ALBERT

Notes: Summary 1 GABAB1 receptor subunit knockout mice were generated and the effects of the GABAB receptor agonist, baclofen, were evaluated within the peripheral nervous system (PNS) of wildtype (+/+), heterozygote (+/–) and knockout (–/–) animals. For this purpose, neuronally-mediated responses were evoked in both the isolated ileum and urinary bladder, using selective electrical field stimulation (EFS). 2 In ileum resected from 4–8-week-old-mice, low frequencies of EFS (0.5 Hz) evoked irregular muscle contractions which were prevented by atropine 1 μM and reduced by baclofen (33.4 ± 5.6%, 100 μm). The latter effect was antagonized by the GABAB receptor antagonist CGP54626 0.2 μm. Baclofen 100 μm did not affect contractions of similar amplitude induced by carbachol, indicating that the ability of baclofen to inhibit cholinergic function in mouse ileum may be due to an action at prejunctional GABAB receptors. 3 To avoid the development of grand mal seizure by GABAB1 (–/–) mice, a behaviour observed when the mice were greater than 3 weeks old, it was necessary to study the effects of this knockout in 1–3-week-old-animals. However, at this age, EFS at 0.5 Hz did not evoke robust muscle contractions. Consequently we used EFS at 5 Hz, which did evoke cholinergically mediated contractions, found to be of similar amplitude in (+/+) and (+/–) mice, of both 1–3 weeks and 4–8 weeks of age. At this frequency of EFS, baclofen reduced the amplitude of the evoked contractions [n=6 (+/+) and n=5 (+/–), IC50 19.2 ± 4.8 μm) and this effect was greatly reduced in the presence of CGP54626 0.2 μm. 4 In urinary bladder from 1–3-week-old-mice, using higher frequencies of EFS to evoke clear, nerve-mediated contractions (10 Hz), baclofen 10–300 μm concentration-dependently inhibited contractions in (+/+) mice (IC50 9.6 ± 3.8 μm). This effect was inhibited by CGP54626 (0.2 μm, 46.2 ± 13.6% inhibition, 300 μm baclofen n=7) a concentration which, by itself, had no effect on the EFS-evoked contractions. 5 The effects of baclofen in both ileum and urinary bladder were absent in the GABAB1 receptor subunit (–/–) mice; however, responses to EFS were unaffected in (–/–) when compared to the (+/+) mice. 6 Our data suggest that, as in the central nervous system (CNS), the GABAB1 receptor subunit is an essential requirement for GABAB receptor function in the enteric and PNS. As such, these data do not provide a structural explanation for the existence of putative subtypes of GABAB receptor, suggested by studies such as those in which different rank-orders of GABAB agonist affinity have been reported in different tissues.

Notes: Rates of solute diffusion fundamentally affect the properties of flooded soils, but the effects of flooding on solute diffusion have not previously been studied in detail. Four soils with widely differing chemical and physical properties were packed to a range of bulk densities, flooded for varying times, and the self-diffusion of chloride through the soils measured. Diffusion impedance factors were derived from the results. In each soil the impedance factor decreased linearly with increase in bulk density, and between soils impedance factors increased with increasing clay content. The impedance factor decreased by up to 20% during the first 3–6 weeks following flooding, but with prolonged flooding it increased to at least its initial value. Concomitantly the cation exchange capacities of the soils increased by between 30 and 100%, there was reductive dissolution of soil iron, probably both structural iron in soil clays and iron oxyhydroxide coatings on clay surfaces, and subsequently there was re-precipitation of ferrous iron, probably as mixed carbonates and hydroxides. The decreases in diffusion impedance factors were consistent with the increases in cation exchange capacity and changes in soil iron, and the subsequent increases were consistent with re-crystallization of mixed ferrous–ferric compounds. We conclude that the effects of changes in redox on diffusion impedance will be important in some soils, although they are smaller than the effects of water content per se.

Notes: The aim of the investigation was to determine the influence of variations in thickness and colour of agricultural plastic film on silage preservation conditions and grass silage quality. For this purpose, 30 cylindrical plastic containers (mini-silos; 0·3 m3) were filled with chopped grass and covered with five films of different thickness and colour: 90 μm, white; 125 μm, green; 150 μm, black; 200 μm, green and 200 μm, white. Four replications of each film type were placed in the open air. Two replications were housed indoors and exposed to an ‘artificial sky’ in a test apparatus. The surface temperature of the films was found to be strongly dependent on film thickness and colour. Results of the chemical analysis of silages did not reveal any significant influence of the films. This was also the case when restricting the analysis to the uppermost silage layer. The results showed that under the conditions of this experiment, well preserved forage can be produced with films of differing colour, as well as of lesser thickness.

Notes: SUMMARY 1. We analysed the vegetation structure of 215 lakes in the flood plain of the river Lower Rhine in relation to environmental variables related to hydrological connectivity, lake morphometry, lake age and land use on adjacent land.2. The frequency distribution of the cover of submerged macrophytes was not normal, implying that submerged macrophytes in any one lake were either scarce or abundant.3. We observed clear water lakes with submerged macrophyte dominance over a wide range of total P concentration (0.020–0.40 mg total P L−1).4. Multiple logistic regression indicated that the probability of dominance by submerged macrophytes decreased markedly with the surface area, depth and age of the lakes. The surface area effect occurred independently of the depth. Further, there was a negative relationship between submerged macrophyte dominance and the long-term annual duration of inundation by the river.5. Nymphaeid cover showed a distinct optimum with respect to mean lake depth, being almost absent in lakes shallower than 0.5 m. In contrast to what was found for submerged plants, the probability of occurrence of nymphaeids increased with lake age.6. The probability of helophyte occurrence increased with lake age, and decreased with the presence of trees, cattle grazing, surface area, use of manure and mean lake depth.7. In all cases the critical level of one factor (e.g. mean lake depth) depended on other factors (e.g. surface area or age of lake). Thus, in the present study, small lakes tended to remain dominated by submerged macrophytes up to a greater depth than large lakes, and helophytes colonised smaller lakes in an earlier phase.8. The effect of inundation by the river was modest. This could be because most of our lakes are rarely inundated during the growing season and experience only moderate current velocities while flooded.9. The results have practical implications for future management of flood plains for conservation purposes. In new water bodies, macrophyte domination will be promoted if many small shallow lakes, rather than few large deep ones, are excavated.

Notes: SUMMARY 1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity.2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS).3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton.4. Water clarity was positively related to lake depth and the presence of vegetation.5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important.6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency.7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes.8. In conclusion, in-lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine.

Notes: The internal conductance from intercellular spaces to the sites of carboxylation (gi) has only been measured in a few tree species and not in conifers, despite the fact it may impose a large limitation on photosynthesis. The present study provides the first estimates of gi for a coniferous species, and examines variation in gi with height and its relationships to anatomical, biochemical and physiological traits. Measurements were made on upper and lower canopy current-year needles of 50-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Needle thickness and specific leaf area decreased by 30% from the top to bottom of the canopy. These anatomical/morphological changes were accompanied by modest variation in allocation of N to chlorophyll and the chlorophyll a/b ratio. Allocation of N to Rubisco did not vary with height, but the ratio of Rubisco to chlorophyll did owing to the aforementioned changes in allocation to chlorophyll. The value of gi was estimated in one tree from concurrent measurements of carbon isotope discrimination and net photosynthesis. To examine the variation in gi among trees a second independent method based on day respiration and the difference between the chloroplastic and intercellular photocompensation points (photocompensation point method) was used. Estimates of gi obtained by the two methods agreed well with values varying between 0.14 and 0.20 mol m−2 s−1. It is estimated that gi limits photosynthesis by approximately 20% as compared to an approximately 30% stomatal limitation (under well-watered conditions). The value of gi scaled approximately with maximum rates of photosynthesis, which were significantly greater in upper canopy needles. Nevertheless, gi did not vary significantly with canopy height, owing to greater variability in gi than photosynthesis.

Notes: The response of the bipolar moss Sanionia uncinata (Hedw.) Loeske to ambient and enhanced UV-B radiation was investigated at an Antarctic (Léonie Island, 67°35′ S, 68°20′ W) and an Arctic (Ny-Alesund, 78°55′ N, 11°56′ E) site, which differed in ambient UV-B radiation (UV-BR: 280–320 nm) levels. The UV-BR effects on DNA damage and photosynthesis were investigated in two types of outdoor experiments. First of all, sections of turf of S. uncinata were collected in an Arctic and Antarctic field site and exposed outdoors to ambient and enhanced UV-BR for 2 d using UV-B Mini-lamps. During these experiments, chlorophyll a fluorescence, chlorophyll concentration and cyclobutyl pyrimidine dimer (CPD) formation were measured. Secondly, at the Antarctic site, a long-term filter experiment was conducted to study the effect of ambient UV-BR on growth and biomass production. Additionally, sections of moss turf collected at both the Antarctic and the Arctic site were exposed to UV-BR in a growth chamber to study induction and repair of CPDs under controlled conditions. At the Antarctic site, a summer midday maximum of 2·1 W m−2 of UV-BR did not significantly affect effective quantum yield (ΔF/Fm′) and the ratio of variable to maximal fluorescence (Fv/Fm). The same was found for samples of S. uncinata exposed at the Arctic site, where summer midday maxima of UV-BR were about 50% lower than at the Antarctic site. Exposure to natural UV-BR in summer did not increase CPD values significantly at both sites. Although the photosynthetic activity remained largely unaffected by UV-B enhancement, DNA damage clearly increased as a result of UV-B enhancement at both sites. However, DNA damage induced during the day by UV-B enhancement was repaired overnight at both sites. Results from the long-term filter experiment at the Antarctic site indicated that branching of S. uncinata was reduced by reduction of ambient summer levels of UV-BR, whereas biomass production was not affected. Exposure of specimens collected from both sites to UV-BR in a growth chamber indicated that Antarctic and Arctic S. uncinata did not differ in UV-BR-induced DNA damage. It was concluded that S. uncinata from both the Antarctic and the Arctic site is well adapted to ambient levels of UV-BR.

Notes: Photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 µmol mol−1 for 10 years using Free Air CO2Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source : sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (Vc,max) and the maximum rate of electron transport (Jmax). Immediately prior to the periodic (every 4–8 weeks) cuts of the L. perenne stands, Vc,max and Jmax, were significantly lower in elevated than in ambient [CO2] in the low-nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation in Vc,max and Jmax in the later years in either nitrogen treatment.

Notes: We investigated the role that manganese superoxide dismutase (MnSOD, EC 1·15·1·1), an important enzyme of the antioxidant pathway, may play in aluminium (Al) toxicity/resistance. A wheat (Triticum aestivum) cDNA (WMnSOD) was cloned and up-regulation of the transcript was observed in root tips after 24 h exposure to 100 µm Al. The WMnSOD1 under the control of the CaMV35S promoter was expressed in canola (Brassica napus). Transgenic plants were phenotypically normal. Northern analyses showed enhanced levels of the WMnSOD1 and WMnSOD1-GUS transcripts and total SOD activity was 1·5- to 2·5-fold greater in transgenic plants than in wild type (WT) plants. Transgenic (T1) leaf discs showed increased retention of chlorophyll and reduced electrolyte leakage when exposed to methyl viologen (MV) as compared with WT leaf discs, suggesting that transgenic plants were more resistant to oxidative stress. When WT canola plants were exposed to aluminium (0–200 µm), inhibition of root growth, higher SOD activity and increased levels of malondialdehyde (MDA, an indicator of lipid peroxidation) were observed in roots. Aluminium-induced inhibition of root growth and accumulation of MDA was lower in homozygous transgenic plants (T2) compared with WT plants. Transgenic lines also showed lower synthesis of 1,3-β-glucans (callose, a sensitive marker for Al injury) as compared with WT. These data suggest that resistance to Al toxicity can be improved by overexpressing WMnSOD1.

Notes: Virtually all current estimates of the maximum carboxylation rate (Vcmax) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the maximum electron transport rate (Jmax) for C3 species implicitly assume an infinite CO2 transfer conductance (gi). And yet, most measurements in perennial plant species or in ageing or stressed leaves show that gi imposes a significant limitation on photosynthesis. Herein, we demonstrate that many current parameterizations of the photosynthesis model of Farquhar, von Caemmerer & Berry (Planta 149, 78–90, 1980) based on the leaf intercellular CO2 concentration (Ci) are incorrect for leaves where gi limits photosynthesis. We show how conventional A–Ci curve (net CO2 assimilation rate of a leaf –An– as a function of Ci) fitting methods which rely on a rectangular hyperbola model under the assumption of infinite gi can significantly underestimate Vcmax for such leaves. Alternative parameterizations of the conventional method based on a single, apparent Michaelis–Menten constant for CO2 evaluated at Ci[Km(CO2)i] used for all C3 plants are also not acceptable since the relationship between Vcmax and gi is not conserved among species. We present an alternative A–Ci curve fitting method that accounts for gi through a non-rectangular hyperbola version of the model of Farquhar et al. (1980). Simulated and real examples are used to demonstrate how this new approach eliminates the errors of the conventional A–Ci curve fitting method and provides Vcmax estimates that are virtually insensitive to gi. Finally, we show how the new A–Ci curve fitting method can be used to estimate the value of the kinetic constants of Rubisco in vivo is presented