Threshold Level Of Damage Research Articles

Incidence and control of the citrus nematode, Tylenchulus semipenetrans Cobb, in the north of Iran

Abstract A survey of citrus orchards of northern Iran revealed that the citrus nematode, Tylenchulus semipenetrans, was present in 89% of soil and root samples collected. The number of juveniles and females averaged 6490/250 ml soil and 214/g root, respectively, in infested samples, and the mean numbers of juveniles and females in soil and root samples of sweet orange and sour orange rootstocks were greater than the damage threshold level in most regions. Population patterns of citrus nematode on sour orange Citrus aurantium and Poncirus, Poncirus trifoliata, rootstocks were monitored for 2 successive years. Nematode population densities peaked once a year during July-August and declined to lower levels during autumn and winter on both rootstocks. The duration of the life cycle of T. semipenetrans was 8 and 9 weeks on sour orange C. aurantium and Troyer citrange, C. sinensis × P. trifoliata, seedlings, respectively, at 24 ± 2°C. In glasshouse experiments, T. semipenetrans was controlled on sour orange seedlings by the granular nematicides, fenamiphos and cadusafos, the doses 4 and 8 ppm having the greatest effect when applied simultaneously with nematodes, whereas the nematicides Vydate and fenamiphos at 8 ppm showed greatest effects on nematodes in roots when applied 6 weeks after nematodes had been added to the soil. The effects of granular fenamiphos and cadusafos (10G) on T. semipenetrans population densities were also investigated in infested citrus orchard with rates 1.0 and 2.0 g a. s. of nematicides/m2 for 1 year. Although the nematicide treatments reduced nematode population densities in soil and roots, fruit weight and size was not affected by these treatments.

Differential sensitivity of polytene chromosomes to Nd:YAG laser

A fundamental understanding of the interaction of Nd:YAG lasers with biological tissues have remained a challenging field demanding further investigation, especially exploring the threshold level of chromatin damage. The present study aims at addressing these questions using an insect derived model tissues, namely salivary glands (SG) and malpighian tubules (MT) of a midge Chironomus ramosus. Both SG & MT have polytenized chromosomes in their nuclei with a variable number of chromatids. Upon Nd:YAG (λ = 1064 nm) laser irradiation, using a fixed dose with variable time of exposure, SG & MT showed different levels of sensitivity. The onset and rate of induction of pyknotic nuclei and the threshold level of laser-induced pyknosis of chromatin material were dependent on exposure time. Parameters used for laser irradiation were non-ablative in order to avoid thermal injury. The temperature of the samples was kept controlled within ± 1 °C and did not vary during laser exposure. Our findings suggest that non-ablative laser-induced cell death is dependent on chromatin organization. International Journal of Biological & Chemical Sciences Vol. 1 (1) 2007: pp. 1-6

Low amounts of herbivory by root-knot nematodes affect microbial community dynamics and carbon allocation in the rhizosphere

Increased carbon translocation to the rhizosphere via 'leakage' induced by low amounts of plant parasitic nematodes can foster microorganisms. The effects of the root-knot nematode Meloidogyne incognita on microbial biomass (C(mic)) and community structure (phospholipid fatty acids) in the rhizosphere of barley were studied. Inoculation densities of 2000, 4000, and 8000 nematodes were well below the threshold level for plant damage. A (13)CO(2) pulse-labelling was performed to assess the distribution of assimilated (13)C in the rhizosphere. Infection with M. incognita increased the carbon concentration in shoots, and enhanced root biomass slightly. The presence of nematodes did not affect microbial biomass, but significantly changed the allocation of the recent photosynthate. Less plant carbon was sequestered by microorganisms with increasing nematode abundance. Microbial community structure was distinctly altered in the early stages of the plant-nematode interactions. Both, bacteria and fungi, showed a positive response with 2000, and a negative one with 4000 and 8000 M. incognita added. The results suggest that low-level root herbivory still imposes a considerable carbon demand, and that proliferation of microorganisms due to increased rhizodeposition may be short-termed. The carbon flow to rhizosphere microbial communities is likely dependent on the specific nematode-plant association and the developmental stage of the nematode in the host.

Stimulated Brillouin scattering in magnetized diffusive semiconductor plasmas

Using the hydrodynamical model and following the coupled mode approach, detailed analytical investigation of stimulated Brillouin scattering is performed in an electrostrictive semiconductor. The total induced current density including diffusion current density and the effective Brillouin susceptibility are obtained under off-resonant laser irradiation. The analysis deals with the qualitative behaviour of the Brillouin gain and transmitted intensity with respect to excess doping concentration and magnetic field. Efforts are directed towards optimizing the doping level and magnetic field to achieve maximum Brillouin gain at pump intensities far below the optical damage threshold level. It is found that by immersing a moderately doped semiconductor in a sufficiently strong magnetic field in transverse direction, one can achieve resonant enhancement of Brillouin gain provided the generated acoustic mode lies in the dispersionless regime.

Patch spraying of weeds in spring cereals: Simulated influences of threshold level and spraying resolution on spraying errors and potential herbicide reduction

A major obstacle to patch spraying of broad-leaved weeds in cereals is a cost-effective method to assess within-field heterogeneity of the weed population. One method could be a camera mounted in front of the spraying vehicle, online image analysis, and field sprayer shifting between ‘on’ and ‘off’ as the predefined weed damage threshold level is reached. Because such a camera will capture a very limited area (<1 m2) compared to the sprayer width (several m), success of this method requires that spraying decisions vary little within boom width, thereby causing few spraying errors. This approach was evaluated by simulations for varying boom widths and three levels of a weed damage threshold model. Potential herbicide reductions compared to blanket application were also simulated. The average potential herbicide reductions estimated as proportions of fields below the threshold, were 60%, 64% and 53% for the original, 25% increased and 25% reduced threshold levels, respectively. The simulated herbicide reductions were not influenced by boom width, but varied significantly between fields, and between threshold levels. As evaluated by spraying errors, the suitability of the suggested approach will increase by decreasing boom width, vary between fields, and in some fields vary between the threshold levels. For boom widths of 15 m and 24 m, the spraying errors were about 10% and 15%, respectively, where omission of spraying areas above the threshold constituted 5% and 8%, respectively.

Electrophysiological correlates of progressive sensorineural pathology in carboplatin-treated chinchillas

Carboplatin produces progressive damage to auditory nerve fibers, spiral ganglion neurons (SGNs) and inner hair cells (IHC) in the chinchilla cochlea but leaves outer hair cells intact. Within 1 h after injection, many afferent terminals beneath IHCs and myelin lamellae surrounding SGN processes are vacuolated. One day after injection, approximately half of the nerve fibers are missing. IHCs are intact at 2 days, but 20–30% are missing at 3 days. We studied the electrophysiological correlates of this progressive morphological damage by recording cochlear microphonics (CM), distortion product otoacoustic emissions (DPOAE), summating potentials (SP), compound action potentials (CAP) and midbrain evoked potentials (IC-EVP) before and 1 h, 12 h, 1 days, 3 days, 5 days, 7 days and 14 days after carboplatin injection (75 mg/kg IP) in four chinchillas. CM and DPOAEs tended to be unchanged or enhanced. CAP and SP showed little change until Day 3, when amplitudes were reduced in all animals and CAP thresholds were elevated by 9 dB; amplitudes declined further between Days 3 and 5 but not thereafter. IC-EVP amplitudes decreased on Days 3 or 5 but thresholds were relatively unchanged. All animals showed some recovery of IC-EVP between Days 7 and 14, including one with 70% enhancement on Day 14. The results indicate that threshold and amplitude measures fail to detect peripheral pathology until some relatively high threshold level of damage has been exceeded. This has important implications for monitoring peripheral damage and interpreting electrophysiological test results in animals and humans.

Sperm DNA damage: importance in the era of assisted reproduction

With the proliferation of assisted reproductive technologies, there is an increasing awareness of the importance of sperm DNA damage. With the recent advent of in-vitro fertilization/intracytoplasmic sperm injection, we are bypassing the normal natural selection barriers and potentially introducing sperm with damaged DNA. To date, these consequences are still largely unknown. Infertile men possess substantially more DNA-damaged sperm than fertile men, and this DNA damage may adversely affect reproductive outcomes. There may be a threshold level of DNA damage beyond which embryo development and subsequent pregnancy are impaired. Protamine deficiency and the effect of reactive oxygen species have been implicated in the etiology of sperm DNA damage. Assays of DNA damage are being used clinically to quantify objectively the degree of DNA damage in the sperm of infertile men in the hope of identifying more accurate ways of gauging fertility potential. There now exists clinical evidence to show that sperm DNA damage is detrimental to reproductive outcomes. Tests for DNA damage may provide better prognostic information and may allow for better decision-making than standard semen parameters when evaluating the infertile couple. The etiology of the DNA damage and the full extent of the damage on reproductive outcomes need further study.

Predicting Sliding Wear Behaviour of a Tin-Based White Metal Under Varying Pressure and Speed Conditions

To determine the effects of loading on the sliding wear behaviour of a tin-based white bearing metal JIS WJ2, experiments were performed over a range of pressures and speeds. A linear relation between stress and cut-off life was obtained on the log-log plot at constant shaft speed. These relations were expressed as different straight lines on the log-log plot, which depend on the shaft speed. However, there is a unique curve in the relation, frictional stress versus cut-off life, regardless of the shaft speeds. To establish a quantitative method of analysing the steady-state sliding wear mode, the following constitutive relation was proposed d w/d N = A(τ – τth)2 where w is the amount of wear, N the number of shaft rotations, A the material constant, τ the interfacial shear stress, and τth the threshold level for wear damage. The aforementioned equation has been used as a basis for estimating wear life. Good agreement between the predictions and the experimental results was obtained.

Type II Chemotherapy-Related Cardiac Dysfunction: Time to Recognize a New Entity

Cancer chemotherapy can affect the heart in a variety of ways. Fluorouracil and capecitabine may initiate coronary artery spasm, high-dose cyclophosphamide can induce a hemorrhagic myonecrosis, and paclitaxel is associated with dysrhythmia. However, the form of chemotherapyrelated cardiac dysfunction (CRCD) of the greatest interest and concern among oncologists and cardiologists is that which directly involves the myocardium, is manifested by a decreased left-ventricular ejection fraction, and which may progress to congestive heart failure; this form is the focus of our brief commentary. CRCD came to the forefront of concerns over chemotherapy in the early 1970s, when anthracyclines were shown to exhibit cumulative– dose-related cardiac dysfunction. Later analyses by Von Hoff et al of cardiac function in more than 2,000 anthracycline-treated patients demonstrated the relative safety of low cumulative dosages and the dramatically increased incidence of congestive heart failure at higher cumulative dosages. A cumulative dose of 550 mg/m of doxorubicin, the most commonly used anthracycline, was felt to balance antitumor benefits and the risk of cardiac dysfunction, keeping this risk at acceptable levels. Billingham et al, and later Mackay et al, correlated anthracycline-associated cardiac failure with structural abnormalities that they identified in electron-microscopic analyses of cardiac biopsy material. Vacuoles, myofibrillar disarray and dropout, and at higher cumulative dosages, myocyte necrosis occurred in the cardiac ultra structure. These anthracycline-associated abnormalities and their related cardiac dysfunction constitute an entity that should now be considered type I CRCD since, as we will demonstrate, it differs from another type of CRCD, which has been described more recently and does not entail the myocardial damage of type I CRCD. Type I CRCD is well understood. Risk factors associated with an increased likelihood of this adverse effect have been identified and commonly are associated with increased left-ventricular end-diastolic pressure. Methods to mitigate type I CRCD include prolonged infusional administration schedules and use of the free-radical scavenger dexrazoxane. Liposomal delivery systems and less-toxic analogs are being actively explored. Relatively recently, doxorubicin was shown to be considerably more cardiotoxic than had been previously recognized; in addition, it is currently believed that type I cardiac damage takes place from the earliest administrations of the drug, that the toxicity follows a simple mathematical relationship, and that once a threshold level of damage takes place, cell death ensues. Although the cardiovascular system may compensate for the cell loss, myocardial damage predominantly expressed clinically by left-ventricular dysfunction remains, making the patient more vulnerable to sequential stresses that may arise from a variety of causes including infections and cardiomyopathies of other etiology. Noninvasive tests to quantify and follow the extent of such cardiac changes have been suboptimal because of their lack of sensitivity and because the heart and the circulatory system have considerable reserves. Until recently, cardiac dysfunction manifested by leftventricular failure in association with any chemotherapeutic agent has been compared with doxorubicin cardiac dysfunction. All drugs with any documented degree of such an effect have been thought to have qualitatively similar cardiac effects and long-term sequelae. This thinking has been challenged recently by a relatively new cancer (of the breast) biotherapy agent, trastuzumab, which is a monoclonal antibody targeted at the molecule HER-2. The initial cardiac toxicity ascribed to trastuzumab was thought to be either less severe than, or as yet below the threshold for, typical cardiac changes associated with anthracyclines. Subsequently, results of extensive tests and clinical experience with trastuzumab revealed true JOURNAL OF CLINICAL ONCOLOGY COMMENTS AND CONTROVERSIES VOLUME 23 NUMBER 13 MAY 1 2005

Compression-induced damage in a muscle cell model in vitro

Soft tissue breakdown can be initiated at the muscle layer associated with bony prominences, leading to the development of pressure ulcers. Both the magnitude and duration of pressure are important factors in this breakdown process. The present study utilizes a physical model, incorporating C2C12 mouse myoblasts in a homogeneous agarose gel, to examine the damaging effects of prolonged applied pressure. Identical cylindrical cores cut from the agarose/cell suspension were subjected to two separate compressive strains, of 10 and 20 per cent. The strain was applied for time periods ranging from 0.5 to 12 hours, using a specially designed loading apparatus. After each compression period, sections taken from the central horizontal plane of the individual constructs were stained using either haematoxylin and eosin or with the fluorescent probes, Calcein AM and ethidium homodimer-1, and assessed for cell damage. It was found that constructs subjected to the higher strain values demonstrated significantly higher values of non-viable cells for equivalent time points compared to the unstrained constructs. Further analysis on sections using the DNA nick-translation method suggested that this increase was primarily due to apoptosis. These findings imply a relationship between the duration of applied compression and damage to muscle cells seeded in the gel, which was particularly apparent at the strain level of 20 per cent, equivalent to a clinically relevant pressure of 32 mmHg (4.3 kPa). Such an approach might be useful in establishing damage threshold levels at a cellular level.

A coupled column generation, mixed integer approach to optimal planning of intensity modulated radiation therapy for cancer

Approximately 40% of all U.S. cancer cases are treated with radiation therapy. In Intensity-Modulated Radiation Therapy (IMRT) the treatment planning problem is to choose external beam angles and their corresponding intensity maps (showing how the intensity varies across a given beam) to maximize tumor dose subject to the tolerances of surrounding healthy tissues. Dose, like temperature, is a quantity defined at each point in the body, and the distribution of dose is determined by the choice of treatment parameters available to the planner. In addition to absolute dose limits in healthy tissues, some tissues have at least one dose-volume restriction that requires a fraction of its volume to not exceed a specified tighter threshold level for damage. There may also be a homogeneity limit for the tumor that restricts the allowed spread of dose across its volume. We formulate this planning problem as a mixed integer program over a coupled pair of column generation processes -- one designed to produce intensity maps, and a second specifying protected area choices for tissues under dose-volume restrictions. The combined procedure is shown to strike a balance between computing an approximately optimal solution and bounding its maximum possible suboptimality that we believe holds promise for implementations able to offer the power and flexibility of mixed-integer linear programming models on instances of practical scale.

Low doses of gamma-radiation induce nonlinear dose responses in Mammalian and plant cells.

The percentage of cells with chromosome aberrations or micronuclei induced by low doses of acute (dose rate of 47 cGy/min) or chronic (dose rate of 0.01 cGy/min) gamma-irradiation was studied in vitro in Chinese hamster fibroblasts, human lymphocytes, and Vicia faba seeds and seedlings. The sensitivity of the indicated biological entities to low doses was greater than expected based on linear extrapolation from higher doses. The dose-response curves for cytogenetic damage that were obtained were nonlinear when evaluated over the full range of the doses used. At very low doses, the dose-response curves appeared linear, followed by a plateau region at intermediate doses. At high doses the dose response curves again appeared linear with a slope different from that for the low-dose region. There was no statistically significant difference between the yields of cells with micronuclei induced by low doses of acute versus chronic irradiation. Similar data were obtained both for human lymphocyte culture and for roots and seeds of Vicia faba. Our experiments revealed that the dose range over which the plateau occurs depends on the type of cells irradiated. We have also shown that the modifying effects of the repair inhibitor caffeine and the radioprotector mercaptoethylenamine (MEA) are absent at low doses of gamma irradiation and that caffeine increased the number of cells with cytogenetic damage when evaluated over the plateau region. In the presence of MEA, the upper end of the plateau region was extended from just above 1 Gy to about 2 Gy. We therefore provide direct evidence that a plateau exists in the dose-response curve for the indicated radiation-induced stochastic effects. Furthermore, our results suggest that, for low linear energy transfer radiation, the induction of DNA repair occurs only after a threshold level of cytogenetic damage and that the higher yield of cytogenetic damage per unit dose at low radiation doses is attributable to an insignificant contribution or the absence of DNA repair processes.

Uranium deposition and retention in a USTUR whole body case.

This report describes a whole body donation from a person with a documented occupational intake of uranium. USTUR Case 1002 was an adult male who died from an acute cerebellar infarct at the age of 83. He worked as a power operator, utility operator, and metal operator for 28 years in a facility that processed and handled radioactive materials. Although he suffered a number of burns from hot metal and acids, cuts, abrasions, and puncture wounds during his many years of work, there were no corresponding health physics or medical records to indicate that these occurrences needed or required excision or decontamination due to the suspicion of the deposition of radioactive material. Over the course of his employment, USTUR Case 1002 submitted numerous urine samples for uranium, plutonium, and fission product analysis. The highest single uranium value measured during this time period was approximately 30 microg L(-1) recorded during the second year of his employment. A urinary bioassay sample taken before termination of employment measured 4.3 microg L(-1). The mean urinary uranium concentration per liter per year calculated from the employee's bioassay records covering the first eleven years of monitoring averaged less than 3 microg L(-1). The ratio of 234/238U activity in the lung tissue was about 1, the same as that found in natural uranium. The highest concentration of uranium was found in a tracheobronchial lymph node. The uranium content in the various tissues of the body followed a rank order lung > skeleton > liver > kidney. Concentration of uranium in the kidney tissue was approximately 1.98 ng g(-1), about 3 orders of magnitude less than the generally accepted threshold level for permanent kidney damage of 3 microg U g(-1) and roughly equal to the 1.4 ng g(-1) reported for Reference Man. The autopsy disclosed findings not uncommon in the aged: severe atherosclerosis, areas of sclerotic kidney glomeruli with stromal fibrous scarring, and moderate to severe arterionephrosclerosis. Lung sections contained parenchymal areas of acute vascular congestion and a mild degree of anthracosis.

Cure effects on post-impact tensile characteristics of 2D epoxy composites

Epoxy and hybrid fabric of aramide and glass fiber was used to obtain quasi-orthotropic composite laminates. They were characterised to tensile and their residual characteristics were determined after impacts from 2 to 8 J. The choice of curing conditions was found to be determinant in the mechanical characteristics. It could be verified that there is one alternative cure treatment in mechanical properties, time and cost to the proposed one by the resin manufacturer for the consolidation of the laminate. The incidence of the impact is barely pronounced at low energies and it was able to establish a threshold level of damage surrounding to 2 J for multi-step cured laminates. The losses in elastic modulus are higher. For strength and toughness losses become very high. Toughness is the indicator with the highest degree of variation. Differences on losses are influenced by cure treatment in all of tensile indicators.

Simultaneous irradiation of laser and ion beams on optical materials

A simultaneous irradiation system of laser and ion beams has been developed to investigate ion beam induced luminescence and optical absorption of crystalline α-Al 2O 3 samples. The luminescence induced by 30 keV Ar + in visible wavelength region had peaks that were attributed to oxygen vacancies. The optical absorption at 633 nm in wavelength was also measured during the ion irradiation. The amount of optical absorption for the irradiated sample with Q-switched Nd:YAG laser pulses was less than that without the laser pulses. The ion irradiation reduced the threshold level of laser damage on the surface region of the sample, and the laser ablation selectively happened on the ion irradiation region.

Review: Rise of the Young Turks: Politics, the Military and Ottoman Collapse M. Naim Turfan

The hemlock woolly adelgid Adelges tsugae Annand (Hemiptera: Adelgidae), is a non-native invasive pest that has caused widespread decline and mortality of eastern hemlock ( Tsuga canadensis (L.) Carr. (Pinales: Pinaceae)) and Carolina hemlock ( T. caroliniana Engelm.) in the eastern United States. Our preliminary experiments evaluated the utility of a rain-down technique to induce artificial infestations of A. tsugae on hemlock seedlings en masse. Experiments were conducted in PVC (1 m 3 ) cages topped with poultry wire for placement of A. tsugae -infested branches, and with 1 m 2 gridded glue sheets and/or hemlock seedlings placed below to capture adelgid abundance, distribution, and infestation rate data. In the March 2011 experiment, the density of progrediens crawlers (adelgid nymphs, first instars) that rained down inside the PVC cages was significantly higher in the high ovisac treatment compared to the low ovisac treatment, with an estimated 513,000 and 289,000 crawlers per m 2 falling beneath each treatment, respectively. Resulting A. tsugae infestation rates on Carolina hemlock seedlings placed inside the cages did not differ between the treatments but were at or above established damage threshold densities for the adelgid. Infestation rates on eastern hemlock seedlings that were placed in cages nine days after the experiment started were below damage threshold levels and did not differ between the treatments. In the May 2011 experiment, the density of sistens crawlers raining down was substantially lower, with 17,000 and 33,000 falling per m 2 in the low and high ovisac treatments, respectively. Resulting infestation rates on Carolina hemlock seedlings were extremely low and well below damage threshold levels. Although A. tsugae crawlers were well distributed across the 1 m 2 gridded glue sheets placed at the bottom of each cage, hot spots of unusually high crawler density did occur in both experiments. This rain-down technique shows potential for use in an operational tree-breeding program where screening large numbers of hemlock seedlings for resistance to A. tsugae is required.

DNA damage-induced G2-M checkpoint activation by histone H2AX and 53BP1.

Activation of the ataxia telangiectasia mutated (ATM) kinase triggers diverse cellular responses to ionizing radiation (IR), including the initiation of cell cycle checkpoints. Histone H2AX, p53 binding-protein 1 (53BP1) and Chk2 are targets of ATM-mediated phosphorylation, but little is known about their roles in signalling the presence of DNA damage. Here, we show that mice lacking either H2AX or 53BP1, but not Chk2, manifest a G2-M checkpoint defect close to that observed in ATM(-/-) cells after exposure to low, but not high, doses of IR. Moreover, H2AX regulates the ability of 53BP1 to efficiently accumulate into IR-induced foci. We propose that at threshold levels of DNA damage, H2AX-mediated concentration of 53BP1 at double-strand breaks is essential for the amplification of signals that might otherwise be insufficient to prevent entry of damaged cells into mitosis.

Simulation of neutron interactions at the single-cell level.

We recently reported that the exposure of cancer cells to 14 MeV neutrons at a very low dose rate (0.8 mGy min(-1)) produced a marked increase in cell killing at 5 cGy, followed by a plateau in survival and chromosomal damage. Simulation of the energy deposition events in irradiated cells may help to explain these unusual cell responses. We describe here a Monte Carlo simulation code, Energy Deposition in Cells Irradiated by Neutrons (EDCIN). The procedure considered the experimental setup and a hemispheric cell model. The simulation data fitted the dosimetric measurements performed using tissue-equivalent ionization chambers, Geiger-Müller counters, fission chambers, and silicon diodes. The simulation showed that 80% of the energy deposited in a single cell came from the interactions of neutrons outside the cell and only 20% came from neutron interactions inside the cell. Thus the "external" interactions that result in the production of recoil protons and secondary electrons may induce most of the biological damage, which may be repaired efficiently at low dose rate. The repair process may be triggered from a threshold level of damage, which would explain an initial increase cell death due to unrepaired sublethal damage, and then may compensate for induced damage, resulting in the plateaus.

Biomonitoring of tropospheric ozone phytotoxicity in rural Catalonia

The ozone (O 3) phytotoxicity in rural areas of Catalonia (NE Spain) and the biomonitoring capacity of Bel-W3 tobacco ( Nicotiana tabacum) cultivars were assessed by determining the percentage of leaf area injured by ozone in plants of this cultivar exposed from spring to autumn since 1995–1999. The study was conducted simultaneously on nine field sites where ground level ozone concentrations and meteorological parameters were continuously monitored. Geographical, seasonal and annual variations of ozone damage rate and their links with meteorological conditions were studied. Ozone concentrations and leaf damage increased at the end of spring and the beginning of summer. Coastal sites generally presented higher O 3 concentrations than inland and mountain sites. These mountain sites were the most sensitive ones to ozone toxicity. The ozone concentrations correlated well with ozone injury. However, at this local scale the ozone levels did not fully account for all the observed injury (only 11%). The response of tobacco plants to ozone concentrations and therefore its biomonitoring capacity depended also on different environmental conditions, mainly those linked to stomatal behaviour such as vapour pressure deficit. The categorization of leaf damage in 10% intervals and its averaging throughout the whole study period and the whole region, strongly improved (99% of variance accounted) the relationship with ozone concentrations expressed as AOT20 (accumulated over a cut-off of 20 ppb v). N. tabacum cultivar Bel-W3 is thus a very good biomonitor of ozone concentrations in the long term at the regional scale. Taking into account the phytotoxical response of this sensitive tobacco cultivar, we propose the 1.28 ppm v h biweekly AOT40 (with a solar radiation threshold of 50 W m −2) as a damage threshold level for sensitive species.

Differential effects of insect herbivory on arbuscular mycorrhizal colonization.

A series of field and laboratory experiments were conducted to examine whether natural levels of insect herbivory affect the arbuscular mycorrhizal (AM) colonization of two plant species. The plant species were the highly mycorrhizal (mycotrophic) Plantago lanceolata, which suffers small amounts of insect damage continuously over a growing season and the weakly mycorrhizal (non-mycotrophic) Senecio jacobaea, which is frequently subject to rapid and total defoliation by moth larvae. Herbivory was found to reduce AM colonization in P. lanceolata, but had no effect in S. jacobaea. Similarly, AM colonization reduced the level of leaf damage in P. lanceolata, but had no such effect in S. jacobaea. AM fungi were found to increase growth of P. lanceolata, but this effect was only clearly seen when insects were absent. AM fungi reduced the growth of S. jacobaea irrespective of whether insects were present. It is concluded that the reduction of AM fungal colonization by herbivory in P. lanceolata is due to the reduced amount of photosynthate available to the symbiont. This may only become apparent at threshold levels of insect damage and, below these, increased photosynthesis elicited by the mycorrhiza is able to compensate for foliage loss to the insects. However, in S. jacobaea, the mycorrhiza appears to be an aggressive parasite and insect attack only exacerbates the reduction in biomass. In mycotrophic plants, insect herbivores may be responsible for poor functioning of the symbiosis in field conditions and there is a symmetrical interaction between insects and fungi. However, in non-mycotrophic plants, the interaction is strongly asymmetrical, being entirely in favour of the mycorrhiza.