Hermann Heilmeier

What contributes to the sensitivity of microalgae to triclosan

Differential sensitivities of microalgae to triclosan have been reported, which may have significant implications for environmental risk assessment of this widely used biocide. Therefore, the aim of this study was to derive a mechanistic understanding of varying microalgal sensitivity to this substance. The toxicity of triclosan was evaluated using microalgal systems varying in biological complexity, exposure time and systematic position (a synchronized culture of the chlorophyte Scenedesmus vacuolatus, a diatom Nitzschia palea cultivated in suspension as well as attached to surfaces and periphyton communities). The results revealed (1) differences in sensitivity of the selected microalgal systems of three orders of magnitude and (2) highest sensitivity of the chlorophyte to triclosan in the range of environmental concentrations. To investigate algal sensitivity to triclosan in more detail, bioavailability was considered by investigating suspended and attached living algae. Differences in the generation time (in comparison to test duration) of the species were addressed by evaluating and modeling concentration-time-effect relationships. However, varying sensitivities of the selected microalgal systems remained unexplained. Comparison of species-specific toxic responses to calculated effect concentrations, derived from quantitative relationships for narcosis and uncoupling mode-of-action, leads us to the conclusion that triclosan may address multiple target sites in different microalgal species.

Abscisic acid in soils : What is its function and which factors and mechanisms influence its concentration ?

Abscisic acid (ABA) was detected in aqueous extracts of a range of different soils, beneath a range of crops, pasture and forest species. Assuming that all the ABA is dissolved in the soil solution concentrations ranged from 0.6–2.8 nM. This is in the range which computer simulations predict is required in soils in order to prevent ABA release from the root hair zones of plant roots. The concentration of ABA in the soil solution was highest in acid soils and in soils with reduced moisture, and was lowest in moist, neutral and moderately alkaline soils. ABA in the soil solution of maize fields increased during the vegetative period. After incubation in soil for 72 h, radioactive ABA was degraded by 30–40%. Tetcyclacis, an inhibitor of the oxidative breakdown of ABA, completely prevented the degradation of ABA in the soil solution. Acid conditions and high salt concentrations significantly retarded ABA breakdown.

Uptake of amino acids by the aquatic resurrection plant Chamaegigas intrepidus and its implication for N nutrition

Chamaegigas intrepidus is a poikilohydric aquatic plant that lives in rock pools on granitic outcrops in Central Namibia. The pools are filled intermittently during the summer rains, and the plants may pass through up 20 rehydration/dehydration cycles during a single wet season. Rehydrated plants also have to cope with substantial diurnal fluctuations in the pH and extreme nutrient deficiency. Ammonium concentrations are normally around 30 μM. Additional nitrogen sources are amino acids. Total free amino acids are up to 15 μM with glycine and serine as the predominant amino acids. Experiments on uptake of radiolabelled amino acids into roots of C. intrepidus showed high␣affinity (KM= 16 μM) and low-affinity (KM= 159 μM) uptake systems. The KM of the high-affinity system is well in accordance with the free amino acid concentration found in the water of the pools. We conclude that amino acids, predominantly glycine and serine, can be utilised by C. intrepidus in its natural habitat. Since glycine uptake showed a strong reduction at pH 10, nitrogen uptake from glycine or serine should occur mainly in the morning when the pH of the pool water is slightly acid. Further experiments with 15N-labelled ammonium in combination with non-labelled glycine demonstrated high 15N values in plant tissues. Under experimental conditions C. intrepidus preferred ammonium as a nitrogen source. The implication of amino acids for nitrogen nutrition of C. intrepidus may depend on the relation of inorganic and organic nitrogen available in the pool water and the preferential utilisation of one or the other nitrogen source may change during the day corresponding with pH changes in the water.

Nitrogen and carbohydrate storage in biennials originating from habitats of different resource availability

Four biennial species (Arctium tomentosum, Cirsium vulgare, Dipsacus sylvester and Daucus carota) which originate from habitats of different nutrient availability were investigated in a 2-year experiment in a twofactorial structured block design varying light (natural daylight versus shading) and fertilizer addition. The experiment was designed to study storage as reserve formation (competing with growth) or as accumulation (see Chapin et al. 1990). We show that (i) the previous definitions of storage excluded an important process, namely the formation of storage tissue. Depending on species, storage tissue and the filling process can be either a process of reserve formation, or a process of accumulation. (ii) In species representing low-resource habitats, the formation of a storage structure competes with other growth processes. Growth of storage tissue and filling with storage products is an accumulation process only in the high-resource plant Arctium tomentosum. We interpret the structural growth of low-resource plants in terms of the evolutionary history of these species, which have closely related woody species in the Mediterranean area. (iii) The use of storage products for early leaf growth determines the biomass development in the second season and the competitive ability of this species during growth with perennial species. (iv) The high-resource plant Arctium has higher biomass development under all conditions, i.e. plants of low-resource habitats are not superior under low-resource conditions. The main difference between high- and low-resource plants is that low-resource plants initiate flowering at a lower total plant internal pool size of available resources.

Extreme weather events and plant–plant interactions: shifts between competition and facilitation among grassland species in the face of drought and heavy rainfall

Biotic interactions play an important role in ecosystem function and structure in the face of global climate change. We tested how plant–plant interactions, namely competition and facilitation among grassland species, respond to extreme drought and heavy rainfall events. We also examined how the functional composition (grasses, forbs, legumes) of grassland communities influenced the competition intensity for grass species when facing extreme events. We exposed experimental grassland communities of different functional compositions to either an extreme single drought event or to a prolonged heavy rainfall event. Relative neighbour effect, relative crowding and interaction strength were calculated for five widespread European grassland species to quantify competition. Single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa but the nature of the shifts varied depending on the community composition. Facilitation by neighbouring plants was observed for Arrhenatherum elatius when subjected to drought. Contrarily, the facilitative effect of neighbours on Lotus corniculatus was transformed into competition. Heavy rainfall increased the competitive effect of neighbours on Holcus lanatus and Lotus corniculatus in communities composed of three functional groups. Competitive pressure on Geranium pratense and Plantago lanceolata was not affected by extreme weather events. Neither heavy rainfall nor extreme drought altered the overall productivity of the grassland communities. The complementary responses in competition intensity experienced by grassland species under drought suggest biotic interactions as one stabilizing mechanism for overall community performance. Understanding competitive dynamics under fluctuating resources is important for assessing plant community shifts and degree of stability of ecosystem functions.

Leaf longevity and drought: avoidance of the costs and risks of early leaf abscission as inferred from the leaf carbon isotopic composition

Plant species with longer leaf longevity tend to maintain lower photosynthetic rates. Among other factors, differences in stomatal limitation have been proposed to explain the negative effects of leaf longevity on photosynthesis, although it is not yet clear why stomatal limitations should be stronger in species with longer leaf longevity. We measured carbon isotopic composition (δ13C) in the fresh leaf litter of several Mediterranean woody species to estimate the mean stomatal limitations during the photosynthetically active part of the leaf life. Interspecific differences in δ13C were best explained by a multiple regression including, as independent variables, the maximum leaf longevity and the annual water deficit. For a similar level of water availability, stomatal limitations were higher in species with longer leaf longevity. We hypothesise that stronger stomatal control of transpiration in longer-living leaves arose as a mechanism to reduce the risk of leaf desiccation and to avoid the high costs for the future C assimilation of anticipated leaf mortality in species with a long leaf life expectancy. This stronger sensitivity to drought should be added to the suite of traits accompanying long leaf longevity and contributes decisively to the overall limitations to C assimilation in long-lived leaves.

Carbon dioxide assimilation in the flowerhead of Arctium

SummaryThe gas exchange of flowerheads was determined in Arctium tomentosum and A. lappa during their development. The light, temperature and CO2 responses were used to estimate flowerhead photosynthesis and the in situ contribution of carbon assimilation to the carbon requirement of the plant for supporting a flowerhead. Changes in vapour pressure deficit had no effect on flowerhead photosynthesis rates and were not included in the model.In both species assimilatory capacity correlated with total bract chlorophyll content. Light, temperature and CO2 response curves were very similar in form between species, differing only in absolute rates. During all stages of development, flowerheads always exhibited a net carbon loss, which was mainly determined by temperature. The respiration rate decreased in the light, the difference of CO2 exchange in the dark and in the light was interpreted as photosynthesis. This rate was larger in A. lappa than in A. tomentosum. 30% of the total C requirement of A. lappa flowerheads was photosynthesized by its bracts, the total contribution offlowerhead photosynthesis in A. tomentosum was only 15%. The potential competitive advantages of variation in flowerhead photosynthesis are discussed.

Ephemeral pools as stressful and isolated habitats for the endemic aquatic resurrection plant Chamaegigas intrepidus

Ephemeral pools are widespread in (semi-)arid regions, where they form unique, mostly isolated habitats in an otherwise dry landscape matrix. They are charac- terized by extremely variable conditions, the most important stress factors being (1) fre- quent and fast desiccation and rehydration, (2) long dry seasons, (3) intensive solar irradi- ation and high temperatures during the dry season, (4) large diurnal oscillations of O2 and CO2 concentration and pH in the pool water, (5) extreme nutrient shortage, especially nitrogen. One functional plant type being highly adapted to these stressful conditions are aquatic resurrection plants, i. e. poikilohydric cormophytes like Chamaegigas intrepidus (Scrophulariaceae). This species grows endemically in ephemeral rock pools on granite outcrops in Central Namibia. Ch. intrepidus shows a number of anatomical, biochemical and physiological adaptations to the complex of extreme habitat conditions such as con- tractive xylem, velamen radicum, accumulation of abscisic acid, dehydrins and carbohy- drates during desiccation. Nitrogen deficiency is partly avoided by acquisition of amino acids and an efficient utilisation of urea. Chamaegigas populations on single inselbergs are genetically very highly isolated, whereas gene flow between sub-populations from different pools on one inselberg is rather high. In accordance with the predominantly outcrossing breeding behaviour, most genetic diversity is found within a pool. A habitat model showed clear niche partitioning between Ch. intrepidus and the less desiccation- tolerant species Limosella grandiflora with respect to depth of the pools. The specific constitutive dehydration tolerance mechanisms displayed by Ch. intrepidus are discussed as a vital adaptation to the low predictability of the occurrence and duration of periods favourable for growth and reproduction.

Survival strategies under extreme and complex environmental conditions: The aquatic resurrection plant Chamaegigas intrepidus

Summary Chamaegigas intrepidus D inter is a tiny poikilohydric member of the Scrophulariaceae growing endemically in ephemeral rock pools on granite outcrops in Central Namibia. Habitat conditions are characterised by (1) frequent and fast desiccation and rehydration during the rainy summer season, (2) complete dehydration during the dry winter season lasting up to 11 months, (3) intensive solar irradiation and high temperatures during the dry season, (4) diurnal oscillations of pH in the pool water up to 6 units, (5) extreme nutrient deficiencies, especially nitrogen. Anatomical, biochemical and physiological adaptations to this complex of extreme environmental conditions are discussed such as contractive xylem, accumulation of abscisic acid, dehydrins and carbohydrates during desiccation, and the role of amino acids, ammonium, urea and urease for nitrogen nutrition.

Abscisic acid relations of plants grown on tungsten enriched substrates

The effect of tungsten as a potential inhibitor of ABA biosynthesis in plants was tested both under controlled conditions in the greenhouse or laboratory and in the field at a site naturally enriched with tungsten. Although as predicted in all cases the activity of aldehyde oxidase was strongly inhibited by tungsten, in well watered, greenhouse-grown maize plants increased concentrations of tungsten did not reduce ABA concentration in tissues. Depending on the time of incubation, mostly an accumulation due to an increased net ABA synthesis could be observed. Under drought stress, however, tungsten may have an inhibitory effect on ABA accumulation. Field experiments with drought-stressed clover and dandelion plants from a habitat close to a former tungsten mine confirm those of the greenhouse and laboratory. The ineffectiveness of tungsten in decreasing ABA levels in well-watered plants and its minimal effects in inhibiting ABA accumulation of droughted plants indicate that it is unsuitable as an inhibitor of ABA biosynthesis.