Herbaceous phytomass and nutrient concentrations of four grass species in Sudanian savanna woodland subjected to recurrent early fire

Fire is an integral ecological factor in African savanna ecosystems, but its effects on savanna productivity are highly variable and less understood. We conducted a field experiment to quantify changes in herbaceous phytomass and nutrient composition in a Sudanian savanna woodland subjected to annual early fire from 1993 to 2004. Fire effects were also assessed on two perennial and two annual grass species during the following growing season. Early fire significantly reduced above‐ground phytomass of the studied species (P = 0.03), their crude protein (P = 0.022), neutral detergent insoluble crude protein (P = 0.016) and concentrations of Ca, Fe and Mn (P < 0.05). Perennial grasses had higher above‐ground phytomass but lower total crude protein and fat than annual grasses. Nonstructural carbohydrates tended to be higher for annuals, while fibre and lignin contents were high for perennials. Except Na and Fe, the concentration of mineral elements varied between species. Fire did not affect measures of digestibility and metabolizable energy, but its effect differed significantly among species. In conclusion, the results illustrate that long‐term frequent fire will counterbalance the short‐term increase in soil fertility and plant nutrient concentrations claimed to be accrued from single or less frequent fire.     

Seasonal gas exchange and water relations in juveniles of two evergreen Neotropical savanna tree species with contrasting regeneration strategies

Colonization dynamics of woody species into grasslands in Neotropical savannas are determined by two main factors: plant-available moisture and fire. Considering seasonality of precipitation and high fire frequency in these ecosystems, vegetative reproduction has been suggested as the main regeneration strategy in woody species. This study examined seasonal variations in water relations and photosynthesis in juveniles of two tree species with contrasting regeneration strategies: Palicourea rigida (sexual reproduction) and Casearia sylvestris (asexual reproduction). The studied species showed similar transpiration rates to deep-rooted adult evergreen tree species during the rainy period, suggesting little water availability limitations on surface soil layers. P. rigida juveniles significantly decreased their leaf water potentials from wet to dry seasons. In C. sylvestris resprouts, there were no seasonal differences in their predawn water potentials and gas exchange parameters, indicating a water deficit avoidance characteristic derived from their connections to deep-rooted adult counterparts allowing access to moist soil at depth even during the drought period. P. rigida rely on strict control of water losses and turgor maintenance through elastic cell walls during the dry season. The iso-hydric behavior of gas exchange and most water relations parameters in C. sylvestris enable turgor maintenance during the dry season which also gives the possibility to achieve foliar expansion under water-stressed conditions for shallow-rooted plants. Nevertheless, in absence of water deficits, P. rigida had the advantage to be physiologically independent individuals, showing an equal or even superior photosynthetic performance that eventually could be translated into a more favorable whole-plant carbon balance and higher growth rates in wet habitats.     

Yield and species composition of a mesic grassland savanna in South Africa are influenced by long‐term nutrient addition

Species composition and productivity of natural grasslands are influenced by soil nutrient status. With high resource availability, productivity is expected to increase, and competition is assumed to gain prominence with predicted exclusion of species of lower competitive ability. During 2010 and 2011 we used the dry weight rank method to measure above‐ground phytomass production of herbage in 96 plots (9 m × 2.7 m) fertilized for 60 years with two forms of nitrogen (N as limestone ammonium nitrate or ammonium sulphate at four levels: 0, 7.1, 14.1, 21.2 g m^?2), phosphorus (P as superphosphate at two levels: 0, 33.6 g m^?2), and lime (two levels: 0, 225 g m^?2). Light attenuation was measured as the proportion of photosynthetically active radiation reaching the lower leaf layers of the grasses and the ground surface. Light conditions beneath the grass layer were reduced by nutrient addition to 30% of full sunlight but remained above 60% in non‐fertilized plots. Grass total above‐ground phytomass production increased with nutrient addition. The strongest yield responses were attained with N plus P addition. Species responses showed that Themeda triandra and Hyparrhenia hirta decreased in above‐ground phytomass production with nutrient addition while Panicum maximum, Eragrostis curvula and E. plana increased. These findings are discussed in terms of competitive interactions among species, their position in the grass canopy and their physiological tolerances to high nitrogen environments.     

Variation in defence strategies in two species of the genus Beilschmiedia under differing soil nutrient and rainfall conditions

The relationships between various leaf functional traits that are important in plant growth (e.g., specific leaf area) have been investigated in recent studies; however, research in this context on plants that are highly protected by chemical defences, particularly resource-demanding nitrogen-based defence, is lacking. We collected leaves from cyanogenic (N-defended) Beilschmiedia collina B. Hyland and acyanogenic (C-defended) Beilschmiedia tooram (F. M. Bailey) B. Hyland at high- and low-soil nutrient sites in two consecutive years that varied significantly in rainfall. We then measured the relationships between chemical defence and morphological and functional leaf traits under the different environmental conditions. We found that the two species differed significantly in their resource allocation to defence as well as leaf morphology and function. The N defended species had a higher leaf nitrogen concentration, whereas the C-defended species had higher amounts of C-based chemical defences (i.e., total phenolics and condensed tannins). The C-defended species also tended to have higher force to fracture and increased leaf toughness. In B. collina, cyanogenic glycoside concentration was higher with higher rainfall, but not with higher soil nutrients. Total phenolic concentration was higher at the high soil nutrient site in B. tooram, but lower in B. collina; however, with higher rainfall an increase was found in B. tooram, while phenolics decreased in B. collina. Condensed tannin concentration decreased in both species with rainfall and nutrient availability. We conclude that chemical defence is correlated with leaf functional traits and that variation in environmental resources affects this correlation.     

Arbuscular mycorrhiza and water and nutrient supply differently impact seedling performance of dry woodland species with different acquisition strategies

Background: We recently demonstrated that dwarf male plants are much more common among mosses than hitherto thought. Dwarf plants, producing functional male sexual organs occur in 10–20% of the moss species worldwide.

Aims: We investigated how an inadvertent omission of dwarf males affects estimates of male sex expression rates and sex ratios.

Methods: We recorded frequencies of specimens with female-expressing plants, male-expressing normal-sized and/or dwarf plants, and sporophytes in herbarium material of five model species from Macaronesia. We analysed how frequency estimates of male occurrence and sex ratios were affected by excluding dwarf males.

Results: Occurrence of dwarf males was positively associated with sporophyte formation. When dwarf males were excluded, male mate availability was missed in 51–61% of specimens (in 74–76% of sporophyte-containing specimens). Specimen-level sex ratio was balanced in material with sporophytes in the three species exhibiting male dwarfism, but strongly female biased if dwarf males were omitted.

Conclusions: We provide the first quantitative evidence that sex-expressing dwarf males are essential for sexual reproduction and sporophyte production in mosses that form such males. Considering its abundance, male dwarfism therefore deserves more attention among plant biologists. Innovative approaches are required to further study the minute male plants.     

Defence against oxidative stress in two species of land snails (Helix pomatia and Helix aspersa) subjected to estivation

During summer, land snails are exposed to estivation/arousal cycles that imposes oxidative stress, but they exhibit different patterns of antioxidant defence. To test the ability of two related species, Helix pomatia and Helix aspersa, to modulate their antioxidant defence mechanism during estivation/arousal cycles, we examined activities of catalase and glutathione-related enzymes and concentrations of glutathione and thiobarbituric acid reactive substances (TBARS; as products of lipid peroxidation). In both species, estivation evoked changes in activity of total and selenium-dependent glutathione peroxidase (GPx), but did not affect activity of catalase, glutathione reductase, and glutathione transferase, and had no effect on concentration of glutathione. Activity of catalase in estivating snails, instead of the expected increase, showed a tendency to diminish. Extremely low activities of catalase in the foot were usually associated with extremely high activities of both forms of GPx. In conclusion, maintenance of relatively high activities of the antioxidant enzymes and accumulation of glutathione, resulting in a low and stable concentration of TBARS, plays an important role in scavenging oxygen free radicals from the organism of both species.     

Fine root responses to temporal nutrient heterogeneity and competition in seedlings of two tree species with different rooting strategies

There is little direct evidence for effects of soil heterogeneity and root plasticity on the competitive interactions among plants. In this study, we experimentally examined the impacts of temporal nutrient heterogeneity on root growth and interactions between two plant species with very different rooting strategies: Liquidambar styraciflua (sweet gum), which shows high root plasticity in response to soil nutrient heterogeneity, and Pinus taeda (loblolly pine), a species with less plastic roots. Seedlings of the two species were grown in sandboxes in inter‐ and intraspecific combinations. Nutrients were applied in a patch either in a stable (slow‐release) or in a variable (pulse) manner. Plant aboveground biomass, fine root mass, root allocation between nutrient patch and outside the patch, and root vertical distribution were measured. L. styraciflua grew more aboveground (40% and 27% in stable and variable nutrient treatment, respectively) and fine roots (41% and 8% in stable and variable nutrient treatment, respectively) when competing with P. taeda than when competing with a conspecific individual, but the growth of P. taeda was not changed by competition from L. styraciflua. Temporal variation in patch nutrient level had little effect on the species’ competitive interactions. The more flexible L. styraciflua changed its vertical distribution of fine roots in response to competition from P. taeda, growing more roots in deeper soil layers compared to its roots in conspecific competition, leading to niche differentiation between the species, while the fine root distribution of P. taeda remained unchanged across all treatments. Synthesis. L. styraciflua showed greater flexibility in root growth by changing its root vertical distribution and occupying space of not occupied by P. taeda. This flexibility gave L. styraciflua an advantage in interspecific competition.     

Divergent and reticulate species relationships in Leucaena (Fabaceae) inferred from multiple data sources: insights into polyploid origins and nrDNA polymorphism

Previous analyses of species relationships and polyploid origins in the mimosoid legume genus Leucaena have used chloroplast DNA (cpDNA) restriction site data and morphology. Here we present an analysis of a new DNA sequence data set for the nuclear ribosomal DNA (nrDNA) 5.8S subunit and flanking ITS 1 and ITS 2 spacers, a simultaneous analysis of the morphology, ITS and cpDNA data sets for the diploid species, and a detailed comparison of the cpDNA and ITS gene trees, which include multiple accessions of all five tetraploid species. Significant new insights into species relationships and polyploid origins, including that of the economically important tropical forage tree L. leucocephala, are discussed. Heterogeneous ITS copy types, including 26 putative pseudogene sequences, were found within individuals of four of the five tetraploid and one diploid species. Potential pseudogenes were identified using two pairwise comparison approaches as well as a tree-based method that compares observed and expected proportions of total ITS variation contributed by the 5.8S subunit optimized onto branches of one of the ITS gene trees. Inclusion of putative pseudogene sequences in the analysis provided evidence that some pseudogenes in allopolyploid L. leucocephala are not the result of post-allopolyploidization gene silencing, but were inherited from its putative diploid maternal progenitor L. pulverulenta.     

Competition under high and low nutrient levels among three grassland species occupying different positions in a successional sequence

To clarify the role of seasonal change, competitive response and nutrient availability in the competitive asymmetry of grassland species a competition experiment was conducted on Holcus lanatus , Anthoxanthum odoratum and Festuca ovina , which represent a successional sequence of decreasing nutrient availability. Seven harvests were taken over two growing seasons. At each harvest the dry weight of plant parts, dead leaves, leaf area and plant height were measured. Three key traits that determine the successional status of the species were studied: specific leaf area, specific shoot height, and dead leaf fraction.
The response of these traits to competition appeared to be limited and insufficient to change the competitive relations in the experiment. However, all three traits showed marked seasonal changes which resulted in superior growth and survival in winter of the species adapted to nutrient-poor environments. The findings support the theory that competitive asymmetry increases at higher nutrient levels. It is postulated that the directionality of light makes it possible for the dominant species to monopolize this resource more easily than nutrients.     

Pathogenicity, virulence factors, and strategies to fight against Burkholderia cepacia complex pathogens and related species

The Burkholderia cepacia complex (Bcc) is a group of 17 closely related species of the β-proteobacteria subdivision that emerged in the 1980s as important human pathogens, especially to patients suffering from cystic fibrosis. Since then, a remarkable progress has been achieved on the taxonomy and molecular identification of these bacteria. Although some progress have been achieved on the knowledge of the pathogenesis traits and virulence factors used by these bacteria, further work envisaging the identification of potential targets for the scientifically based design of new therapeutic strategies is urgently needed, due to the very difficult eradication of these bacteria with available therapies. An overview of these aspects of Bcc pathogenesis and opportunities for the design of future therapies is presented and discussed in this work.     

New insights into reactive oxygen species and nitric oxide signalling under low oxygen in plants

Plants produce reactive oxygen species (ROS) when exposed to low oxygen (O₂). Much experimental evidence has demonstrated the existence of an oxidative burst when there is an O₂ shortage. This originates at various subcellular sites. The activation of NADPH oxidase(s), in complex with other proteins, is responsible for ROS production at the plasma membrane. Another source of low O₂‐dependent ROS is the mitochondrial electron transport chain, which misfunctions when low O₂ limits its activity. Arabidopsis mutants impaired in proteins playing a role in ROS production display an intolerant phenotype to anoxia and submergence, suggesting a role in acclimation to stress. In rice, the presence of the submergence 1A (SUB1A) gene for submergence tolerance is associated with a higher capacity to scavenge ROS. Additionally, the destabilization of group VII ethylene responsive factors, which are involved in the direct O₂ sensing mechanism, requires nitric oxide (NO). All this evidence suggests the existence of a ROS and NO – low O₂ mechanism interplay which likely includes sensing, anaerobic metabolism and acclimation to stress. In this review, we summarize the most recent findings on this topic, formulating hypotheses on the basis of the latest advances.     

Growth, carboxylate exudates and nutrient dynamics in three herbaceous perennial plant species under low, moderate and high phosphorus supply

Background and aims

Australian herbaceous native species have evolved in phosphorus (P) impoverished soils. Our objective was to explore shoot and root adaptations of two of these species with potential to be developed as pasture plants, at low, moderate and high P supply after 4 and 7?weeks of growth.

Methods

A glasshouse experiment examined the effect of 5, 20 and 80?mg?P?kg^?1 air-dry soil on growth, rhizosphere carboxylate content, and mineral nutrition of two Australian native perennials, Kennedia nigricans (Fabaceae) and Ptilotus polystachyus (Amaranthaceae), and the exotic Medicago sativa (Fabaceae).

Key results

Leaf P concentrations at P80 were 6, 14 and 52?mg?P?g^?1 leaf dry weight for M. sativa, K. nigricans and P. polystachyus, respectively. As soil P concentration increased, rhizosphere carboxylate content decreased for M. sativa, increased and then decreased for K. nigricans and was unchanged for P. polystachyus. For all species, the contribution of malic acid declined at the second harvest. For all species and P treatments, the amount of rhizosphere carboxylates per unit root length decreased as root length of a plant increased. Plant P content was determined more by P uptake rate per unit root length and time than by root length. Uptake of Mo for all species, and uptake of K, Mg and Mn for P. polystachyus, increased with soil P concentration. Uptake of Fe and S was higher when the content of carboxylates in the rhizosphere was higher.

Conclusion

Root physiological adaptations (i.e. rhizosphere carboxylate content and P-uptake rate) are more important than morphological adaptations (i.e. root length and diameter) to enhance the uptake of P and cations.     

Tissue and species conservation of the vertebrate and arthropod forms of the low molecular weight (16–18000) proteins of gap junctions

Summary Gap junctions have been isolated from four murine tissues, from rat and Xenopus laevis liver, and from Nephrops norvegicus (Norway lobster) hepatopancreas. The preparations of gap junctions from each vertebrate tissue contain a single major protein, M_r 16000, and those from Nephrops hepatopancreas a protein, M_r 18000. Immunocytochemical studies using affinity-purified antibodies raised against gap junctions from Nephrops show the junctional origin of the 18k protein. Immunological studies using Western blotting and biochemical studies using tryptic peptide mapping show no significant differences between the 16k junctional proteins of mouse and hence provide no evidence of tissue variation. These studies also suggest that the mouse, rat, and Xenopus 16 k proteins and the Nephrops 18 k protein share some common structural features.     

Nitrogen supply effects on leaf dynamics and nutrient input into the soil of plant species in a sub-arctic tundra ecosystem

Global warming will lead to increased nitrogen supply in tundra ecosystems. How increased N supply affected leaf production, leaf turnover and dead leaf N input into the soil of Empetrum nigrum and Andromeda polifolia (evergreens), Eriophorum vaginatum (graminoid) and Betula nana (deciduous) in a sub-arctic tundra in northern Sweden between 2003 and 2007 was experimentally investigated. There was considerable interspecific variation in the response of leaf production to N addition, varying from negative, no response to a positive response. Nitrogen addition effects on leaf turnover also showed considerable variation among species, varying from no effect to increased leaf turnover (up to 27% in Eriophorum). Nitrogen addition resulted in a four to fivefold increase in N content in the dead leaves of both evergreens and a 65% increase in Eriophorum. Surprisingly, there was no increase in Betula. The response of dead leaf P contents to N addition was rather species specific. There was no response in Empetrum, whereas there were significant increases in Andromeda (+214%) and Eriophorum (+32%), and a decrease of 47% in Betula. As an overall result of the changes in leaf production, leaf turnover and dead leaf N and P contents, nitrogen addition increased in all species except Betula the amount of N and, for Andromeda and Eriophorum the amount of P transferred to the soil due to leaf litter inputs. However, the way in which this was achieved differed substantially among species due to interspecific differences in the response of the component processes (leaf production, leaf turnover, dead leaf nutrient content).     

Coping with low nutrient availability and inundation: root growth responses of three halophytic grass species from different elevations along a flooding gradient

We describe the responses of three halophytic grass species that dominate the low (Spartina anglica), middle (Puccinellia maritima) and high (Elymus pycnanthus) parts of a salt marsh, to soil conditions that are believed to favour contrasting root-growth strategies. Our hypotheses were: (1) individual lateral root length is enhanced by N limitations in the soil but restricted by oxygen limitations, (2) the density of root branching within a species is inversely related to the length of the lateral roots, and (3) species from high elevations (i.e. the driest parts of a marsh) are the most responsive to changing soil conditions. Plant growth responses and soil parameters showed that the contrasting but uniformly applied soil treatments were effective. All three species showed a small but significant shift towards a finer root diameter distribution when N was limiting, partly because of the finer diameters of the laterals (Elymus and Spartina) and partly because of increased length of individual 1st-order laterals (Elymus and Puccinellia). The increased length of the 1st-order laterals of Elymus and Puccinellia grown under low N indicates that the first part of hypothesis 1 may be true. However, lack of effect of flooding and reduced soil conditions lead us to reject the second part of hypothesis 1. Hypothesis 2 was rejected for these three halophytes, as the branch density of 1st- and 2nd-order laterals appears to be controlled by other factors than length of individual laterals. Hypothesis 3 may be true for specific root characteristics (e.g. length of individual 1st-order laterals), but cannot be generalised (e.g. branch density and topological index). In conclusion, the present data on root growth in contrasting but homogeneous soil conditions indicate that morphological responsiveness of the root systems of these halophytic grass species is limited, regardless of their location along the elevational gradient.     

Trait‐abundance relation in response to nutrient addition in a Tibetan alpine meadow: The importance of species trade‐off in resource conservation and acquisition

In competition‐dominated communities, traits promoting resource conservation and competitive ability are expected to have an important influence on species relative abundance (SRA). Yet, few studies have tested the trait‐abundance relations in the line of species trade‐off in resource conservation versus acquisition, indicating by multiple traits coordination. We measured SRA and key functional traits involving leaf economic spectrum (SLA, specific leaf area; LDMC, leaf dry matter content; LCC, leaf carbon concentration; LNC, leaf nitrogen concentration; LPC, leaf phosphorus concentration; Hs, mature height) for ten common species in all plots subjected to addition of nitrogen fertilizer (N), phosphorus fertilizer (P), or both of them (NP) in a Tibetan alpine meadow. We test whether SRA is positively related with traits promoting plant resource conservation, while negatively correlated with traits promoting plant growth and resource acquisition. We found that species were primarily differentiated along a trade‐off axis involving traits promoting nutrient acquisition and fast growth (e.g., LPC and SLA) versus traits promoting resource conservation and competition ability (e.g., large LDMC). We further found that SRA was positively correlated with plant height, LDMC, and LCC, but negatively associated with SLA and leaf nutrient concentration irrespective of fertilization. A stronger positive height‐SRA was found in NP‐fertilized plots than in other plots, while negative correlations between SRA and SLA and LPC were found in N or P fertilized plots. The results indicate that species trade‐off in nutrient acquisition and resource conservation was a key driver of SRA in competition‐dominated communities following fertilization, with the linkage between SRA and traits depending on plant competition for specific soil nutrient and/or light availability. The results highlight the importance of competitive exclusion in plant community assembly following fertilization and suggest that abundant species in local communities become dominated at expense of growth while infrequent species hold an advantage in fast growth and dispersals to neighbor meta‐communities.     

A proteomic characterization of water buffalo milk fractions describing PTM of major species and the identification of minor components involved in nutrient delivery and defense against pathogens

Water buffalo has been studied in relation to the exclusive use of its milk for the manufacture of high-quality dairy products. Buffalo milk presents physicochemical features different from that of other ruminant species, such as a higher content of fatty acids and proteins. We report here a detailed proteomic analysis of buffalo skim milk, whey and milk fat globule membrane fractions. Notwithstanding the poor information available on buffalo genome, identification of protein isoforms corresponding to 72 genes was achieved by a combined approach based on 2-DE/MALDI-TOF PMF and 1-DE/muLC-ESI-IT-MS-MS. Major protein components, i.e. alpha(Sl)-, alpha(S2)-, beta-, kappa-caseins, alpha-lactalbumin and beta-lactoglobulin, were characterized for PTM, providing a scientific basis to coagulation/cheese making processes used in dairy productions. Minor proteins detected emphasized the multiple functions of milk, which besides affording nutrition to the newborn through its major components, also promotes development and digestive tract protection in the neonate, and ensures optimal mammary gland function in the mother. Defense against pathogens is guaranteed by an arsenal of antimicrobial/immunomodulatory proteins, which are directly released in milk or occur on the surface of secreted milk-lipid droplets. Proteins associated with cell signaling or membrane/protein trafficking functions were also identified, providing putative insights into major secretory pathways in mammary epithelial cells.     

Competitive growth of slow growingRhizobium japonicum against fast growingEnterobacter andPseudomonas species at low concentrations of succinate and other substrates in dialysis culture

A cultivation system with simultaneous growth of six bacterial cultures in separate bags in dialysis culture was developed. In a medium with no added carbon source (one half concentrated Hoagland solution, water deionized and distilled), cell number ofRhizobium japonicum increased during a 7 day period by a factor of 35, whereas the number ofEnterobacter aerogenes cells decreased to one half. With a concentration of 100 nM succinate as an additional carbon source in the inflow,Rhizobium japonicum 61-A-101 cell number increased by a factor of 50 during an 8 day period, whereas cell number ofEnterobacter cloacae NCTC 10005 only doubled and ofEnterobacter aerogenes NCTC 10006 decreased. At 10 mM concentration of succinate in the inflow, doubling time the twoEnterobacter strains was about 12 h, compared to about 24 h for theRhizobium japonicum strain. Varying the succinate concentration from 10 mM to 100 nM in the inflow,Rhizobium japonicum 61-A-101 surpassed theEnterobacter aerogenes strains in the growth rate between 1 mM and 100 M succinate in the inflowing medium. Three otherRhizobium japonicum strains (fix⁺ and fix^-) did grow with a similar rate as strain 61-A-101 at very low concentrations of substrate. Growth rates for the strains were confirmed by protein data per culture. Growing in competition with twoPseudomonas strains,Rhizobium japonicum RH 31 Marburg (fix^-) did overgrow alsoPseudomonas fluorescens, was however outgrown byPseudomonas putida. In utilizing low concentrations of a¹⁴C labelled organic acid (malonate), three strains ofRhizobium japonicum left 2–4 times smaller amounts of¹⁴C in the medium than two species ofPseudomonas and two species ofArthrobacter.On sabbatical leave at ANU