions

Earlier studies have speculated that missing enzyme steps in fragmented endosymbiont metabolic pathways are completed by adaptable endosymbiont enzymes from other pathways. Here, we experimentally demonstrate completion of a fragmented endosymbiont vitamin biosynthesis pathway by recruitment of a substrate ambiguous enzyme from another pathway. In addition, this work extends host/symbiont metabolic collaboration in the aphid/Buchnera symbiosis from amino acid metabolism to include vitamin biosynthesis.

     

Host genotype affects the relative success of competing lines of aphid parasitoids under superparasitism

1. In solitary parasitoids, only one individual can complete development in a given host. Therefore, solitary parasitoids tend to prefer unparasitised hosts for oviposition, yet under high parasitoid densities, superparasitism is frequent and results in fierce competition for the host's limited resources. This may lead to selection for the best intra‐host competitors. 2. Increased intra‐host competitive ability may evolve under a high risk of superparasitism if this trait exhibits genetic variation, and if competitive differences among parasitoid genotypes are consistent across environments, e.g. different host genotypes. 3. These assumptions were addressed in the aphid parasitoid Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae) and its main host, the black bean aphid, Aphis fabae (Scopoli) (Hemiptera: Aphididae). Three parthenogenetic lines of L. fabarum were allowed to parasitise three aphid clones singly and in all pairwise combinations (superparasitism). The winning parasitoid in superparasitised aphids was determined by microsatellite analysis. 4. The proportions of singly parasitised aphids that were mummified were similar for the three parasitoid lines and did not differ significantly among host clones. 5. Under superparasitism, significant biases in favour of one parasitoid line were observed for some combinations, indicating that there is genetic variation for intra‐host competitive ability. However, the outcome of superparasitism was inconsistent across aphid clones and thus influenced significantly by the host clone in which parasitoids competed. 6. Overall, this study shows that the fitness of aphid parasitoids under superparasitism is determined by complex interactions with competitors as well as hosts, possibly hampering the evolution of improved intra‐host competitive ability.     

Effect of ambient temperature on mechanosensory host location in two parasitic wasps of different climatic origin

Abstract.  Several parasitic wasps of the Pimplinae (Ichneumonidae) use self-produced vibrations transmitted through plant substrate to locate their concealed immobile hosts (lepidopteran pupae) by reflected signals. This mechanosensory mechanism of host location, called vibrational sounding, depends on the physical characteristics of the plant substrate and the wasp's body and is postulated to depend on ambient temperature. Adaptations of two parasitoid species to thermal conditions of their habitats and the influence of temperature on the trophic interaction during host location are investigated in the tropical Xanthopimpla stemmator (Thunberg) and compared with the temperate Pimpla turionellae (L.). Plant-stem models with hidden host mimics are offered to individual wasps under defined temperature treatments and scored for the number and location of ovipositor insertions. Significant effects of temperature are found on host-location activity and its success. The tropical species possesses an optimum temperature range for vibrational sounding between 26 and 32 °C, whereas the performance decreases both at low and high temperatures. The temperate species reveals substantial differences with respect to performance at the same thermal conditions. With increasing temperature, P. turionellae shows a reduced response to the host mimic, reduced numbers of ovipositor insertions, and decreased precision of mechanosensory host location. In the tropical X. stemmator , the female wasps are able to locate their host with high precision over a broad range of ambient temperatures, which suggests endothermic thermoregulation during vibrational sounding. Environmental physiology may therefore play a key role in adaptation of the host location mechanism to climatic conditions of the species' origin.     

Seed production and seed quality in a calcareous grassland in elevated CO2

In diverse plant communities the relative contribution of species to community biomass may change considerably in response to elevated CO₂. Along with species‐specific biomass responses, reproduction is likely to change as well with increasing CO₂ and might further accelerate shifts in species composition. Here, we ask if, after 5 years of CO₂ exposure, seed production and seed quality in natural nutrient‐poor calcareous grassland are affected by elevated CO₂ (650 μ L L^?1 vs 360 μ L L^?1) and how this might affect long‐term community dynamics. The effect of elevated CO₂ on the number of flowering shoots (+ 24%, P < 0.01) and seeds (+ 29%, P = 0.06) at the community level was similar to above ground biomass responses in this year, suggesting that the overall allocation to sexual reproduction remained unchanged. Compared among functional groups of species we found a 42% increase in seed number (P < 0.01) of graminoids, a 33% increase (P = 0.07) in forbs, and no significant change in legumes (? 38%, n.s.) under elevated CO₂. Large responses particularly of two graminoid species and smaller responses of many forb species summed up to the significant or marginally significant increase in seed number of graminoids and forbs, respectively. In several species the increase in seed number resulted both from an increase in flowering shoots and an increase in inflorescence size. In most species, seeds tended to be heavier (+ 12%, P < 0.01), and N‐concentration of seeds was significantly reduced in eight out of 13 species. The fraction of germinating seeds did not differ between seeds produced in ambient and elevated CO₂, but time to germination was significantly shortened in two species and prolonged in one species when seeds had been produced in elevated CO₂. Results suggest that species specific increases in seed number and changes in seed quality will exert substantial cumulative effects on community composition in the long run.     

Late Cretaceous follicular fruits from southern Sweden with systematic affinities to early diverging eudicots

Eight new species in seven new genera of follicular fruits are described from the Late Cretaceous of southern Sweden. They are Agapitocarpus emisxus , Chontrocarpus pachytoichus , Maiandrocarpus moirasmenus , Malliocarpus batrachoides , Mitocarpus elegans , Xylocarpus rhitidoides , Zeugarocarpus adroagathus and Z. leptoagathus . The fossil follicles are borne along infructescence axes or more commonly occur as isolated, dispersed fruits. Three genera, Maiandrocarpus , Mitocarpus and Zeugarocarpus , have follicles in distinct pairs borne spirally on an infructescence axis. Each follicle of a pair faces the axis with its ventral slit, which strongly indicates that the paired follicles were derived from two separate, monocarpellate flowers rather than from a single, bicarpellate flower. One genus ( Malliocarpus ) has follicles borne individually in the infructescence and three others ( Agapitocarpus , Chontrocarpus and Xylocarpus ) are known only as dispersed follicles. The paired follicles share many features with most of the dispersed follicles, including a sessile stigmatic area, a three-layered simple follicle wall, a simple vascular system composed of one dorsal and two lateral bundles, as well as a marginal-linear placentation bearing several anatropous ovules. Accordingly they are thought to belong to the same complex of taxa. The general structure of the fossils and comparison with modern angiosperms suggest that the fossils might represent an extinct lineage within or close to basal eudicots and many characters are shared with members of extant Proteaceae.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 148 , 377–407.