Mechanism of parasitization by Cuscuta reflexa: RNase activity in the haustorial region of Cuscuta and infected host tissues

In a study of host-parasite interrelationship and the mechanism of parasitization, ribonuclease activity was determined in Cuscuta reflexa Roxb, and in infected and control host plants of Lantana camara L. In the haustoria-bearing region of Cuscuta , the concave half of the vine bearing the haustorial site, always showed significantly higher RNase activity than the convex half, irrespective of the differing enzyme activity of the parasite tissue growing on different hosts — Brassica campestris L., Helianthus animus L., Lantana camara L., Medicago saliva L. and Solatium nigrum L. The uninfected host branch of L. camara showed the maximum specific RNase activity in the apical region which decreased toward the base, while the infected host branch showed minimum specific RNase activity in the apical region, gradually increasing towards the infected region.     

Disturbance, invasion and re-invasion: managing the weed-shaped hole in disturbed ecosystems

We aim to develop a simple model to explore how disturbance and propagule pressure determine conditions for successful invasion in systems where recruitment occurs only in disturbed sites. Disturbance is often thought to favour invaders as it allows recruitment; however, the effects of disturbance are more complicated when it results in mortality of the invader. When disturbance rates in both invader occupied and unoccupied sites are the same, recruitment and mortality effects are exactly balanced, and successful invasion is independent of the disturbance regime. Differences in the disturbance rates between invader occupied and unoccupied sites can occur through invader modification or management of disturbance. Under these conditions, we found a novel mechanism for the generation of an Allee effect, which occurs when the invader promotes disturbance in sites it already occupies. When Allee effects occur one-off, large-scale disturbances can result in permanent, dramatic shifts in invader abundance; and conversely, reducing the population below a critical threshold can cause extinction.     

Some dynamics of spread and infection by aeciospores of Puccinia punctiformis,a biological control pathogen of Cirsium arvense

Systemic disease of Cirsium arvense caused by Puccinia punctiformis depends on teliospores, from telia that are formed from uredinia, on C. arvense leaves. Uredinia result from infection of the leaves by aeciospores which are one main source of dispersal of the fungus. However, factors governing aeciospore spread, germination, infection, and conversion to uredinia and telia have not been extensively investigated. In this study, effective spread of aeciospores from a source area in a field was fitted to an exponential decline model with a predicted maximum distance of spread of 30 m from the source area to observed uredinia on one leaf of one C. arvense shoot. However, the greatest number of shoots bearing leaves with uredinia/telia was observed within 12 m of the source area, and there were no such shoots observed beyond 17 m from the source area. Aeciospore germination under laboratory conditions was low, with a maximum of about 10%. Temperatures between 18 °C and 25 °C were most favorable for germination with maximum germination at 22 °C. Temperature and dew point data collected from the Frederick, MD airport indicated that optimum temperatures for aeciospore germination occurred in late spring from about May 18 to June 20. Dew conditions during this period were favorable for aeciospore germination. A total of 122 lower leaves, 2 per shoot, on 61 C. arvense shoots were individually inoculated in a dew tent in a greenhouse by painting suspensions of aeciospores onto the leaves. Of these inoculated leaves, 47 produced uredinia within an average of 21.2 ± 6.9 days after inoculation. Uredinia were also produced, in the absence of dew, on 17 non-inoculated leaves of 12 shoots. These leaves were up to 4 leaves above leaves on the same shoots that had been individually and separately inoculated. Results of PCR tests for the presence of the fungus in non-inoculated leaves that were not bearing uredinia, showed that 44 leaves above inoculated leaves on 27 shoots were positive for the presence of the fungus. These leaves were up to 5 leaves above inoculated leaves on the same shoot. Uredinia production and positive PCR results on leaves above inoculated leaves on the same shoot indicated that aeciospore infection was weakly systemic. In other tests in which all leaves of plants were spray-inoculated with aeciospores, uredinia were produced by 10 days after inoculation and converted to telia and sole production of teliospores in about 63 days after inoculation. Successful systemic aeciospore infections in late spring would be expected to result in uredinia production in excess of a 1:1 ratio of aeciospore infections to uredinia and ultimately telia production in late summer. In this manner, systemic aeciospore infections would promote increased density of telia that lead to systemic infections of roots in the fall.     

Alternatives for Reintroducing a Rare Ecotone Species: Manually Thinned Forest Edge versus Restored Habitat Remnant

Species native to ecotones are often overlooked in restoration efforts despite the increasing rarity of ecotone habitat. In fragmented, fire-suppressed landscapes, true ecotone may no longer exist. Restoration biologists interested in reintroducing ecotone species must decide whether to plant them in historic ecotones maintained by manual thinning or whether to opt for discrete restoration areas that are easier to maintain. We investigated these two alternatives with Lantana canescens , a rare tropical shrub native to the ecotone between pine and hardwood forests of Miami-Dade County, Florida, U.S.A. Our short-term findings show that after 15 and 18 months, survival of transplants was 69% in a restored site and 65% and 84% in two historic ecotone sites. The restored site had significantly higher photosynthetically active radiation (PAR) (75%) than the historic ecotones (25–39%). Correspondingly, 267 seedlings have recruited at the restored site, whereas only 8 have emerged at both historic ecotone sites. Seedling establishment was associated with higher PAR at the restored site. We found that overall population sustainability was higher at the restored site where there is the additional benefit of less maintenance. Our work suggests that, by reducing succession, a discrete restoration area can approach the historic conditions of hardwood/pine forest ecotone more closely than degraded historic ecotones themselves. We present a viable solution for conserving rare ecotone species when their natural habitat and the processes that maintained it no longer exist.     

马缨丹叶提取液对水葫芦叶片中超氧物歧化酶活性和过氧化氢积累的影响

水葫芦[Eichhornia crassipes（Mart）Solms]是世界上繁殖最快、危害最严重的多年生水生杂草之一。为了避免化学除草剂对水体的污染,生物防治已成为当前水葫芦治理的重要方向。马缨丹（Lantana camara）是马鞭草科的一种植物,其叶片提取物对水葫芦有很强的毒性。研究结果表明：经马缨丹叶提取液处理的水葫芦叶片中,超氧物歧化酶（SOD）活性与H2O2浓度均显著升高,但过氧化氢酶的活性受到抑制,膜脂过氧化程度明显增加。H2O2的组织化学染色结果表明H2O2在气孔细胞中有异常高的积累,H2O2过量产生同时导致水葫芦叶片失绿与细胞死亡。因此,氧胁迫可能是马缨丹提取液对水葫芦毒害的主要原因之一。     

Effects of elevated CO2 and temperature on development and consumption rates of Octotoma championi and O. scabripennis feeding on Lantana camara

We carried out a factorial experiment to explore the effect of doubled CO₂ concentration and a 3 °C temperature increase on the development of a complete generation of the beetles Octotoma championi Baly and O. scabripennis Guérin‐Méneville (Coleoptera: Chrysomelidae). These species are biological control agents of Lantana camara L. (Verbenaceae), with a leaf‐mining larval phase and free‐living, leaf‐chewing adults. Plants grown at elevated CO₂ had enhanced above‐ground biomass, thicker leaves, reduced nitrogen concentration, and increased C:N ratios. Under the high temperature treatment, plants grown at ambient CO₂ suffered wilting and premature leaf loss, despite daily watering; this effect was ameliorated at elevated CO₂. The wilting of plants in the ambient CO₂/high temperature treatment reduced the emergence success of the beetles, particularly O. championi. Development time was accelerated by approximately 10–13 days at the higher temperature, but was not affected by CO₂. Neither CO₂ nor temperature affected adult beetle weight. Consumption rates of free‐living beetles were not affected by either CO₂ or temperature. By contrast, in the short‐term trials using excised foliage, beetles given no choice between ambient and elevated CO₂‐grown foliage, consumed more from ambient plants. When beetles were offered a choice between foliage grown at the two CO₂ levels, O. championi did not display a significant preference but O. scabripennis consumed more ambient CO₂‐grown foliage when feeding at the lower temperature. This study indicates that under future conditions of higher temperatures, amelioration of water stress in host plants growing in elevated CO₂ may benefit some endophagous insects by reducing premature leaf loss. Under some circumstances, this benefit may outweigh the deleterious effects of lower leaf nitrogen. Our results also indicate that foliage consumption under elevated CO₂ by mobile, adult insects on whole plants may not be significantly increased, as was previously indicated by short‐term experiments using excised foliage.     

Additions to the mycobiota of the weed Lantana camara (Verbenaceae) in southeastern Brazil

A sequel to the work of systematic surveying the mycobiota of Lantana camara aimed at finding potential biocontrol agents, was carried out during 1995–1996 covering part of its centre of origin in Brazil (state of Minas Gerais). Fifty-eight sampling sites, representing the four main climatic types in the state of Minas Gerais, were surveyed. Additional ad hoc collections were made in the states of Bahia, Espírito Santo, Paraná Rio de Janeiro and São Paulo. Fifteen fungal species were recorded in association with L. camara including the previously undescribed species Phomopsis lantanae-glutinosae sp. nov. Five fungi are also newly recorded on this host in Brazil: Cercospora lantanicola Corynespora cassiicola Meliola ambigua Mycovellosiella lantaniphila and Phomopsis lantanae. The following fungi, previously recorded on L. camara in Brazil, are recorded here for the first time in Minas Gerais: Dendryphielia aspera Micropustulomyces mucilaginosus Mycovellosiella lantanae Pseudocercospora guianensis and Puccinia lantanae.     

嗅觉和视觉在中华微刺盲蝽对马缨丹定向行为中的作用

对马缨丹(Lantana camara)、黄花马缨丹(Lantana camara cv.‘Flava')和紫花马缨丹(Lantanamontevidensis)花序上中华微刺盲蝽(Campylomma chinensis)的密度调查结果表明,中华微刺盲蝽在3种颜色马缨丹花序上的分布与花序颜色无关(偏相关系数为0.240,P=0.147,n=40),但与花序中的黄胸蓟马数量和花序中小花数的偏相关系数分别为0.512(P＜0.0001,n=0)和0.451(P＜0.0001,n=40),达到极显著水平.中华微刺盲蝽对不同颜色粘卡的试验也表明,该盲蝽寻找植物寄主时,视觉刺激并不起作用.四臂嗅觉仪的试验结果表明,中华微刺盲蝽对马缨丹的气味有强烈的趋向作用.采用固相微萃取和气相-质谱联用技术分析表明,3种马缨丹花序的挥发物有7种相同的化学成分.     

Inhibitory effects of some invasive alien species leaf extracts against tomato (Lycopersicon esculentum Mill.) bacterial wilt (Ralstonia solanacearum)

The inhibitory activity of selected invasive alien species leaf extracts against tomato bacterial wilt was studied in vitro and greenhouse. In vitro inhibitory activity was undertaken in a disc diffusion sensitivity test. Furthermore, aqueous extracts of Eichhorina crassipes, Mimosa diplotricha and Lantana camara and methanolic extract of Prosopis juliflora which showed better inhibitory effect in vitro were evaluated against R. solanacearum in greenhouse on tomato. The plant extracts were applied and evaluated at three different times of application (at the time of inoculation; two days before pathogen inoculation; and two days after pathogen inoculation). The result showed that most of the treatment combinations significantly reduce disease incidence and area under disease progress curve and increase biomass of tomato plants, but the effectiveness of tested plant species depends on the type of plant species and application time. The application of plant extracts at the time of pathogen inoculation resulted in highest reduction of disease development on tomato plants. Aqueous extract of E. crassipes was found to be the most effective plant extract in disease suppression and increase above-ground biomass compared to inoculated control. The study revealed that the tested plant species have a potential of inhibiting the development of bacterial wilt on tomato.     

土壤不同镉水平对马缨丹生长及其抗氧化酶活性的影响

采用盆栽试验方法,分别设置0(对照,不添加镉)、30、60、90、120、150、180、210mg·kg-1共8个土壤镉处理水平,研究土壤不同镉水平对马缨丹(Lantana comara L.)生长及其抗氧化酶活性的影响,以探讨马缨丹对镉胁迫的生理响应机制。结果显示:(1)随着土壤镉处理浓度的升高,马缨丹干重呈先升高后降低的趋势,30mg·kg-1镉处理能促进植株的生长,而浓度高于60mg·kg-1时显著抑制马缨丹的生长。(2)马缨丹叶片和根系中O-·2产生速率、H2O2和MDA含量及电解质渗漏率均随土壤镉处理浓度的升高和胁迫时间的延长逐渐升高,胁迫90d时,叶片和根系中O-·2产生速率、H2O2和MDA含量及电解质渗漏率分别在镉浓度高于60和30mg·kg-1时显著低于对照。(3)叶片和根系抗氧化酶SOD、POD、APX和CAT活性随着土壤镉处理浓度的增加大体呈先升高后降低的趋势,并在镉浓度分别高于90和60mg·kg-1时,叶片和根系抗氧化酶活性显著低于对照。研究表明,低浓度镉处理土壤能促进马缨丹植株生长,而高浓度镉处理土壤显著降低了马缨丹体内抗氧化酶活性,导致活性氧大量积累,引起严重的膜脂过氧化伤害,从而显著抑制马缨丹植株的生长。