Gene flow inAgrostis tenuis (Poaceae)

The copper tolerance of both adult plants and their seedlings, ofAgrostis tenuis from sites within and outside, but neighbouring the Parys Mountain mine (North Wales) were studied. Intensive sampling was carried out along a line starting from the mine boundaries to a distance of five miles away in the direction of the prevailing wind, and of two miles in an upwind direction. The test have shown that mine populations are very tolerant. In outside populations on soils with reduced of very low copper content the mean tolerance level of the seedlings is considerably higher than that of corresponding adult plants, while both show a gradual decrease in tolerance with distance from the mine. The gene flow moves in the direction of the prevailing wind transfering tolerant characters from the mine to the normal populations. The selective pressures on the toxic soil of the mine are quite strong and permit ± tolerant plants only, while on normal soils these pressures are not so severe, so that populations may be rather heterogenous regarding their tolerance towards copper.     

Behaviour of Agrostis tenuis Populations against Copper Ions in Combination with the Absence of some Macro-Nutrient elements

The influence of the toxicity of copper, connected with the absence of a number of macro-elements, was studied in two different populations of Agrostis tenuis Sibth, The tolerant mine population was adapted in contrast to the non-tolerant population to a toxic (heavy metals), acid and low pH soil. The comparison of these two different populations has shown an increase in tolerance in the absence of macro-nutrient elements in the tolerant population. It was thus concluded that the adaptation of the mine populations to the adverse conditions of their surroundings results in their being tolerant not only to the toxicity of the existing metals but also to other unfavourable edaphic factors to which the non-tolerant populations show but minor resistance. As a result, the tolerant populations can withstand longer the selective pressure of their immediate surroundings, to which the non-tolerant populations succumb.     

Selection for tolerance to copper during pollen formation in Mimulus guttatus Fischer ex DC

In Mimulus guttatus, copper tolerance is determined largely by a single gene and is expressed in both the sporophyte and microgametophyte. This study explores the extent to which selection during pollen formation affects copper tolerance in the sporophytic generation. Two sets of plants heterozygous for copper tolerance, produced by reciprocal crosses between different copper-tolerant or sensitive families, and the plant on which the original observations were based, were cloned and grown in control or copper-supplemented solutions. Pollen viability and the number of tolerant progeny produced in backcrosses to sensitive plants were compared. In addition, the effect of copper treatment on pollen viability in vitro was compared for plants tolerant, sensitive and heterozygous for copper tolerance. The extent to which in vitro pollen viability decreased in response to copper treatment corresponded to the copper tolerance of the pollen source. When grown with added copper, four of the five plants showed significant reductions in pollen viability, ranging from 18% to 48% of control values. The reductions in pollen viability were correlated with an increase in tolerant progeny (r= 0.679, p=0.004). Increases in tolerant progeny could be large, ranging from 119% to 170% of that of controls, but were usually smaller than was predicted from the reductions in viable pollen. In addition, plants derived from reciprocal crosses differed significantly in the extent to which pollen viability was decreased and sporophytic tolerance increased. Thus, while selection during pollen formation could increase sporophytic tolerance, sporophytic factors, perhaps including cytoplasmic or epigenetic ones, moderated the effectiveness of pollen selection for copper tolerance.     

Heavy metal tolerance in Festuca ovina L. from contaminated sites in the Eifel Mountains,Germany

Extremely high degrees of lead tolerance, measured by comparing rates of root extension in culture solutions, are reported from populations of Festuca ovina growing at two lead-mining sites (Westschacht and Keldenich-II) near Mechernich in the Eifel Mountains, Germany. Other populations from nearby heavy metal-contaminated areas show a considerably smaller degree of lead tolerance. Samples of Festuca ovina collected in the field at Westschacht and Keldenich-II contain higher levels of lead in their aerial organs than do those from other lead-contaminated sites. The main soil factor determining the high degree of lead tolerance is the high Pb/Ca ratio. Populations from soils with a low Pb/Ca ratio display a very low degree of tolerance. It is therefore concluded that in Westschacht and Keldenich-II plants, a genuine intracellular tolerance mechanism is present, allowing the accumulation of lead in aerial organs.Leaf samples of zinc-tolerant Festuca contain higher levels of zinc than do samples of non-tolerant plants. Lead and zinc amounts in leaves are correlated with the soil ratios of Pb/Ca and Zn/Ca, respectively, rather than with the absolute soil-metal levels.In a slightly lead-tolerant, but highly zinc-tolerant clone of Festuca ovina from a site contaminated with large amounts of lead and zinc (Plombières), lead was found to be the major factor affecting the inhibition of root extension with combined treatments of lead and zinc in culture solutions. In the highly lead-tolerant, zinc-sensitive population from Westschacht, zinc governs the response of root growth to combinations of the two metals. The results are discussed in terms of discriminating distinct types of heavy-metal tolerance.     

Zinc and copper effects on metal-tolerant and non-tolerant clones ofAgrostis tenuis (Poaceae)

The toxicity of various Cu and Zn concentrations, as well as their mutual influence upon tolerant and non-tolerant clones ofAgrostis tenuis originating from soils of different metal content has been studied. Specific resistance has been clearly established. Co-existence of toxic metals causes an increase in toxicity. Uptake of metals (Cu and Zn) obviously occurs rather independently, and the toxic activity of the one is not affected competitively by the presence of the other. Above normal concentrations of Cu and Zn negatively affect cell division and growth in roots of non-tolerant genotypes. There is less ability to store metals than in tolerant genotypes.     

On the evolution of heavy-metal tolerant populations in Armeria maritima: evidence from allozyme variation and reproductive barriers

The process of ecological differentiation leading to the evolution of heavy-metal tolerant populations in Armeria maritima was studied by comparing population genetic structure and pattern of gene flow between populations growing on heavy-metal contaminated against non-contaminated sites using allozyme markers. In addition the evolution of reproductive isolation among populations was studied by measuring pollen fertility in interpopulational hybrids. The allozyme data suggested that in A. maritima multiple independent evolutionary origins of heavy-metal tolerant populations have occurred in the absence of strong genetic bottlenecks. The pattern of gene flow among populations was consistent with the model of isolation by distance with considerable gene flow between neighbor populations, and no reduction of gene flow between tolerant and non-tolerant populations. Hence it appears that substantial gene flow has not hampered genetic differentiation, probably because of the high selection pressure for heavy-metal tolerance. The pattern of reproductive isolation among populations suggests that evolution of heavy-metal tolerant populations has not triggered the development of reproductive barriers against non-tolerant populations. However, partial reproductive isolation has occurred under geographic separation.     

Genetic basis of Cd tolerance and hyperaccumulation in Arabidopsis halleri

The genetic basis of Cd tolerance and hyperaccumulation was investigated in Arabidopsis halleri. The study was conducted in hydroponic culture with a backcross progeny, derived from a cross between A. halleri and a non-tolerant and non-accumulating related species Arabidopsis lyrata ssp. petraea, as well as with the parents of the backcross. The backcross progeny segregates for both cadmium (Cd) tolerance and accumulation. The results support that (i) Cd tolerance may be governed by more than one major gene, (ii) Cd tolerance and Cd accumulation are independent characters, (iii) Cd and Zn tolerances co-segregate suggesting that they are under pleiotropic genetic control, at least to a certain degree, (iv) the same result was obtained for Cd and Zn accumulation.     

In vitro selection and characterization of Ni-tolerant callus lines of Setaria italica L

Nickel tolerant callus lines of Setaria italica L. were developed from callus cultures grown on MS medium supplemented with 0.5 mg·dm⁻³ kinetin+2.0 mg·dm⁻³ 2,4-D+2.0 mg·dm⁻³ Ni⁺². Standard growth parameters such as callus fresh and dry weight, growth tolerance index were used as indicators of nickel toxicity. Measurements as early as 2 weeks after the beginning of the treatments did not yield consistent results. However, growth tolerance index at 4, and 8 weeks after the beginning of treatments yielded significant differences among the non-tolerant and tolerant calli. The tolerant calli has enhanced growth at 2.0 mg·dm⁻³ Ni⁺² while non-tolerant calli showed a reverse trend in growth in the presence of 2.0–2.5 mg·dm⁻³ of nickel. The tolerant calli differentiated into mass of embryogenic calli within 4 weeks of culture which could be maintained for prolonged period without loss of regenerative capacity.     

Polymorphism and physiology of arsenate tolerance in Holcus lanatus L. from an uncontaminated site

The polymorphism of arsenate tolerance in a Holcus lanatus L. population from an uncontaminated soil was investigated and a high percentage of tolerant individuals (65%) was found in the population studied. Influx of arsenate was highly correlated to arsenate tolerance within the population, with the most tolerant individuals having the lowest rates of arsenate influx. Isotherms for the high affinity arsenate uptake systems were determined in six tolerant and six non-tolerant genotypes. Tolerant plants had the lowest rates of arsenate influx. This was achieved by adaptation of the V_max of arsenate influx with the V_max of the high affinity uptake system saturating at lower substrate concentrations in the tolerant plants. The polymorphism is discussed with relation to adaptation to the extreme environments to which the plants are subjected on mine-spoil soils.     

Phytochelatin is not a primary factor in determining copper tolerance

Phytochelatin (PC) is involved in the detoxification of harmful, non-essential heavy metals and the homeostasis of essential heavy metals in plants. Its synthesis can be induced by either cadmium (Cd) or copper (Cu), and can form stable complexes with either element. This might suggest that PC has an important role in determining plant tolerance to both. However, this is not clearly apparent, as evidenced by a PC-deficient and Cd-sensitiveArabidopsis mutant (cad1-3) that shows no significant increase in its sensitivity to copper. Therefore, we investigated whether the mechanism for Cu tolerance differed from that for Cd by analyzing copper sensitivity in Cd-tolerant transgenics and Cd-sensitive mutants ofArabidopsis. Cadmium-tolerant transgenic plants that over-expressedA. thaliana phytochelatin synthase 1 (AtPCS1) were not tolerant of copper stress, thereby supporting the hypothesis that PC is not primarily involved in this tolerance mechanism. We also investigated Cu tolerance incad2-1, a Cd-sensitive and glutathione (GSH)-deficientArabidopsis mutant. Paradoxically,cad2-1 was more resistant to copper stress than were wild-type plants. This was likely due to the high level of cysteine present in that mutant. However, when the growth medium was supplemented with cysteine, the wild types also exhibited copper tolerance. Moreover,Saccharomyces cerevisiae that expressedAtPCS1 showed tolerance to Cd but hypersensitivity to Cu. All these results indicate that PC is not a major factor in determining copper tolerance in plants.     

PCR based fingerprinting of Westiellopsis cultures with short tandemly repeated repetitive (STRR) and highly iterated palindrome (HIP) sequences

The presence of repetitive DNA sequences viz., short tandemly repeated repetitive (STRR) and highly iterated palindrome (HIP), in the cyanobacterial genome were used to generate a PCR-based fingerprint pattern of nine cyanobacterial cultures (both stress tolerant and non-tolerant), belonging to the genus Westiellopsis. By this method it was possible to generate distinguishing fingerprint patterns for all the isolates and cluster isolates with similar stress tolerance properties. This study reveals the utility of repetitive DNA sequences in the cyanobacterial genome, for differentiation of Westiellopsis cultures and clustering strains that posses similar stress tolerance properties.     

Root exudation and Fe uptake and transport in wheat genotypes differing in tolerance to Zn deficiency

Tolerance to Zn deficiency in wheat germplasm may be inversely related to uptake and transport of Fe to shoots. The present study examined eight bread (Triticum aestivum) and two durum (T. turgidum L. conv. durum) wheat genotypes for their capacity to take up and transport Fe when grown under either Fe or Zn deficiency. Bread wheat genotypes Aroona, Excalibur and Stilleto showed tolerance to Zn and Fe deficiency, while durum wheat genotypes are clearly less tolerant to either deficiency. Roots of bread wheats tolerant to Zn deficiency exuded more phytosiderophores than sensitive bread and durum genotypes. Greater amounts of phytosideophores were exuded by roots grown under Fe than Zn deficiency. A relatively poor relationship existed between phytosiderophore exudation or the Fe uptake rate and relative shoot growth under Fe deficiency. At advanced stages of Zn deficiency, genotypes tolerant to Zn deficiency (Aroona and Stilleto) had a greater rate of Fe uptake than other genotypes. Zinc deficiency depressed the rate of Fe transport to shoots in all genotypes in early stages, while advanced Zn deficiency had the opposite effect. Compared with Zn-sufficient plants, 17-day-old Zn-deficient plants of genotypes tolerant to Zn deficiency had a lower rate of Fe transport to shoots, while genotypes sensitive to Zn deficiency (Durati, Yallaroi) had the Fe transport rate increased by Zn deficiency. A proportion of total amount of Fe taken up that was transported to shoots increased with duration of either Fe or Zn deficiency. It is concluded that greater tolerance to Zn deficiency among wheat genotypes is associated with the increased exudation of phytosiderophores, an increased Fe uptake rate and decreased transport of Fe to shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Increase of glutathione in mine population of Sedum alfredii: a Zn hyperaccumulator and Pb accumulator

Phytochelatins (PCs) have been induced in a large range of plant species, but their role in heavy metal tolerance is unclear. Sedum alfredii is a new zinc (Zn) hyperaccumulator and lead (Pb) accumulator found in an old Pb/Zn mine in the Zhejiang Province of China. Until now, the mechanisms of its hyperaccumulation/accumulation and tolerance were poorly understood. The aim of this work was to investigate whether PCs were differentially produced in mine populations of S. alfredii compared with a non-mine control of the same species. The results showed that plants from the mine site were more tolerant to increasing Zn and Pb concentrations than those from the control site. No PCs and cysteine (Cys) were detected by pre-column derivatization with HPLC fluorescence in any tissues of two populations at any treatment, which in turn indicated they were not responsible for Zn and Pb tolerance in the mine population. Instead, Zn and Pb treatments resulted in the increase of glutathione (GSH) for both populations in a tissue-dependent manner. Significant increases were observed in leaf, stem and root tissues of plants grown on the mine site. The results suggest that GSH, rather man PCs, may be involved in Zn and Pb transport, hyperaccumulation/accumulation and tolerance in mine population of S. alfredii.     

The parallel expression of metal tolerance in pollen and sporophytes of Silene dioica (L.) Clairv., S. alba (mill.) krause and Mimulus guttatus DC

Summary The purpose of this paper was to determine if heavy metal tolerance was expressed in pollen and if its expression was correlated with the tolerance of the pollen source. Clones of Silene dioica, tolerant to zinc, closely related but nontolerant S. alba and clones of Mimulus guttatus tolerant and sensitive to copper were grown in the greenhouse in either standard potting soil or nutrient culture. Pollen was collected shortly after dehiscence, hydrated, and tested over a broad range of metal concentrations. The tolerance of the pollen source was determined by comparing root growth in solutions with and without heavy metals. In both Silene species and M. guttatus, the tolerance of the parental clone was expressed in its pollen. Pollen from tolerant individuals was able to germinate and grow at concentrations of metals which markedly inhibited pollen from nontolerants.     

Physiological responses in two populations of Andropogon glomeratus Walter B.S.P. to short-term salinity

Summary Andropogon glomeratus is a C₄ nonhalophytic grass which exhibits population differentiation for tolerance to short-term salinity exposure. To investigate possible physiological mechanisms whch enable salt-tolerant individuals to survive short-term inundation, gas exchange and water relations parameters were measured before and during a 5-day watering treatment of half-strength synthetic seawater in plants from a tolerant and a non-tolerant population. Photosynthetic recovery was followed for 10 days after the salinity treatment. Photosynthetic CO₂ uptake was substantially inhibited in both populations. Stomatal conductances decreased and intercellular CO₂ concentrations increased, indicating non-stomatal factors were primarily responsible for the decrease in CO₂ uptake. After termination of the salinity treatment photosynthetic capacity increased more rapidly in the tolerant population and reached the pretreatment level after 6 days, whereas the nontolerant population did not recover fully after 10 days. A-Ci curves measured before and after the salinity treatment indicated a decrease in the carboxylation efficiency, and suggested a proportionately greater metabolic inhibition relative to the increase in the stomatal limitation. Osmotic adjustment occurred in a 2-day period in the tolerant population, but there was no change in the osmotic potentials or the water potential at the point of turgor loss in the nontolerant population. Thus short-term salt tolerance in the marsh population is associated with rapid osmotic adjustment and recovcry of photosynthetic capacity shortly after the end of the salinity exposure, rather than maintenance of greater photosynthesis during the salinity treatment.     

Tolerance and bioaccumulation of copper in Phragmites australis and Zea mays

The effects of copper on the growth, tolerance indices, mineral composition (N, P, K, Fe, Zn and Mn) and metal uptake of reed (Phragmites australis [Cav. Trin. ex Steudel]) and maize (Zea mays L.) were investigated in hydroponic experiments at copper concentrations ranging from 0.5 to 157 M Cu. A reduction in root length was shown to be a good indicator of copper toxicity, concentrations of 15.7 and 78.7 M Cu inhibiting root growth in maize and reed, respectively. The reed was significantly more tolerant of copper than maize and at 7.85 M Cu (external concentration), reed can be described as a Cu tolerant plant, and maize as a Cu non-tolerant species. As a result of Cu toxicity, the concentrations of macronutrients N, P and K decreased in both shoot and root of maize, while the concentrations were hardly affected in reed tissues. Fe concentration increased in shoots and roots of maize and in roots of reed with increasing Cu treatments, leading to highly significant (p<0.01) linear relationships between tissue Fe and Cu concentrations. The bioconcentration factor (BCF) of Cu was higher in roots than in shoots of both plant species, ranging from 612 to 1592 in reed for the Cu treatments tested. In the roots of maize, BCF of Cu increased from 349 to 1931 when increasing Cu in nutrient solution from 7.85 M to 78.5 M. Therefore, reed could be useful in wastewater treatments for the removal of Cu. However, the use of reed in phytoextraction of Cu from contaminated soils is limited by the low accumulation rate in shoots and although reed can be more efficient than maize for Cu phytoextraction, harvesting the full biomass, including roots, may be required.     

Relationship between plant phosphorus status and the kinetics of arsenate influx in clones ofdeschampsia cespitosa (L.) beauv. that differ in their tolerance to arsenate

Uptake kinetics of arsenate were determined in arsenate tolerant and non-tolerant clones of the grassDeschampsia cespitosa under differing root phosphorus status to investigate the mechanism controlling the suppression of arsenate influx observed in tolerant clones. Influx was always lower in tolerants compared to non-tolerants. Short term influx of arsenate by the high affinity uptake system in both tolerant clones was relatively insensitive to root phosphorus status. This was in contrast to the literature where the regulation of the phosphate (arsenate) uptake system is normally much more responsive to plant phosphorus status. The low affinity uptake system in both tolerant and non-tolerant clones, unlike the high affinity uptake system, was more closely regulated by root phosphate status and was repressed to a much greater degree under increasing root phosphorus levels than the high affinity system.     

In vitro selection for 2,4-D tolerance in red clover

Summary In vitro, selection is a viable method of selecting herbicide-tolerant crops. This research was to evaluate in vitro selection techniques for enhancing 2,4-D [(2,4-dichlorophenoxy) acetic acid] tolerance in red clover (Trifolium pratense L.). In vivo and in vitro responses to 2,4-D of eight diverse red clover populations were correlated (r=0.77), justifying in vitro selection for 2,4-D tolerance. Suspension cultures of a red clover genotype capable of regeneration were plated onto agar-based nutrient media supplemented with 0.18 mM 2,4-D for selection experiments. After two cycles of selection, 16 2,4-D tolerant callus lines were identified based on visual growth assessment. These lines were evaluated for 2,4-D tolerance (based on 2,4-D content), using a 2,4-D bioassay procedure which consisted of placing selected callus tissue pieces on top of oat (Avena sativa L.) coleoptile or internode sections. The relative amount of 2,4-D in the callus tissue was estimated by the amount of oat section elongation after 24 h. Two of the more tolerant callus lines had 61% and 83% less 2,4-D in their tissues than the susceptible control tissue. These studies indicated that in vitro selection can enhance the levels of 2,4-D tolerance in red clover callus tissue.Florida Agricultural Experiment Station Journal Series No. 8943     

Differential antioxidant enzyme and thiol responses of tolerant and non-tolerant clones of <Emphasis Type="Italic">Chloris barbata</Emphasis> to cadmium-stress

Tolerant and non-tolerant clones of Chloris barbata Sw. obtained, respectively, from an erstwhile mercury contaminated solid waste dump site near a chloralkali plant and a non-contaminated (control) site were subjected to cadmium-stress by growing the rooted cuttings in water containing CdSO₄, 13 and 130 μM. Differences between the two clones in their response to cadmium-stress were noted in root growth, and also with respect to certain biochemical parameters. Whereas catalase activity decreased and non protein-thiol levels increased in the non-tolerant clone, the level of protein-thiol alone increased significantly in the tolerant clone in response to cadmium-stress. No remarkable differences between the clones, however, were noted with respect to total soluble protein, peroxidase activity and lipid peroxidation. Remarkably the two clones responded differently to buthionine sulfoximine, an inhibitor of glutathione and/or phytochelatin synthesis, which inhibited root growth significantly in non-tolerant clone but not in the tolerant clone. Buthionine sulfoximine, nonetheless, could potentate cadmium toxicity in either of the clones, but more effectively in the tolerant clone. The high sensitivity of tolerant-clone to the combined treatment of BSO and Cd in the present study could, therefore, be attributed to the cumulative oxidative stress generated synergistically by BSO and Cd.