Compensatory growth response of the legume, Medicago sativa, to defoliation and denodulation

A laboratory study was conducted to determine the effects of defoliation and denodulation on compensatory growth of Medicago sativa (L.). Plants grown hydroponically in clear plastic growth pouches were subjected to 0 and 50% nodule pruning, and 0, 25, 50, and 75% defoliation by clipping trifoliate leaves. An additional experiment was conducted to determine if clipping leaves simulated herbivory by Hypera postica (Gyllenhal) larvae. Previously, we determined that nodule pruning accurately simulated herbivory by Sitona hispidulus (L.) larvae (Quinn & Hall, 1992). Results indicated that denodulation stimulated nodule growth and caused exact compensation in standing and total number of nodules per plant within 15 days and in standing nodule biomass within 22 days of treatment. Denodulation caused a significant reduction (13%) in final shoot biomass, but did not affect significantly final root biomass. Percentage of change in number of trifoliate leaves per plant increased with the level of defoliation. Within 22 days of treatment, total number of trifoliate leaves per plant was similar to controls. However, final standing shoot biomasses were significantly less that controls, indicating undercompensatory growth. Shoot biomasses of the 25-, 50-, and 75%-defoliated plants were 18, 20, and 36% lower than controls, respectively. Nodule biomass per plant was reduced by 24 and 32% in 50- and 75%-defoliated plants, respectively, but was not affected significantly by 25% defoliation. Root biomass was affected by all levels of defoliation. Clipping trifoliate leaves accurately simulated defoliation by H. postica larvae. Our results indicated that partial defoliation affected shoot, root, and nodule biomass of M. sativa, but that partial denodulation only affected shoot biomass.     

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

Nodule distribution on the roots of soybean and a supernodulating mutant in sand-vermiculite

The distribution of nodules of soybean (Glycine max (L.) Merr.) cultivar Bragg and the supernodulating mutant derivative nts382 was examined on the primary root relative to the first emerging lateral root, and on laterals relative to the base of the roots of plants grown in sand-vermiculite. Mutant nts382 nodulates profusely even in the presence of nitrate and appears defective in a systemic autoregulatory response that regulates nodule number in soybean. Nodules were clustered on primary roots about the first 4 cm down from the first emerging lateral root in both genotypes. Nodulation profiles showed reduced nodulation in younger and older regions of the primary root. Similarly, nodules appeared clustered close to the base of the lateral roots. Decreasing inoculum dose shifted nodule emergence to younger regions of the primary root and to lateral roots emerging in younger portions of the primary root. Our results indicate that the supernodulating mutant is able to regulate nodule number in both primary and lateral roots in the particulate matrix.     

Spontaneous nodules induce feedback suppression of nodulation in alfalfa

A small subpopulation of alfalfa (Medicago saliva L.) plants grown without fixed nitrogen can develop root nodules in the absence of Rhizobium. Cytological studies showed that these nodules were organized structures with no inter- or intracellular bacteria but with the histological characteristics of a normal indeterminate nodule. Few if any viable bacteria were recovered from the nodules after surface sterilization, and when the nodular content was used to inoculate alfalfa roots no nodulation was observed. These spontaneous nodules were formed mainly on the primary roots in the region susceptible to Rhizobium infection between 4 and 6 d after seed imbibition. Spontaneous nodules appeared as early as 10 d after germination and emerged at a rate comparable to normal nodules. The formation of spontaneous nodules on the primary root suppressed nodulation in lateral roots after inoculation with R. meliloti RCR2011. Excision of spontaneous nodules at inoculation eliminated the suppressive response. Our results indicate that the presence of Rhizobium is not required for nodule organogenesis and the elicitation of feedback regulation of nodule formation in alfalfa.Abbreviation RT root tip This work was supported by an endowment to the Racheff Chair of Excellence of the University of Tennessee, and the Soybean Promotion Board, Haskinsville, Tenn., USA. We are indebted to Noel Gerahty for performing the acetylene-reduction assays, and Dr. E.T. Graham for allowing the use of microscope facilities.     

Effects of Zn<Superscript>2+</Superscript> on nodulation and growth of a South American actinorhizal plant, <Emphasis Type="Italic">Discaria americana</Emphasis> (Rhamnaceae)

Discaria americana is a xerophytic shrub which lives in symbiosis with an actinomycete of the genus Frankia. The objective of this paper was to investigate the effects of high soil Zn²⁺ concentrations on growth and nodulation on the association Discaria americana–Frankia with the aim of determining if this association is suitable for improving contaminated soils. Two experiments were performed in 1 dm³ pots containing soil and different Zn additions, from 0 to 2,000 mg Zn²⁺ kg⁻¹ dry soil, with or without N fertilization. Zn additions strongly delayed shoot and root growth, but once growth was initiated, the biomass production of the plants supplied with moderate Zn amounts did not differ from the control plants. Zn reduced the final nodule number, but not the total nodule biomass. At the end of the experiment only the highest Zn treatments showed a lower nodule weight than the control plants, while N addition completely inhibited nodulation. It is concluded than Zn reduces the number of Frankia infections, but once the actinomycete is inside the roots, nodules can continue growing according to plant demand for N, compensating the reduced nodule number with more biomass. On the other hand, there is a toxic effect of Zn itself on plants when present in very high concentrations.     

Morphogenesis of lucerne root nodules incited by Rhizobium meliloti in the presence of combined nitrogen

Combined light and transmission electron microscopy were used to examine the effect of nitrate on the development of root nodules in lucerne (alfalfa, Medicago sativa L.) following induction by the nitrogen-fixing symbiont, Rhizobium meliloti. The timing of NO ₃ ^- addition was varied in order to study its effect on all of the recognized morphogenetic steps of nodule formation. Roots of plants inoculated in the presence of 18 mM NO ₃ ^- had straight root hairs which were devoid of adherent rhizobia and infection threads, and developed no nodules. However, nodules were formed on roots if 18 mM NO ₃ ^- was added 5 d after inoculation. At this time, the initiation of nodule primordia had already commenced in the root cortex. The histology and ultrastructure of young nodules which had developed for 5 d in the absence of NO ₃ ^- and another 5 d in the presence of 18 mM NO ₃ ^- resembled nodules developing under N-free conditions, except that in the infection threads within the infection zone of the nodule 1) some bacteria tended to loose their normal shape and gain more electron density, indicating premature degradation, and 2) the matrix of the infection threads was abnormally enlarged. In the presence of high NO ₃ ^- levels in the medium, lysis and degeneration of the bacteria released from the infection threads were observed in the infection and bacteroid zones of developing nodules, indicative of premature senescence. On the other hand, the nodule meristems continued to proliferate even after 12 d of exposure of 18 mM NO ₃ ^- . This was the only morphogenetic step of root nodulation which was insensitive to levels of combined nitrogen that completely prevented infection if present at the time of inoculation. These data indicate that all of the recognized steps of root nodule morphogenesis in which the bacteria play a key role are sensitive to the inhibitory effect of combined nitrogen.     

Effetto della rimozione dei fiori sulla crescita delle piante e sul contenuto in leghemoglobina dei tubercoli radicali di pisello e fava

Abstract

Observations on vegetative growth and leghemoglobin contents of root nodules of pea and bean plants after flower bud removal. — These studies found their origin in the papers by MATTIROLO (1899) on the effect of the removal of flowers as they formed in bean plants; he observed that deflowering resulted in extraordinary plant growth, stem branching and flower buds formation as well as in a delayed root nodule senescence. In the light of modern knowledge of the leghemoglobin role in symbiotic nitrogen fixation, the aim of the present research was to ascertain any possible relation between flower bud removal and haeme pigment contents in the root nodules. The experiments were carried out during two growing seasons (1966 and 1970) using Vicia faba L. cv. Regina and Pisum sativum L. cv. Senatore during 1966 and cv. Vittoria in 1970. In both control and test plants the seasonal trends of average plant height, fresh and dry weight of vegetative portions, fresh and dry weight of root system, fresh weight of nodules, root nodule leghemoglobin concentration and total leghemoglobin content per plant, were determined. The data obtained are quoted in Table 2 and reported in Figures 1, 2, and 3. The removal of flower buds caused in both species: an increased plant growth, a marked stem branching, a longer blooming period, an increased flower number, an increased root nodule number and a certain delay in root nodule reabsorption. Deflowering did not significantly extend — at least in the species studied — life span (senescence was delayed only of one week). On the basis of these and of other Authors' results, we conclude that deflowering may actually delay senescence; the size of this delay, however, depends on the plant species considered and is fairly negligible both in pea and bean. The different effects of deflowering and of preventing floral induction on life span extension, are discussed, and these facts lead to consider floral induction as the onset of a chain of processes leading annual plants toward senescence in a more or less delayable, but definitive way. After having stressed the generally accepted importance of leghemoglobin concentration as an index of nodule nitrogen fixing ability, a correlation between biomass increase of test plants and number and total weight increase of root nodules, is put in evidence. No correlation between test plant biomass and the leghemoglobin concentration in root nodules, was however observed. Leghemoglobin concentration in root nodules is known to change in connection with various factors depending either on host plants and on Rhizobium strains and also in connection with several environmental conditions. Any prevented flower onset (ROPONEN and VIRTANEN, 1968) and deflowering (our data) however exerted no significant influence. The effects of flower bud removal were therefore the following: increased stem, leaf and root weights and increased root nodule number; no difference between control and test plants was however observed as regards size and leghemoglobin concentration of root nodules and hence probably no difference as regards their nitrogen fixing ability.     

Effect of seed size and plant growth on nodulation and nodule development in lima bean (Phaseolus lunatus L.)

Lima bean (Phaseolus lunatus L.) cultivars vary widely in their growth habit and seed size. Preliminary experiments indicated that a large-seeded pole cultivar (King of the Garden) formed many more nodules than a small-seeded bush cultivar (Henderson). The relative importance of seed size and shoot mass in determining nodule number and mass was assessed in five lima bean cultivars differing in seed size and growth habit. Between cultivars, significant positive correlations between initial seed mass, plant weight and nodule number and mass were observed during the first four weeks after planting. Comparisons within cultivars indicated a strong correlation between nodule mass and shoot dry weight. The influence of plant morphology on nodule formation and mass was secondary to the effects of seed and shoot mass. As plants matured, the increase in nodule mass paralleled the increase in plant mass, while nodule number was relatively stable after day 18. These results suggest that the highly regulated process of nodule formation was under the influence of seed derived factors, while the continued accumulation of nodule tissue was related to shoot growth.     

Interactions Between High pH and Iron Supply on Nodulation and Iron Nutrition of Lupinus albus L. Genotypes Differing in Sensitivity to Iron Deficiency

Poor growth of white lupin (Lupinus albus L.) on alkaline soils may result from its sensitivity to iron deficiency and poor nodulation. This study examined interactive effects of iron supply and high pH on the growth and nodulation of three genotypes differing in their sensitivity to iron deficiency. Three genotypes (P27486, Ultra and WTD180) were grown for 17 days in buffered solutions with Fe supply of 0.2, 2 and 20 μM. Solution pH was adjusted to 5.2, 6.5 or 7.5. Plant growth, nodulation and nutrient concentrations in plants were measured. Decreasing Fe supply decreased chlorophyll concentration in young leaves by up to 92%. Increasing pH decreased chlorophyll concentration by an average of 40% at pH 6.5 and by 47% at pH 7.5. The decrease of chlorophyll was less obvious in P27485 than in Ultra or WTD180. Shoot biomass was reduced by up to 18% by Fe deficiency, with such decrease being less for P27486. Increasing pH exacerbated the effect of Fe deficiency on shoot biomass only of Ultra. Decreasing Fe supply decreased nodule number by an average of 54%, and increasing pH decreased nodule number by 80%. P27486 formed the greatest number of nodules while WTD180 the least. P27486 had high Fe uptake and low internal requirement. Irrespective of genotype, leaf chlorosis positively correlated with cluster root formation. The results suggest that a combination of Fe deficiency and high pH impaired nodulation in L. albus, and that selection of genotypes for both tolerance of iron deficiency and good nodulation at high pH is important for a successful lupin crop on alkaline soils.     

Concentration-dependent influence of quercetin on nodulation process and the main characteristics of soybean inoculated with Bradyrhizobium japonicum

Soybean plants cv. Corsoy were grown in greenhouse conditions on sterilized quartz sand. They were inoculated with Bradyrhizobium japonicum, strain 542. The plants were treated with different concentrations of quercetin (ranging from 10 nM to 1M) at regular intervals during the experiment. The experiment was terminated at flower development. The following parameters, important for symbiosis efficiency were determined: shoot, root and nodule weights, nodule number, total leghemoglobin in the nodules,total nitrogen and soluble protein concentrations in shoots and roots, as well as chlorophyll concentration in the leaves.The results obtained partly confirmed the earlier findings that quercetin inhibits nodulation since increasing quercetin concentration decreased the number of nodules. However, at very low concentrations, quercetin stimulated the number of nodules. Quercetin also exerted a stimulating influence on other characteristics of the plant and nodules which did not correlate with nodule number and quantity of N fixed. These are: nodule weight, leghemoglobin concentration, total soluble protein content in shoots and roots as well as shoot and root weight.     

Counteractive biomass allocation responses to drought and damage in the perennial herb Convolvulus demissus

Herbivory and water shortage are key ecological factors affecting plant performance. While plant compensatory responses to herbivory include reallocation of biomass from below‐ground to above‐ground structures, plant responses to reduced soil moisture involve increased biomass allocation to roots and a reduction in the number and size of leaves. In a greenhouse study we evaluated the effects of experimental drought and leaf damage on biomass allocation in Convolvulus demissus (Convolvulaceae), a perennial herb distributed in central Chile, where it experiences summer drought typical of Mediterranean ecosystems and defoliation by leaf beetles and livestock. The number of leaves and internode length were unaffected by the experimental treatments. The rest of plant traits showed interaction of effects. We detected that drought counteracted some plant responses to damage. Thus, only in the control watering environment was it observed that damaged plants produced more stems, even after correcting for main stem length (index of architecture). In the cases of shoot : root ratio, relative shoot biomass and relative root biomass we found that the damage treatment counteracted plant responses to drought. Thus, while undamaged plants under water shortage showed a significant increase in root relative biomass and a significant reduction in both shoot : root ratio and relative shoot biomass, none of these responses to drought was observed in damaged plants. Total plant biomass increased in response to simulated herbivory, apparently due to greater shoot size, and in response to drought, presumably due to greater root size. However, damaged plants under experimental drought had the same total biomass as control plants. Overall, our results showed counteractive biomass allocation responses to drought and damage in C. demissus. Further research must address the fitness consequences under field conditions of the patterns found. This would be of particular importance because both current and expected climatic trends for central Chile indicate increased aridity.     

百脉根小G蛋白Rac1基因的克隆与功能分析

小G蛋白Rop在植物细胞信号转导中发挥着重要的分子开关功能。该实验通过RT-PCR方法克隆了百脉根的一个Rop编码基因LjRac1,并对LjRac1基因序列进行生物信息学分析,然后采用半定量RT-PCR检测LjRac1基因在百脉根不同组织中的表达,用荧光实时定量PCR方法检测百脉根接种根瘤菌后LjRac1基因在不同阶段根系中的表达,构建过表达重组质粒,利用发根农杆菌介导的遗传转化法对LjRac1基因功能进行分析。结果表明:(1)序列分析显示,LjRac1完整编码区的cDNA序列长度为594bp,编码197个氨基酸,其编码蛋白具有典型的Rop家族保守结构域;同源分析显示,百脉根LjRac1与大豆GmRac1、野大豆GsRac1的一致性最高(94.42%)。(2)LjRac1基因在百脉根的根、茎、叶、根瘤和花中均有表达,且在根和根瘤中的表达水平较高;接种根瘤菌0.5h后,LjRac1基因在根系中的表达量呈显著升高趋势。(3)过表达转基因植株中LjRac1mRNA的表达水平为对照植株的14.3倍,且过表达植株的结瘤数目较对照明显增加。研究认为,LjRac1基因是一个受根瘤菌诱导增强表达的基因,过表达LjRac1基因可以引起植株结瘤数目的增加,说明LjRac1基因可能参与早期结瘤信号转导途径,从而在根瘤的发育中发挥一定作用。     

Effect of time of inoculation on in vitro ectomycorrhizal colonization and nodule initiation in Acacia holosericea seedlings

The complex interactions that occur in systems with more than one type of symbiosis were studied using one isolate of Bradyrhizobium sp. and the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker and Couch inoculated on to the roots of Acacia holosericea A. Cunn. ex G. Don in vitro. After a single inoculation with Bradyrhizobium sp., bacteria typically entered the roots by forming infection threads in the root hair cells via the curling point of the root hair and/ or after intercellular penetration. Sheath formation and intercellular penetration were observed on Acacia roots after a single inoculation with Pisolithus tinctorius but no radial elongation of epidermal cells. Simultaneous inoculation with both microorganisms resulted in nodules and ectomycorrhiza on the root system, occasionally on the same lateral root. On lateral roots bearing nodules and ectomycorrhiza, the nodulation site was characterized by the presence of a nodule meristem and the absence of an infection thread; sheath formation and Hartig net development occurred regularly in the region of the roots adjacent to nodules. Prior inoculation with Bradyrhizobium sp. did not inhibit ectomycorrhizal colonization in root segments adjacent to nodules in which nodule meristems and infection threads were clearly present. Conversely, in ectomycorrhizae inoculated by bacteria, the nodule meristem and the infection thread were typically absent. These results show that simultaneous inoculation with both microorganisms inhibits infection thread development, thus conferring an advantage on fungal hyphae in the competition for infection sites. This suggests that fungal hyphae can modify directly and/or indirectly the recognition factors leading to nodule meristem initiation and infection thread development.     

Shoot apex removal does not alter autoregulation of Modulation in soybean

The suppression of new nodule development in soybean (Glycine max (L.) Merr.) has been previously demonstrated to involve the shoot through reciprocal grafts between the wild-type cultivar Bragg and its supernodulating mutant nts382. Using the same grafting technique, but modified through the excision of the shoot apex region and emerging lateral shoots, we show here that autoregulation of nodule number still existed despite apex removal. This radical treatment lowered total nodule number per plant as well as root, shoot and nodule dry weight. Bragg shoots grafted onto nts382 roots gave wild-type nodulation (26 nodules, 15mg total nodule mass) as compared to nts382 shoots grafted onto Bragg roots (340 nodules, 277 mg total nodule mass). Specific nodule mass differed between supernodulating (about 0·5-1·0mg per nodule) and wild-type nodulating (2·3 mg per nodule) plants. In contrast to other growth characteristics, apex removal did not affect specific nodule size, except in plants with wild-type shoots and nts382 (supernodulation) roots. Apex removal only slightly affected the percentage of nodule weight per total root weight in nts382, but had a severe effect in wild type. Growth reductions varied between the normal and supernodulating plants. The fact that autoregulation of nodulation still functions in plants devoid of functional shoot apices suggests that the autoregulation signal may not be derived from the apex regions and that the leaf may be a likely source.     

Allocating nitrogen away from a herbivore: a novel compensatory response to root herbivory

Centaurea maculosa, an invasive North American plant species, shows a high degree of tolerance to the root-boring biocontrol herbivore, Agapeta zoegana. For example, infested individuals of C. maculosa often exhibit more rigorous growth and reproduction compared with their non-infested counterparts. Compensatory responses to aboveground herbivores often involve increases in leaf area and/or photosynthetic capacity, but considerably less is known about root system compensatory responses to belowground herbivory. We used a ¹⁵N labeling approach to evaluate whether compensatory adjustments in N acquisition via changes in root morphology and/or physiological uptake capacity could explain the ability of C. maculosa to tolerate root herbivory. Root herbivory reduced whole plant N uptake by more than 30% and root uptake capacity by about 50%. Despite a marked reduction in N procurement, herbivory did not affect total biomass or shoot N status. Infested plants maintained shoot N status by shifting more of the acquired N from the root to the shoot. To our knowledge, shifting N allocation away from a root herbivore has not been reported and provides a plausible mechanism for the host plant to overcome an otherwise devastating effect of a root herbivore-induced N deficit.     

Spatial relationships between uninfected and infected cells in root nodules of soybean

In soybean (Glycine max (L.) Merr.) the uninfected cells of the root nodule are responsible for the final steps in ureide production from recently fixed nitrogen. Stereological methods and an original quantitative method were used to investigate the organization of these cells and their spatial relationships to infected cells in the central region of nodules of soybean inoculated with Rhizobium japonicum strain USDA 3I1B110 and grown with and without nitrogen (as nitrate) in the nutrient medium. The volume occupied by the uninfected tissue was 21% of the total volume of the central infected region for nodules of plants grown without nitrate, and 31% for nodules of plants grown with nitrate. Despite their low relative volume, the uninfected cells outnumbered the much larger infected cells in nodules of plants grown both without and with nitrate. The surface density of the interface between the ininfected and infected tissue in the infected region was similar for nodules in both cases also, the total range being from 24 to 26 mm²/mm³. In nodules of plants grown without nitrate, all sampled infected cells were found to be in contact with at least one uninfected cell. The study demonstrates that although the uninfected tissue in soybean nodules occupies a relatively small volume, it is organized so as to produce a large surface area for interaction with the infected tissue.     

Frankia in the rhizosphere of nonhost plants: A comparison with root-associated N2-fixing Enterobacter,Klebsiella and Pseudomonas

Bacterial growth in the rhizosphere and resulting changes in plant growth parameters were studied in small aseptic seedlings of birch (Betula pendula and B. pubescens) and grasses (Poa pratensis and Festuca rubra). The seedlings were inoculated with three Frankia strains (Ai1a and Ag5b isolated from native Alnus root nodules and Ai17 from a root nodule induced by soil originating from a Betula pendula stand), and three associative N₂-fixing bacteria (Enterobacter agglomerans, Klebsiella pneumoniae and Pseudomonas sp., isolated from grass roots). Microscopic observations showed that all the Frankia strains were able to colonize and grow on the root surface of the plants tested without addition of an exogenous carbon source. No net growth of the associative N₂-fixers was observed in the rhizosphere, although inoculum viable counts were maintained over the experimental period. Changes in both the biomass and morphology of plant seedlings in response to bacterial inoculation were recorded, which were more dependent on the plant species than on the bacterial strain.     

基于高通量测序分析西藏沙棘根瘤内生菌的多样性

根瘤是微生物侵染植物根部并与之形成的共生结构,这些微生物都可被称为植物内生菌。豆科植物根瘤中的内生菌常常又被称为根瘤菌,而侵染非豆科植物形成根瘤的主要是放线菌弗兰克氏菌,这些非豆科植物又被称为放线菌结瘤植物。西藏沙棘是一种典型的放线菌结瘤植物,由于其分布生境的特殊性,对其根瘤内生菌的研究具有重要的生态意义。对于西藏沙棘根瘤内生菌的研究,培养方法因难以模拟自然条件而不易获得纯培养,高通量测序技术对其多样性的研究提供了便利。因此,本研究以生长在甘肃省天祝县金强河河滩地的西藏沙棘根瘤为材料,采用16S rRNA基因扩增子高通量测序方法,结合OTU分析,对西藏沙棘根瘤内生菌的多样性进行探讨。实验结果表明,西藏沙棘根瘤内生菌具有丰富的多样性,根瘤内的优势属为共生固氮的弗兰克氏菌属（Frankia）,其相对丰度为47.63%,共检测到7个弗兰克氏菌属的OTUs;根瘤内除弗兰克氏菌外,还存在大量的非弗兰克氏菌,共检测到1523个OTUs,隶属于22个门、33个纲、69个目、113个科和202个属,相对丰度排名前9的属中有25个非弗兰克氏菌属的OTUs。该研究也表明,西藏沙棘根瘤内生菌具有丰富的多样性,西藏沙棘根瘤中不仅存在着可共生固氮的弗兰克氏菌,并且还分布着非弗兰克氏菌;在同一根瘤样品中,弗兰克氏菌属还具有不同的物种。本研究不仅拓展了西藏沙棘根瘤内生菌多样性的研究方法,还为同一寄主植物中弗兰克氏菌多样性的研究提供了分析思路。     

Response of an invasive liana to simulated herbivory: implications for its biological control

Pre-release evaluation of the efficacy of biological control agents is often not possible in the case of many invasive species targeted for biocontrol. In such circumstances simulating herbivory could yield significant insights into plant response to damage, thereby improving the efficiency of agent prioritisation, increasing the chances of regulating the performance of invasive plants through herbivory and minimising potential risks posed by release of multiple herbivores. We adopted this approach to understand the weaknesses herbivores could exploit, to manage the invasive liana, Macfadyena unguis-cati. We simulated herbivory by damaging the leaves, stem, root and tuber of the plant, in isolation and in combination. We also applied these treatments at multiple frequencies. Plant response in terms of biomass allocation showed that at least two severe defoliation treatments were required to diminish this liana's climbing habit and reduce its allocation to belowground tuber reserves. Belowground damage appears to have negligible effect on the plant's biomass production and tuber damage appears to trigger a compensatory response. Plant response to combinations of different types of damage did not differ significantly to that from leaf damage. This suggests that specialist herbivores in the leaf-feeding guild capable of removing over 50% of the leaf tissue may be desirable in the biological control of this invasive species.     

Influence of migrant tundra swans (Cygnus columbianus) and Canada geese (Branta canadensis) on aquatic vegetation at Long Point, Lake Erie, Ontario

Numerous studies have shown that large, herbivorous waterfowl can reduce quantity of aquatic plants during the breeding or wintering season, but relatively few document herbivory effects at staging areas. This study was done to determine if feeding activities of tundra swans (Cygnus columbianus columbianus) and Canada geese (Branta canadensis) had a measurable additive influence on the amount of aquatic plants, primarily muskgrass (Chara vulgaris), wild celery (Vallisneria americana), and sago pondweed (Potamogeton pectinatus), removed during the fall migration period at Long Point, Lake Erie, Ontario. Exclosure experiments done in fall 1998 and 1999 showed that, as compared to ducks and abiotic factors, these two large herbivorous waterfowl did not have any additional impact on above or below ground biomass of those aquatic plants. As expected, however, there were substantial seasonal reductions in above-ground and below-ground biomass of aquatic plants in wetlands that were heavily used by all waterfowl. We suggest that differences in large- and small-scale habitat use, feeding activity, and food preferences between tundra swans and other smaller waterfowl as well as compensatory herbivory contributed to our main finding that large waterfowl did not increase fall reductions of Chara spp, V. Americana, and P. pectinatus biomass.