Root porosity, radial oxygen loss and iron plaque on roots of wetland plants in relation to zinc tolerance and accumulation

Background and aims

Wetland plants have been widely used in constructed wetlands for the clean-up of metal-contaminated waters. This study investigated the relationship between rate of radial oxygen loss (ROL), root porosity, Zn uptake and tolerance, Fe plaque formation in wetland plants.

Methods

A hydroponic experiment and a pot trial with Zn-contaminated soil were conducted to apply different Zn level treatments to various emergent wetland plants.

Results

Significant differences were found between plants in their root porosities, rates of ROL, Zn uptake and Zn tolerance indices in the hydroponic experiment, and concentrations of Fe and Mn on roots and in the rhizosphere in the pot trial. There were significant positive correlations between root porosities, ROL rates, Zn tolerance, Zn, Fe and Mn concentrations on roots and in the rhizosphere. Wetland plants with higher root porosities and ROL tended to have more Fe plaque, higher Zn concentrations on roots and in their rhizospheres, and were more tolerant of Zn toxicity.

Conclusions

Our results suggest that ROL and root porosity play very important roles in Fe plaque formation, Zn uptake and tolerance, and are useful criteria for selecting wetland plants for the phytoremediation of Zn-contaminated waters and soils/sediments.     

Enumeration of Fe(II)-oxidizing and Fe(III)-reducing bacteria in the root zone of wetland plants: Implications for a rhizosphere iron cycle

Iron plaque occurs on the roots of most wetland and submersed aquatic plant species and is a large pool of oxidized Fe(III) in some environments. Because plaque formation in wetlands with circumneutral pH has been largely assumed to be an abiotic process, no systematic effort has been made to describe plaque-associated microbial communities or their role in plaque deposition. We hypothesized that Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB) are abundant in the rhizosphere of wetland plants across a wide range of biogeochemical environments. In a survey of 13 wetland and aquatic habitats in the Mid-Atlantic region, FeOB were present in the rhizosphere of 92% of the plant specimens collected (n = 37), representing 25 plant species. In a subsequent study at six of these sites, bacterial abundances were determined in the rhizosphere and bulk soil using the most probable number technique. The soil had significantly more total bacteria than the roots on a dry mass basis (1.4 × 10⁹ cells/g soil vs. 8.6 × 10⁷ cells/g root; p < 0.05). The absolute abundance of aerobic, lithotrophic FeOB was higher in the soil than in the rhizosphere (3.7 × 10⁶/g soil vs. 5.9 × 10⁵/g root; p < 0.05), but there was no statistical difference between these habitats in terms of relative abundance (1% of the total cell number). In the rhizosphere, FeRB accounted for an average of 12% of all bacterial cells while in the soil they accounted for < 1% of the total bacteria. We concluded that FeOB are ubiquitous and abundant in wetland ecosystems, and that FeRB are dominant members of the rhizosphere microbial community. These observations provide a strong rationale for quantifying the contribution of FeOB to rhizosphere Fe(II) oxidation rates, and investigating the combined role of FeOB and FeRB in a rhizosphere iron cycle.     

湿地植物根表铁膜研究进展

为了适应渍水环境,许多湿地植物都具有根系泌氧、形成铁膜的能力。因铁膜具有特殊的物理或化学结构,可以通过吸附和共沉淀作用影响元素在土壤中的化学行为和生物有效性,在植物吸收营养元素和重金属中起重要作用。综述了湿地植物根表铁膜的形成、影响因素以及根表铁膜对营养元素和重金属的生态环境效应,从表征技术方面阐述了根表铁膜的作用机制。对今后的研究方向给出如下建议:(1)扩大研究领域;(2)铁膜形成的动态变化过程;(3)铁膜对植物生理形态的影响;(4)利用先进的表征技术以确定铁膜的作用机制。     

Interactions among Fe2+, S2-, and Zn2+ tolerance, root anatomy, and radial oxygen loss in mangrove plants

Root anatomy, radial oxygen loss (ROL), and tolerances to ferrous (Fe(2+)), sulphide (S(2-)), and zinc (Zn(2+)) ions were investigated in seedlings of eight species of mangrove, including three pioneer species, three rhizophoraceous and two landward semi-mangrove species. The results showed an interesting co-tolerance to Fe(2+), S(2-), and Zn(2+). The three rhizophoraceous species (Bruguiera gymnorrhiza, Kandelia obovata and Rhizophora stylosa), which possessed the thickest lignified exodermis and the 'tightest barrier' in ROL spatial pattern, consistently exhibited the highest tolerance to Fe(2+), S(2-), and Zn(2+). B. gymnorrhiza could directly reduce ROL by increasing lignification within the exodermis. Such an induced barrier to ROL is a probable defence response to prevent further invasion and spread of toxins within plants. The data also indicated that, in B. gymnorrhiza, Fe(2+) or S(2-), or both, induced a lignified exodermis that delayed the entry of Zn(2+) into the roots and thereby contributed to a higher tolerance to Zn(2+). This study provides new evidence of exclusive strategies of mangrove seedling roots in dealing with contaminations. The information is also important in the selection and cultivation of tolerant species for the bioremediation of contaminated waters or soils.     

Responses of plants to iron,zinc and copper deficiencies

Iron, copper and zinc are essential metals for cell metabolism. Plants have evolved different schemes to efficiently mobilize low-solubility nutrients such as metals from their environment and to transport them between organs. In this review we highlight the divergences and convergences of the iron, copper and zinc uptake, transport and homoeostatic pathways.     

根系活动对湿地植物根际铁异化还原的影响及机制研究进展

根系活动是影响湿地植物根际铁异化还原速率的关键因素之一。以往国内外湿地铁异化还原的研究多为分析和比较各类中宏观生境中铁异化还原能力的差异。近年来,湿地植物根际微域铁的生物地球化学行为也日益成为该领域的研究热点。综述了根际铁异化还原研究概况,梳理了根系活动对根际铁异化还原关键因子的作用机制,分析了根际铁异化还原和其他有机质代谢途径的竞争关系,探讨了根际铁异化还原对根系活动动态变化和异质性的响应,提出了根际铁异化还原的概念模型,并指出了未来我国湿地植物根际铁异化还原研究应加强的工作。     

Dynamic process,spatial pattern and species coexistence in plants

The significance of dynamic processes of individual genets/ramets for the spatial pattern of plant species and community structure is discussed. It is suggested that under a different mode of competition (symmetric vs. asymmetric), spatial distribution of individuals, initial size distribution at the establishment stage and boundary conditions as recruitment influence differently the species coexistence pattern. It is therefore important to consider the mode of competition for the study of community structure. To know the mode and degree of intra- and interspecific competition, the dynamic processes of individual genets/ramets must be studied by following the growth, mortality and recruitment of each genet/ramet of each component species in a plant community. The models and methods of plant population ecology are therefore useful also for plant community ecology.     

Effect of wastewater discharge on root anatomy and radial oxygen loss (ROL) patterns of three mangrove species in southern China

The effects of wastewater discharge on radial oxygen loss (ROL) and root anatomy varied among mangrove species. ROL of Bruguiera gymnorrhiza (L) increased from 22.44 ng cm(-2) min(-1) in the control (just fresh water) to 31.09 ng cm(-2) min(-1) when received normal wastewater (NW) and to 44.22 ng cm(-2) min(-1) when treated with strong wastewater (10NW). However, discharge of both NW and 10NW caused 28% decreases of ROL in the root tip of Excoecaria agallocha L., and the decreases in Acanthus ilicifolius L were even more significant, with 45% when treated by 10NW The changes of ROL were related to the root anatomy. Among three species, A. ilicifolius had the highest proportional cross-sectional area of aerenchyma air spaces, suggesting that the internal oxygen transfer to root tip was the fastest. However, the area of aerenchyma air spaces in the root tip of 10NW treated A. ilicifolius was significantly reduced while area of epidermis and hypodermis (E + H) increased leading to less oxygen supply to root tip. Compared to B. gymnorrhiza and E. agallocha, the (E + H) layer of A. ilicifolius was the thinnest, and the cells without suberized walls were loosely packed in all three treatments. These results suggested that the root anatomy and ROL of B. gymnorrhiza was least affected by wastewater discharge, followed by E. agallocha, and A. ilicifolius was the most susceptible species thus was not suitable for treating strong wastewater.     

南四湖湿地景观空间格局动态演变

以山东省南四湖湿地为例,选取1987、2002、2014年3期遥感影像,应用景观格局指数、动态度、景观梯度和格网化模型等方法,对南四湖湿地景观格局的动态变化进行研究.结果表明: 1987—2014年,研究区景观蔓延度指数和聚集度指数逐渐减小,景观多样性指数和均匀度指数逐渐增大,景观类型面积趋于均匀且分布格局间断零散,人类活动对南四湖湿地景观的干扰度呈现增强趋势.研究期间,南四湖湿地总面积逐渐增加,其中,湖泊面积先减少后增加,2014年面积达到最大;湖滨池塘面积也呈现持续增加趋势,但增速减缓;河流面积基本保持稳定;沼泽面积持续减少.南四湖地区的农业活动、南四湖自然保护区的成立及南水北调工程在南四湖湿地景观格局的变化中起着重要作用.     

Fructose-induced modifications of myoglobin: Change of structure from met (Fe3+) to oxy (Fe2+) form

We studied structural modifications of metmyoglobin (Mb) after short-term (6 days) and long-term (30 days) glycation by fructose (fructation). Fructation caused gradual changes in the structure of the protein with respect to increased absorbance at 280 nm, enhanced fluorescence emission (with excitation at 285 nm), increased surface accessible tryptophan residues and reduced α-helix content and change in tertiary structure. However, long-term fructation changed Mb to oxymyoglobin (MbO2), as demonstrated by different spectroscopic (absorption, fluorescence, circular dichroic and electron paramagnetic resonance) studies and trifluoperazine-induced oxygen release experiment. Fructation appeared to modify Arg139 to arg-pyrimidine, which exhibited antioxidative activity and might be involved in the conversion of met (Fe3+) to oxy (Fe2+) form of myoglobin.     

Fe(2+)-catalyzed oxidation and cleavage of sarcoplasmic reticulum ATPase reveals Mg(2+) and Mg(2+)-ATP sites

Fe(2+) can substitute for Mg(2+) in activation of the sarcoplasmic reticulum (SR) ATPase, permitting approximately 25% activity in the presence of Ca(2+). Therefore, we used Fe(2+) to obtain information on the binding sites for Mg(2+) and the Mg(2+)-ATP complex within the enzyme structure. When the ATPase is incubated with Fe(2+) in the presence of H(2)O(2) and/or ascorbate, specific patterns of Fe(2+)-catalyzed oxidation and cleavage are observed in the SR ATPase, depending on its Ca(2+)-bound (E1-Ca(2)) or Ca(2+)-free conformation (E2-TG), as well as on the presence of ATP. The ATPase protein in the E1-Ca(2) state is cleaved efficiently by Fe(2+) with H(2)O(2) and ascorbate assistance, yielding a 70-75 kDa carboxyl end fragment. Cleavage of the ATPase protein in the E2-TG state occurs within the same region, but with a more diffuse pattern, yielding multiple fragments within the 65-85 kDa range. When Fe(2+) catalysis is assisted by ascorbate only (in the absence of H(2)O(2)), cleavage at the same protein site occurs much more slowly, and is facilitated by ATP (or AMP-PNP) and Ca(2+). Amino acid sequencing indicates that protein cleavage occurs at and near Ser346, and is attributed to Fe(2+) bound to a primary Mg(2+) site near Ser346 and neighboring Glu696. In addition, incubation with Fe(2+) and ascorbate produces Ca(2+)- and ATP-dependent oxidation of the Thr441 side chain, as demonstrated by NaB(3)H(4) incorporation and analysis of fragments obtained by extensive trypsin digestion. This oxidation is attributed to bound Fe(2+)-ATP complex, as shown by structural modeling of the Mg(2+)-ATP complex at the substrate site.     

The relation of root systems to shoot systems in vascular plants

The roots and shoots of vascular plants may be positionally and developmentally related in various ways. However, botanical teaching and research are strongly influenced by the paradigmatic annual dicotyledon, whose bipolar embryo develops into a plant with root and shoot meeting only at the hypocotyl. In 1930 Goebel criticized this example as a general model for plants, proposing instead the opposed concepts “allorhizy” (referring to plants whose root and shoot are related as above) and “homorhizy”(referring to plants without a bipolar embryo, all of whose roots are shoot-borne, e.g., pteridophytes). Goebel’s approach permeates the extensive German morphological literature, but has been virtually ignored in English-language literature. The allorhizy/homorhizy dichotomy has proved heuristic. However, it suggests a correlation between embryo type and mature morphology that does not always hold. Furthermore, it does not take into account the root-borne shoots typical of many plant species. Finally, Goebel’s presentation of the terms (which he does not explicitly define) creates ambiguity as to whether they designate structural concepts or the attributes of evolutionary groups. The alternative proposed here is a structural analysis of the possible topological relationships among root and shoot systems. Each structural class is then considered with regard to embryo types, potential for clonal growth and other ecological correlates, and phylogenetic distribution. This approach provides both a test of Goebel’s concepts and a basis for further comparative study of wholeplant form.     

Similarity and diversity in adventitious root anatomy as related to root aeration among a range of wetland and dryland grass species

Assessments of the anatomy, porosity and profiles of radial O₂ loss from adventitious roots of 10 species in the Poaceae (from four subfamilies) and two species in the Cyperaceae identified a combination of features characteristic of species that inhabit wetland environments. These include a strong barrier to radial O₂ loss in the basal regions of the adventitious roots and extensive aerenchyma formation when grown not only in stagnant but also in aerated nutrient solution. Adventitious root porosity was greater for plants grown in stagnant compared with aerated solution, for all 10 species in the Poaceae. The ‘wetland root’ archetype was best developed in Oryza sativa and the two species of the Cyperaceae, in which the stele contributed less than 5% of the root cross‐sectional area, the cells of the inner cortex were packed in a cuboidal arrangement, and aerenchyma was up to 35–52%. Variations of this root structure, in which the proportional and absolute area of stele was greater, with hexagonal arrangements of cells in the inner cortex and varying in the extent of aerenchyma formation, were present in the other wetland species from the subfamilies Pooideae, Panicoideae and Arundinoideae. Of particular interest were Vetiveria zizanoides and V. filipes, wetland grass species from the tribe Andropogoneae (the same tribe as sorghum, maize and sugarcane), that had a variant of the root anatomy found in rice. The results are promising with regard to enhancing these traits in waterlogging intolerant crops.     

A re-examination of the root cortex in wetland flowering plants with respect to aerenchyma

AIMS: We review literature and present new observations on the differences among three general patterns of aerenchyma origin and their systematic distributions among the flowering plants, and we clarify terminology on root aerenchyma. SCOPE: From our own previous works and some new observations, we have analysed the root cortex in 85 species of 41 families in 21 orders of flowering plants that typically grow in wetlands to determine the characteristic patterns of aerenchyma. FINDINGS: A developmental and structural pattern that we term expansigeny, as manifested by honeycomb aerenchyma, is characteristic of all aquatic basal angiosperms (the Nymphaeales) and basal monocots (the Acorales). Expansigenous aerenchyma develops by expansion of intercellular spaces into lacunae by cell division and cell expansion. Schizogeny and lysigeny, so often characterized in recent reviews as the only patterns of root cortex lacunar formation, are present in most wetland plants, but are clearly not present in the most basal flowering plants. CONCLUSION: We conclude that expansigeny is the basic type of aerenchyma development in roots of flowering plants and that the presence of expansigenous honeycomb aerenchyma in root cortices was fundamental to the success of the earliest flowering plants found in wetland environments.     

Effect of phosphorus,potassium and zinc fertilizers on iron toxicity in wetland rice (Oryza sativa L.)

During the period January–August 1996, an investigation was carried out in La Mata, Cotuí, Dominican Republic with the objective to study the effect of P, K and Zn fertilizers on Fe toxicity in the rice varieties JUMA-57 (sensitive to Fe toxicity), ISA-40 and PSQ-4 (both tolerant to Fe toxicity). The rate of fertilizer application was 22 and 62 kg P ha^–1; 58 and 116 kg K ha^–1; 3 and 7 kg Zn ha^–1 and a constant dose of 140 kg N ha^–1 and 40 kg S ha^–1 on all fertilized plots. The control received no fertilizer. JUMA-57 was the only variety that showed symptoms of Fe toxicity. The observed symptoms showed a yellow to orange colour. Symptoms of Fe toxicity appeared first one week after transplanting (WAT), decreased at the fourth WAT, but returned six WAT and continued until the end of the experiment. Fertilizer application reduced symptom intensity and increased grain yield in all varieties, but only JUMA-57 did not reach the maximum yield typical for that variety. Fertilizer application did not completely overcome the toxicity effect, i.e. in symptom intensity and grain yield. The positive effect of fertilizer application could not be attributed to a specific nutrient. Intensity of symptoms was not related to Fe concentration in the leaves. The average Fe concentration of 108 mg kg^–1 was not high enough to be considered toxic. Symptoms could not be explained through Mn toxicity (average Mn concentration in the leaves was 733 mg kg^–1) nor Zn deficiency (average Zn concentration in the leaves was 20 mg kg^–1). There was a clear relationship, though, between soil DTPA extractable Fe and symptom intensity or grain yield. The toxic effect was observed when the DTPA extractable Fe in the flooded soil was above 200 mg kg^–1. From these results, we concluded that the Fe toxicity resulted from high Fe in the root zone and not from high Fe concentrations in the leaves.     

苏干湖湿地植被覆盖度时空变化格局

植被覆盖度是反映群落外貌特征和影响植被生态系统稳定性的重要因子,其时空异质性演化规律研究有助于认识湿地群落的结构功能及其环境响应机制。采用湿地群落学调查和遥感技术相结合的方法,分析了苏干湖内陆盐沼湿地近30年植被覆盖度的时空变化及其影响因素。结果表明:像元二分模型在内陆盐沼湿地植被覆盖度研究方面具有较高的模拟精度;苏干湖湿地的植被覆盖度在1987—2017年间总体呈上升趋势,年际增幅为0.162%/5a,与气温和降水呈正相关关系;在空间上植被覆盖度与地下水位埋深呈正相关,与土壤全盐量呈负相关,但不同等级植被覆盖度与地下水位埋深、土壤全盐量间的相关性各有差异。苏干湖湿地植被覆盖度受地下水位埋深、土壤全盐量空间异质性的影响,呈现出斑块状镶嵌分布。     

Fluorescent pseudomonads in the rhizosphere of plants and their relation to root exudates

Fluorescent pseudomonads were present in chernozem soil not influenced by plant roots (10(3)-10(4) per g dry soil) in the rhizosphere soil of various plants (10(4)-10(5) per g soil) and on roots (10(3) to 10(7) per g fresh roots), depending on the species and age of the plant. Relative species representation of fluorescent pseudomonads changed on the roots and in the plant rhizosphere as compared with free soil. Pseudomonas fluorescens, representing 60-93% of the population of fluorescent pseudomonads predominated on the roots of all plants investigated. Somewhat different results were obtained in rhizosphere soil. Relatively higher numbers of P. fluorescens were detected in the rhizosphere soil of cucumber and maize, numbers in the rhizosphere soil of wheat were practically the same as in free soil and higher numbers of P. putida were found in the rhizosphere soil of barley. Almost all components contained in the root exudates of the plants studied, including beta-pyrazolylalanine from the root exudates of cucumbers were utilized as carbon and energy sources. Root exudates of wheat and maize were utilized by the strain P. putida K2 with an efficiency of 73-91%, depending on species and age of the plant.