Wilfried H. O. Ernst

Copper-induced Damage to the Permeability Barrier in Roots of Silene cucubalus

Summary The mechanism of copper-induced damage to cell plasma membrane functioning in intact roots of Silene cucubalus was studied by comparing the effects of high copper ion concentrations with those of Nethylmaleimide (NEM), a sulfhydryl-reagent, and of cumene hydroperoxide (CHP), an organic peroxide known to induce lipid peroxidation by free radical formation. Leakage of potassium ions from roots (a measure of cell permeability) was detected directly upon supply of copper or NEM, whereas with CHP a 45 minutes time-lag was observed. In copper or CHP treated plants, leakage was constant for at least three hours and, as demonstrated by trypan blue staining after a 24 hour exposure, the plasma membranes of root cells were severely damaged, especially in the tips. A marked increase of lipid peroxidation products was detected in roots treated with these reagents, although the increase was highest with copper. In contrast, with NEM potassium leakage diminished after about 1 hour, dye staining of root cells was negligible and the level of lipid peroxidation products did not increase. When NEM plus CHP were added, the results were the same as those obtained with high copper supply, suggesting a combined effect of thiol depletion and lipid peroxidation by copper ions. In addition, it was shown in vitro that copper ions stimulate lipid peroxidation in microsomes isolated from roots of plants grown at a normal low copper supply. These results suggest that, apart from thiol depletion, direct free radical formation leading to lipid peroxidation might play an important role in the mechanism of copper-induced damage to the permeability barrier in roots of higher plants in vivo.

Phytochelatins in Cadmium-Sensitive and Cadmium-Tolerant Silene vulgaris (Chain Length Distribution and Sulfide Incorporation)

In response to a range of Cd concentrations, the root tips of Cd-tolerant plants of Silene vulgaris exhibit a lower rate of PC production accompanied by a lower rate of longer chain PC synthesis than those of Cd-sensitive plants. At the same Cd exposure level, stable PC-Cd complexes are more rapidly formed in the roots of Cd-sensitive plants than in those of tolerant plants. At an equal PC concentration in the roots, the PC composition and the amount of sulfide incorporated per unit of PC-thiol is the same in both populations. Although these compounds might play some role in mechanisms that contribute to Cd detoxification, the ability to produce these compounds in greater amounts is not, itself, the mechanism that produces increased Cd tolerance in tolerant S. vulgaris plants.

Combination toxicology of metal-enriched soils : Physiological responses of a Zn- and Cd-resistant ecotype of Silene vulgaris on polymetallic soils

Plants of an ecotype of Silene vulgaris (Caryophyllaceae) originating from a Cd-Pb-Zn mine at Plombiere (Belgium) were grown on 15 polymetallic soils for a full life-cycle to investigate physiological responses which can help explain previously reported disorders in plants. The degree of regulation of the metal concentration in the young seedlings was a very reliable indicator of the subsequent plant performance. Uptake of Zn could be regulated up to 200 nmol water-soluble Zn g−1 dry soil without surpassing 7 μmol Zn g−1 dry leaf tissue supporting the hypothesis of a high regulating potential of Zn-resistant ecotypes. As soon as a certain ecotype-specific threshold was surpassed the Zn concentration in all plant tissues strongly increased. The Cu concentration in roots and shoots had no threshold and showed the tendency to increase near linearly with the external Cu soil concentration. Similar behaviours were found for Cd, Mn and Pb. The metal concentration of seeds was the lowest of all plant parts; nevertheless it increased linearly with increasing concentration of Fe and Zn in the soil. From visible symptoms the degree of chlorosis was positively related with the concentration of Zn, but not with that of Cu, and interrelated with Fe availability. High cyanidin concentrations in leaves were not indicating a surplus of heavy metals, but deficiency in phosphorus and to a lesser degree in nitrogen. Phytochelatins (PCs) were only present in measurable amounts in leaves of plants grown on soils rich in Cu; but PCs amounts in the early vegetative phase could not be related to vegetative and seed biomass at the reproductive stage. Therefore, it is concluded that PCs are a less reliable indicator of metal toxicity during a full life-cycle than the metal concentration of young seedlings.

Life-cycle phases of a zinc- and cadmium-resistant ecotype of Silene vulgaris in risk assessment of polymetallic mine soils

Short-term exposure of plants to heavy metals is often used for risk assessment of metal-enriched soils (OECD guideline 208) without considering the reliability of the assessment for long-term exposure, i.e. for the completion of a plants life-cycle. In the present study with 15 orogenic soils three phases of the life-cycle of a Zn-Cd-resistant ecotype of Silene vulgaris were studied to improve risk assessment of metal-enriched soils. The first phase, i.e. emergence of seedlings was not related to the water-soluble or total metal concentration of the soils. Seedling mortality was low as long as the water-soluble metal concentration did not surpass 0.15 micromol Zn and 0.04 micromol Cu g(-1) dry soil. Curtailment of the life-cycle prior to flowering, i.e. the vegetative growth as second phase, occurred on those soils where roots and shoots were heavily enriched by Zn already in the seedling phase. In the third phase, i.e. the generative phase, time to flowering and yield differences between orogenic soils were substantial, but soil metal concentrations could not be directly related to timing of reproduction or biomass. Ranking of data showed a high inconsistency of the responses to metal exposure during the first phases of the life-cycle. It is concluded that total plant mass and seed mass are the only realistic endpoints of life-cycle bioassays in risk assessment as long as ranks are inconsistent between two successive early phases of the life-cycle.

Uptake and Transport of Zinc in Zinc-sensitive and Zinc-tolerant Silene vulgaris

Summary The uptake and transport of zinc in zinc-sensitive and zinc-tolerant Silene vulgaris were compared in a short-term experiment. Although the roots of tolerant plants contained more CaCl2-exchangeable zinc, tolerance can not result from exclusion through binding of zinc to the cell wall, because the total uptake of zinc was not significantly different between the populations. Compared at equal levels of zinc injury, measured by root-length growth inhibition, the uptake of zinc was much higher in the tolerant plants. Up to the NOEC for root-length growth of sensitive plants, an equal zinc concentration of the roots was found in sensitive and tolerant plants, but at a higher supply the zinc concentration of the roots was higher in tolerant plants. In sensitive plants the zinc concentration of the shoot was higher than in tolerant ones, except after 3 and 7 days at the highest zinc supply, the percentage of zinc allocated to the shoot was higher at all external zinc concentrations. The results of a split-root experiment clearly show that zinc has a direct inhibitory effect on root growth. Zinc tolerance seems to result from an efficient sequestration of zinc inside the root cells. This probably causes a lower transport of zinc to the shoot.

Sulfur metabolism in higher plants: potential for phytoremediation.

Sulfur is a major nutrient for all organisms. Plant species have a high biodiversity in uptake, metabolization and accumulation of sulfur so that there are potentials to use plants for phytoremediation of sulfur-enriched sites. A survey of soils enriched with sulfur either naturally or by human activities shows that a surplus of sulfur is mostly accompanied with a surplus of other chemical elements which may limit phytoremediation because these co-occurring elements are more toxic to plants than sulfur. In addition, the accumulation of the other elements makes the plant material (phyto-extraction) less suitable for the use as fodder and for human consumption.

Does the seed bank contribute to the restoration of species-rich vegetation in wet dune slacks?

Abstract Questions: What is the contribution of the seed bank to restoration of species-rich vegetation in oligotrophic wet dune slacks? Does the restoration management affect the seed bank? Location: Calcareous coastal dune slacks at the west coast of The Netherlands. Methods: Species composition of the seed bank and the above-ground vegetation was sampled in dune slacks that had a variable extent of groundwater level rise in combination with either topsoil removal or mowing. Results: The seed bank had a high potential for restoration of species-rich vegetation: 60 species were found in the seed bank of which 14 were characteristic of oligotrophic, wet dune vegetation. While topsoil removal almost completely removed the seed bank, groundwater level rise did not permanently submerge the seed bank of species of oligotrophic, wet conditions. Changes in abundance in the established vegetation were unrelated to species abundance in the seed bank. Of all new species establishments in the vegetation relevés, 76% occurred where the species was not found in the seed bank. The chance that presence of a species in the seed bank led to establishment the following year was only 11%. Conclusion: The seed bank was not the dominant source for newly establishing species following the large disturbance that was induced by restoration management. Changes in species abundance after the restoration impact were not related to species abundance in the seed bank, but to ongoing succession and current year dispersal. To attain a high number of new establishments, restoration projects should preferably be planned in the proximity of refuge populations, rather than relying on the seed bank alone. Nomenclature: van der Meijden (1996).

Ultraviolet-B (280 − 320 nm) absorbing pigments in the leaves of Silene vulgaris : their role in UV-B tolerance

Summary The UV-B radiation tolerant perennial herb Silene vulgaris was tested on the influence of incident UV-B fluxes during growth on the synthesis of UV-B absorbing pigments in the leaves. Analysis of methanolic leaf extracts showed a stimulating effect of UV-B on the absorbing ability of leaf extracts. HPLC analysis made clear that UV-B radiation stimulated extractable flavonoid concentrations in leaves, but that UV-B absorption could only be partly attributed to these flavonoids. The contribution of flavonoids to UV-B absorption diminishes if plants mature. Other possible functions of flavonoids in plants growing under elevated UV-B conditions are discussed.

Allocation patterns of zinc and cadmium in heavy metal tolerant and sensitive Silene vulgaris

Summary The allocation patterns of Zn and Cd were studied in a tolerant and a sensitive ecotype of Silene vulgaris . Within mature leaves, the allocation pattern of Zn was similar to that reported earlier for Cd, with the highest concentrations found in the lower epidermis. Part of the Zn present in the leaf seemed to be bound to cell walls of the mesophyll tissue, which is different from the allocation of Cd. At low concentrations, both metals inhibited the growth of the sensitive plants, and the tolerant ecotype was also affected at higher concentrations and longer exposure times. However, Zn and Cd had different effects on morphological parameters such as root/shoot ratio and leaf thickness. The various metal concentrations found in the leaves of different ages could for the most part be explained by these effects on plant growth. It is concluded that the allocation of metals in leaves of S. vulgaris can be largely explained by the different effects of the metals on plant growth and leaf morphology, and not primarily as a specific result of tolerance mechanisms.

THE IMPACT OF ELEVATED UV-B (280-320 NM) RADIATION LEVELS ON THE REPRODUCTION BIOLOGY OF A HIGHLAND AND A LOWLAND POPULATION OF SILENE VULGARIS

A highland (altitude 1600 m) and a lowland (altitude –2 m) population of the perennial herb Silene vulgaris were tested on the effects of elevated levels of UV-B radiation on their reproductivity. Highland populations receive higher natural UV-B doses than lowland populations. Therefore adaptation to high UV-B levels of the highland population is to be expected. The lowland population showed a decrease in the number of seed producing flowers and the number of seeds produced per plant under elevated UV-B levels. The highland population increased the number of seeds per plant under elevated UV-B levels. In both populations individual seed mass as well as seed germination percentages were unaffected by the UV-B flux received by the parental plant. Possible effects of UV-B induced alterations in reproductivity on the geographical distribution of the different populations are discussed.