Early effects of excess cadmium uptake in Phaseolus vulgaris

Abstract. Seedlings of Phaseolus vulgaris were exposed to solutions containing Cd²⁺ in the range 0 to 1 molm⁻³. Ethylene formation started following 3 h of exposure to 10⁻², 10⁻¹ and 1 mol m⁻³ Cd²⁺, peaked at 18 h and returned to a relatively low rate after 24 h. Cadmium-induced ethylene formation depended on the formation of 1-aminocyclopropane-1-carboxylic acid (ACC). Aminoethoxyvinylglycine (AVG, 0.1 mol m⁻³) inhibited ACC accumulation and ethylene production during exposure to 0.2 mol m⁻³ Cd²⁺.
Activity of soluble and ionically-bound peroxidase increased after 18 h of exposure to Cd²⁺ concentrations above 10⁻³ mol m⁻³ due to an increase in activity of cathodic isoperoxidases. Stimulation of soluble and ionically-bound peroxidase by 0.2 mol m⁻³ Cd²⁺ was reduced in the presence of 0.1 mol m⁻³ AVG.
Accumulation of soluble and insoluble ('ligninlike') phenolics was found in plants exposed to Cd²⁺ (10⁻² mol m⁻³ or above) in the presence or absence of AVG. Deposition of insoluble (autofluorescing) material occurred in cell walls around vessels and was associated with reduced expansion and water content of leaves.     

Further Evidence for the Relationship between Light-Induced Changes of Plasmalemma Transport and Transthylakoid Proton Uptake

The simultaneous measurement of the induction curves of chlorophyllfluorescence, its responses to saturating flashes, light-scatteringat 532 nm, and plasmalemma voltage supports previous findings(Hansen, Kolbowski, and Dau, 1987), that light-induced uptakeof protons into the inner thylakoid space causes the rapid (5to 20 s) light-induced depolarization at the plasmalemma viasubstrate depletion of the electrogenic H⁺-pump. These conclusionsare based on kinetic studies which enable the separation ofindividual components in complex signals by means of their assignmentto different time-constants. In contrast to the previous investigation,binary noise was used for modulation of the actinic light. Thenew input signal not only increased the reliability of the previousresults obtained by sine-waves, but also led to the detectionof three additional time-constants. One of these is probablyrelated to the action of light on the potassium channel of theplasmalemma. The others are assigned to the quencher Q and toa still unknown process. Key words: Chlorophyll fluorescence, plasmalemma potential, proton fluxes, noise, scattering, spinach, state-transitions, thylakoid membrane     

Heat-induced electrical signals affect cytoplasmic and apoplastic pH as well as photosynthesis during propagation through the maize leaf

Combining measurements of electric potential and pH with such of chlorophyll fluorescence and leaf gas exchange showed heat stimulation to evoke an electrical signal (propagation speed: 3–5 mm s⁻¹) that travelled through the leaf while reducing the net CO₂ uptake rate and the photochemical quantum yield of both photosystems (PS). Two-dimensional imaging analysis of the chlorophyll fluorescence signal of PS II revealed that the yield reduction spread basipetally via the veins through the leaf at a speed of 1.6 ± 0.3 mm s⁻¹ while the propagation speed in the intervein region was c. 50 times slower. Propagation of the signal through the veins was confirmed because PS I, which is present in the bundle sheath cells around the leaf vessels, was affected first. Hence, spreading of the signal along the veins represents a path with higher travelling speed than within the intervein region of the leaf lamina. Upon the electrical signal, cytoplasmic pH decreased transiently from 7.0 to 6.4, while apoplastic pH increased transiently from 4.5 to 5.2. Moreover, photochemical quantum yield of isolated chloroplasts was strongly affected by pH changes in the surrounding medium, indicating a putative direct influence of electrical signalling via changes of cytosolic pH on leaf photosynthesis.     

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     

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.

     

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.