Investigations on the parasitism ofOtiorrhynchus salicicola andO. Sulcatus [Col.: Curculionidae] byHeterorhabditis sp. [Nematoda]

Experiments were conducted to study the efficacy of the insect parasitic nematodeHeterorhabditis sp. (HW79) as a biological control agent ofOtiorrhynchus salicicola. This weevil species is reported as a pest of ornamental plants in Switzerland and Italy. Dipping plastic boxes containing heavily infested cuttings of laurel (Prunus laurocerasus) in a nematode suspension resulted in approximately 100% parasitisation of full-grown larvae, pupae and non-emerged young adults. The average dose resulting from dipping varied between 56,000 and 62,000 nematodes per liter soil. This experiment was run under natural outdoor conditions. In a further outdoor experiment, pottedLigustrum plants were inoculated with eggs ofO. salicicola and later 20,000 infective juvenile nematodes per liter soil were added to the soil surface. The resulting weevil mortality in the treated pots was 78%. In seven greenhouse tests using the same nematode dose in pots with horticultural soil to which weevil larvae had been added, weevil mortality varied between 76% and 100%, the arithmetic average being 90%. These results indicate that Heterorhabditid nematodes may provide an effective means of controllingO. salicicola. In an other experiment usingO. sulcatus larvae, the influence of application time on nematode efficacy was investigated. When nematodes were added a few days before weevil larvae had hatched from the eggs, no parasitic effect was obtained. Nematode applications done shortly after larval hatching however, resulted in complete weevil control. These results are of significance in timing nematode applications in practice.      

Dependency of cassava (Manihot esculanta Crantz) on vesicular-arbuscular mycorrhizal fungi

Cassava plants were started in the greenhouse either from small cuttings (2.0 mg P/cutting) or large cuttings (20.2 mg P/cutting) in a subsurface Oxisol not inoculated or inoculated with Glomus aggregatum at target soil solution P concentrations of 0.003–0.2 mg l^-1. Vesicular-arbuscular mycorrhizal (VAM) fungal colonization levels in excess of 60% were attained on cassava roots irrespective of the size of cutting material used or target soil solution P status. However, plants started from large cuttings grew faster and better than those started from smaller ones. Cassava was found to be very highly dependent on VAM fungi if grown from small cutting but only marginally dependent if grown from large cuttings. The lower dependence of cassava on VAM fungi when started from larger cuttings appears to be related to the high P reserve in these cuttings and hence the low requirement of the plants for soil P until the P reserve in the cuttings is significantly depleted.A contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3896     

Use of soil transfer for reforestation on abandoned mined lands in Alaska

Soil transfers from an intermediate successional site and a mature forest site were applied to Populus balsamifera L. cuttings and Alnus crispa (Ait.) Pursh seedlings placed on an abandoned mined site in south central Alaska to improve plant establishment. Mycorrhizal fungi in the soil transfers from the two successional stages were hypothesized to have different effects on plant species that colonize disturbed sites at different times or on different substrates. The site consisted of coarse, dry, low-nutrient spoils and was naturally colonized by scattered P. balsamifera but not A. crispa, although seed sources for both were adjacent to the site. Physical dimensions of the transplanted seedlings and cuttings were measured at the beginning and end of each growing season. Selected plants were harvested at the end of the 2-year study and examined for mycorrhizal formation, current growth, and leaf tissue nutrient concentrations. Both plant species were taller when treated with the soil transfers from the mature forest than with soils from the intermediate site although the increase for A. crispa was greater. Physical dimensions, current growth, and nutrient concentrations were greater when A. crispa was treated with the mature soil transfer compared with the intermediate soil transfer. Mycorrhizae which infected Alnus were predominantly a brown woody type, while other types accounted for greater relative mycorrhizal infection percentage on Populus. Insufficient quantities of mycorrhizal inoculum of suitable species, as well as low moisture and low nutrient conditions, may be factors limiting A. crispa colonization on primary disturbed sites in south central Alaska.     

Effects of rhizosphere soil,vesicular-arbuscular mycorrhizal fungi and phosphate onPlantago major L. ssp.pleiosperma Pilger

Biotic factors in the rhizosphere and their effect on the growth ofPlantago major L. ssp.pleiosperma Pilger (Great plantain) were studied. In a pot experiment the effect on shoot growth of the addition of 2.5% rhizosphere soil at four levels of phosphate was highly dependent on the availability of phosphate: a promoting effect at low phosphate levels was observed while a reducing effect occurred at higher phosphate levels. As the roots were infected with vesicular-arbuscular mycorrhizal (VAM) fungi in the treatment with rhizosphere soil, two other experiments were set up to separate effects of the indigenous VAM fungi from effects of the total rhizosphere population. The uptake of phosphate and shoot growth was not decreased at higher phosphate availability when VAM inoculum was added alone or in combination with rhizosphere soil. The growth reducing effect of the rhizosphere soil could therefore not be ascribed only to mycorrhizal infection. The results suggest that biotic factors in the rhizosphere soil affect the phosphate uptake ofPlantago major ssp.pleiosperma. This may, under conditions of phosphate limitation, lead to an increase of phosphate stress and, subsequently, a growth reduction. Futhermore, it is concluded that VAM fungi, as part of the rhizosphere population, may compensate this phosphate stress by enhancing the phosphate uptake.Grassland Species Research Group Publication No. 148.     

Bioindicator capacity of trees towards dryland salinity

To investigate plant–soil reactions towards site salinity we chose Ruprechtia triflora as the dominant tree species in its natural habitat in dry forests of Paraguay. We applied freezing point osmometry to measure tree leaves and soil samples identically on the basis of colligative solute properties. In order to substantiate the first field findings, the relation of tree and soil in terms of salinity was further investigated under controlled conditions in the greenhouse. R. triflora shows extreme osmotic adaptability. In three independent greenhouse experiments with NaCl application, Ruprechtia seedlings and later Eucalyptus dunnii seedlings from E Australia (for inter-species comparison) showed highly significant responses to their soil salinities. We conclude that tree leaves can serve as bioindicator for soil salinity within the tree’s rhizosphere.     

Introduction ofPhytoseiulus persimilis for twospotted spider mite control in greenhouses in Egypt [Acari: Phytoseiidae,tetranychidae]

The exotic predaceous mite,Phytoseiulus persimilis Athias-Henriot and the 2 native onesPhytoseius finitimus Ribaga andAmblyseius gossipi Elbadry were released on greenhouse cucumber plants in Egypt to examine their efficiency to control the twospotted spider miteTetranychus urticae Koch.P. persimilis proved to be sufficiently effective for the twospotted spider mite control under greenhouse conditions in Egypt. The 2 before-mentioned native predators were lost soon after release in the greenhouse although they are key mite predators on outdoor crops in this area.      

Test of local adaptation to biotic interactions and soil abiotic conditions in the ant-tended <Emphasis Type="Italic">Chamaecrista fasciculata</Emphasis> (Fabaceae)

Few previous studies have assessed the role of herbivores and the third trophic level in the evolution of local adaptation in plants. The overall objectives of this study were to determine (1) whether local adaptation is present in the ant-defended plant, Chamaecrista fasciculata, and (2) the contribution of ant-plant-herbivore interactions and soil source to such adaptation. We used three C. fasciculata populations and performed both a field and a greenhouse experiment. The first involved reciprocally transplanting C. fasciculata seedlings from each population-source to each site, and subsequently applying one of three treatments to one-third of the seedlings of each population-source at each site: control, reduced ant density and reduced folivory. The greenhouse experiment involved reciprocal transplants of population-sources with soil sources to test for a soil-source effect on flower production and local adaptation to soil conditions. Field results showed that ant and herbivore treatments reduced ant density (increasing folivory) and herbivore damage relative to controls, respectively; however, these manipulations did not impact C. fasciculata reproduction or the likelihood of survival. In contrast, greenhouse results showed that soil source significantly affected flower production. Overall, plants in both experiments, regardless of population-source, always had higher reproductive output at one specific site. Native populations did not outperform nonnative ones, causing us to reject the hypothesis of local adaptation. The absence of treatment effects on plant reproduction and the likelihood of survival suggest a limited effect of ants and folivores on C. fasciculata fitness and local adaptation during the study year. Temporally inconsistent effects of biotic forces across years, coupled with the young age of populations, relative proximity of populations and possible counter effects of seed predators may reduce the likelihood of local adaptation in the populations studied.     

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.     

Measurement of bacterial growth rates on the rhizoplane using 3H-thymidine incorporation into DNA

Bacterial growth rates on the rhizoplane of rape seedlings grown in sand were determined using ³H-thymidine incorporation into DNA. Axenic roots incorporated thymidine into DNA, which had to be subtracted from values for roots with associated bacteria. Thymidine incorporation into rhizoplane bacterial DNA ranged between 0.6 and 1.4 pmol thymidine h^–1 root^–1 for 6 to 26-day-old plants. Using a conversion factor, the turnover time of bacteria was calculated to decrease from 9.2 h for 6-day-old plants to 160h for 26-day-old plants. A similar value was found for rhizosphere bacteria of plants grown for 26 days in natural soil.     

Formation of nodular structures on the non-legumes Brassica napus,B. campestris,B. juncea and Arabidopsis thaliana with Bradyrhizobium and Rhizobium isolated from Parasponia spp. or legumes grown in tropical soils

Strains of Bradyrhizobium formed nodule-like structures on Arabidopsis and species of Brassica in pots with sandvermiculite and in glass tubes on a nitrogen-free mineral salts agar. Broad-host-range Rhizobium strains NGR234 from Lablab purpureus and NGR76 from Phaseolus vulgaris formed similar nodule-like structures on Brassica spp. The size of these structures on plants in pots were large, often reaching 10 mm in diameter.The frequency of inoculated Brassica plants in pots with nodule-like structures was 25–50%, depending on the inoculum strain. The inheritable nature of factors involved in the formation of the nodule-like structures was demonstrated when the structures occurred on 100% of inoculated B. napus seedlings derived from plants with the nodule-like structures.Nodule-like structures occurred without, but not with, the application of a cellulase-pectolyase-PEG treatment to the roots. Attempts to isolate Bradyrhizobium or Rhizobium from the nodule-like structures failed. Internal infection of these structures could not be detected using either the light or electron microscope. The inoculum strains of root-nodule bacteria were detected in high numbers in the rhizosphere of plants 5 months after inoculation. On agar plates bacterial colonies could be seen, with undiminished growth, over the surface of the agar extending to the root surface. However, ground root tissue of Brassica was toxic to Bradyrhizobium strains. This suggested that Bradyrhizobium strains would not survive after infecting the roots of Brassica spp. Nitrogen fixation was associated with high rhizosphere populations of Azospirillum and not with Bradyrhizobium induced nodule structures of Brassica spp.     

Forest encroachment into a Californian grassland: examining the simultaneous effects of facilitation and competition on tree seedling recruitment

Competition and facilitation are both considered major factors affecting the structure of plant assemblages, yet few studies have quantified positive, negative, and net effects simultaneously. In this study, we investigated the positive, negative, and net effects of tree saplings on the encroachment of two tree species, Douglas fir (Pseudotsuga menziesii) and tanoak (Lithocarpus densiflora), into a coastal California grassland. The study involved three components: sampling the spatial distributions of P. menziesii and L. densiflora in the grasslands, a field experiment examining seedling survival in different grassland environments, and a greenhouse experiment examining the effects of soil moisture on early seedling performance. The field experiment was conducted over a 2-year period, using Pseudotsuga in 2002 and both species in 2003. Seedlings were separated into four treatment groups: those planted in open grassland, in shaded grassland, under artificial (plastic) conifer saplings, and under natural Pseudotsuga saplings. Air temperature, relative humidity, soil moisture, incident radiation levels and fog water inputs were measured for each treatment group in 2003. In the greenhouse experiment, Pseudotsuga and Lithocarpus seedlings were grown for 13 weeks in watering treatments simulating the summer soil moisture conditions of the open grasslands and under Pseudotsuga saplings. Surveys of naturally established seedlings found that Lithocarpus occurred only under Pseudotsuga saplings, while most Pseudotsuga seedlings were located near but not directly under conspecific saplings. In the field experiment, positive effects of tree saplings were much larger than negative effects, resulting in strong net facilitation of seedling establishment. Survival for both species was always higher under the plastic and live trees than in the open or shade plots. The primary mechanism facilitating seedling survival appeared to be increased soil moisture caused by input of fog precipitation coupled with reduced microsite evaporation. The greenhouse experiment further showed that soil moisture strongly affected seedling performance, with both species having much higher photosynthetic rates in the higher moisture treatment. In the lower moisture treatment, Pseudotsuga seedlings had higher photosynthetic rates and stomatal conductance than Lithocarpus, suggesting they may be able to better tolerate the environmental conditions found in the open grasslands. Our combined results suggest that rate and patterning of woody plant encroachment can be strongly influenced by facilitation and that fog precipitation may play a key role in plant interactions.     

Characterization of diazotrophic bacteria associated with maize: effect of plant genotype, ontogeny and nitrogen-supply

Summary The population size of diazotrophic bacteria naturally associated with the maize rhizosphere can be affected by biotic and environmental factors. In this work we have evaluated the effect of two genotypes of maize, with and without nitrogen fertilization, on the population dynamics and distribution of diazotrophic bacteria associated with maize plants over different plant ontogenic stages. The study was carried out in a field experiment with and without nitrogen fertilization, using two maize cultivars (Santa Helena 8447 and Santa Rosa 3063) previously selected from 32 maize cultivars for the lowest and highest response to nitrogen fertilization, respectively. Microbiological and molecular approaches were used to characterize the diazotrophic bacterial population structure. Bacterial population was assessed by the most probable number using semi-solid N-free media, and by DNA isolation from soil and plant tissue followed by amplification of nifH gene fragments using nested PCR, and by RFLP analysis using the restriction endonucleases TaqI and HaeIII. The dynamics of the diazotrophic bacterial population were affected by the ontogenic stage of the maize plants, but not by the cultivar type. Roots were the preferred site of colonization, independent of cultivar type and growth stage. During the first stage of maize growth, addition of nitrogen fertilizer negatively affected the diazotrophic bacterial population.     

Structure and regeneration patterns of the <Emphasis Type="Italic">Juniperus excelsa</Emphasis> Bieb. stands in the central part of the Nestos valley in the northeast of Greece,in the context of anthropogenic disturbances and nurse plant facilitation

In order to analyse the structure and regeneration patterns of the Juniperus excelsa Bieb. stands on the slopes of the Nestos valley, 40 sample plots were established and more than 120 J. excelsa trees and seedlings were cut and collected. The main results of this research indicate that stand structure, species mixture, age structure and stand development patterns were strongly determined by the interaction of anthropogenic disturbances such as grazing and illegal cuttings, and the facilitation of adult J. excelsa trees in the establishment of the J. excelsa seedlings. Their effect was influenced by site quality and J. excelsa together with other species ecology. Under present conditions, almost all J. excelsa seedlings, except for a small number of plants, are found under the facilitation of nurse plants that belong to the same species. It seems that the layer of litter underneath the nurse plants combined with the protection of seedlings against grazing play an important role in the establishment and growth of J. excelsa seedlings.     

Effects of VA-mycorrhizal fungi on growth and nutrient uptake of cuttings ofRosa multiflora in two container media with three levels of fertilizer application

Rooted cuttings ofRosa multiflora ‘Brooks 56’ were grown in a medium of 1 mineral soil: 1 sand (v/v) or bark: 1 sand (v/v) inoculated with the VA-mycorrhizal (VAM) fungiGlomus mosseae (Nicol. and Gerd.) Gerd. and Trappe andG. fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe or left as noninoculated controls. The slow release fertilizer osmocote was applied at rates of either 0, 1.2, or 4.2 kg/m³ (18N-6P-12K) and incorporated into the container medium. After 180 days plants were evaluated for growth, development and chemical leaf analysis. Greatest growth responses occurred after the highest fertilizer application of 4.2 kg/m³, and the poorest one after 0 kg/m³. Combination bark: sand medium was superior to mineral soil: sand medium for growth of mycorrhizal plants. At 0 kg/m³, mycorrhizal plants in bark: sand medium had longer shoots than other treatments. At 1.2 kg/m³, VAM plants compared to nonmycorrhizal plants in park:sand medium had greater effect on growth parameters. At the highest fertilizer application of 4.2 kg/m³, greatest growth responses occurred with VAM plants in bark:sand medium. Mycorrhizal plants compared to nonmycorrhizal plants in bark:sand medium had greater K and Zn uptake at 0 kg/m³, and greater K, Ca, S, Mn and Zn uptake at 1.2 kg/m³. Texas Agr. Expt. Sta. Journal Series No. TA-22264.     

The effects of Fusarium oxysporum f.sp. lycopersici (Sacc.) Snyder & Hansen on tomato in diphenamid-treated soil

Pre-emergence soil application of the herbicide diphenamid in concentrations exceeding the normal field rate increased the resistance of tomato plants towards infection by the wilt fungus Fusarium oxysporum f.sp. lycopersici. This was detected as significant increases in the percentage emergence of seedlings although growth parameters of the raised seedlings were reduced. Treated plants exhibited no wilt symptoms, although the pathogen maintained its population at detectable levels in the rhizosphere of tomato plants. However, the growth inhibition caused by diphenamid alone was much less than that reported for the combined application of pathogen and herbicide. Growth activities of F. oxysporum f.sp. lycopersici were inhibited by high concentrations of diphenamid in vitro. It is possible that the biodegradation of this herbicide by species such as Aspergillus candidus (present in substantial counts in treated rhizospheres) was one of the causes of increased tolerence of the pathogen to the herbicide in situ.     

Within-population variation in susceptibility to Agrobacterium tumefaciens A281 in Picea abies (L.) Karst

Summary The purpose of this study was to investigate the genetic variation of susceptibility to Agrobacterium within a Picea abies population. Tumor formation was studied in 16 open-pollinated families belonging to a central Swedish population of Picea abies. Strain A281 of Agrobacterium tumefaciens was used to infect 3-monthold seedlings in a five-block greenhouse experiment. The analysis of variance showed strong significance for the between-family variation of tumor-formation percentages, varying from 28% to 73%. The most susceptible material will be suitable for experiments on the production of transgenic plants in vitro using disarmed Agrobacterium strains.     

Growth reduction in Plantago lanceolata in relation to biotic factors in the soil environment

Summary To investigate the influence of soil-borne pathogens on the population biology of a grassland species in natural vegetation, soil samples were collected, partly disinfected or sterilized and then planted with Plantago lanceolata. Those on some of the untreated soils showed growth reduction due to a biotic factor in the soil. This growth reduction was not associated with the density of P. lanceolata plants at the sampling sites. It was only visible when differences in abiotic factors — especially nutrient levels —were eliminated. In the natural situation the nutritional status of the soil has a far greater impact than biotic soil factors. Micro-organisms harmful to Plantago roots are considered to be of minor importance in the distribution of the individuals in the P. lanceolata population studied.Study performed by Grassland Species Research Group No 113     

Characterization of plant-growth promoting diazotrophic bacteria isolated from field grown Chinese cabbage under different fertilization conditions

Diazotrophic bacteria isolated from the rhizosphere of Chinese cabbage were assessed for other plant growth promoting characteristics viz., production of IAA, ethylene, ACC deaminase, phosphate solubilization, and gnotobiotic root elongation. Their effect on inoculation to Chinese cabbage was also observed under growth chamber conditions. A total of 19 strains that showed higher nitrogenase activity identified by 16S rRNA gene sequence analysis were found to be the members of the genera Pseudomonas and Agrobacterium belonging to α- and γ-Proteobacteria groups. These strains were also efficient in producing IAA and ACC deaminase though they produced low levels of ethylene and no phosphate solubilization. In addition, inoculation of selected diazotrophic bacterial strains significantly increased seedling length, dry weight, and total nitrogen when compared to uninoculated control. The colonization of crop plants by diazotrophic bacteria can be affected by many biotic and abiotic factors, and further studies are oriented towards investigating the factors that could influence the establishment of a selected bacterial community.     

Facilitation of seedling establishment by an endemic shrub in tropical coastal sand dunes

Facilitation is predicted to occur on coastal sand dunes as these ecosystems have the harsh physical conditions common during primary succession. In a tropical dune system on the Gulf of Mexico the spatial patterns of plant distribution were analyzed and the hypothesis that facilitation is the responsible mechanism was tested by monitoring changes in the micro-environment and performing a seedling planting experiment under natural conditions. Densities of seedlings and adults of late colonizer grasses were two to six times greater beneath the early colonizer shrub, Chamaecrista chamaecristoides, than in the exposed areas. Temperatures on the sand surface, wind speed and sand accretion were significantly reduced by the shade of Chamaecrista. Only phosphate contents during the dry season were significantly higher in sand in the shade than in exposed sand. In the field experiment, successful establishment of seedlings of two late colonizer grasses (Trachypogon plumosus and Schizachyrium scoparium) was low but was concentrated exclusively beneath the shrubs. The introduced plants were reproductive one year after onset of the experiment and remained vigorous after three years. The observed spatial aggregation among the target species may be a result of improved conditions in the shade of the shrub, which facilitated the survival and establishment of late colonizers. On top of the environmental amelioration, protection from recurrent disturbances such as substrate mobility is an additional beneficial effect of the shrub.     

Interaction of Glomus mosseae and Paecilomyces lilacinus on Meloidogyne javanica of tomato

The effects of Glomus mosseae and Paecilomyces lilacinus on Meloidogyne javanica of tomato were tested in a greenhouse experiment. Chicken layer manure was used as a carrier substrate for the inoculum of P. lilacinus. The following parameters were used: gall index, average number of galls per root system, plant height, shoot and root weights. Inoculation of tomato plants with G. mosseae did not markedly increase the growth of infected plants with M. javanica. Inoculation of plants with G. mosseae and P. lilacinus together or separately resulted in similar shoots and plant heights. The highest root development was achieved when mycorrhizal plants were inoculated with P. lilacinus to control root-knot nematode. Inoculation of tomato plants with G. mosseae suppressed gall index and the average number of galls per root system by 52% and 66%, respectively, compared with seedlings inoculated with M. javanica alone. Biological control with both G. mosseae and P. lilacinus together or separately in the presence of layer manure completely inhibited root infection with M. javanica. Mycorrhizal colonization was not affected by the layer manure treatment or by root inoculation with P. lilacinus. Addition of layer manure had a beneficial effect on plant growth and reduced M. javanica infection.