Below-ground herbivory promotes soil nutrient transfer and root growth in grassland

Extremely little is known about the ecosystem-level implications of below-ground herbivory, which often represents the dominant form of consumption of primary productivity. We provide the first empirical evidence that low levels of below-ground herbivory may promote soil nutrient flux and root growth of both host plants and companion plants. Low levels of white clover ( Trifolium repens L.) root infection by clover cyst nematodes ( Heterodera trifolii Goffart) increased root growth by 141% and 219% in the host plant and the uninfected neighbouring grass ( Lolium perenne L.), respectively. Root infection increased the size of the soil microbial biomass in the root zone and the transfer of ¹⁵N from the host plant to soil and the neighbouring grass. These data suggest that low amounts of below-ground herbivory may increase the transfer of plant carbon and nitrogen below-ground, leading to increases in root growth and soil nutrient recycling in grasslands. Presumably, such interactions will influence the competitive interactions between plant species, altering plant community structure in grasslands.     

Insect herbivory in relation to dynamic changes in host plant quality*

Evidence that chemical changes in plants following insect feeding can lead to reduced grazing levels, enhanced insect movement and selective leaf avoidance is briefly reviewed. A simple model is constructed in which changes in damaged and/or adjacent leaves lead to effects on herbivore performance. The model reveals that as the density of herbivore larvae/plant increases from one to twenty-four, wound-induced changes in the leaves reduce larval survival by up to 40%, treble the number of movements of the larvae and increase their development time by c. 10%. The distribution of grazing between leaves changes in the direction of more leaves with lower grazing levels but overall grazing levels are not greatly affected by the above changes in larval performance. The model's output is discussed in relation to recent views concerning the relative roles of intra-specific competition and predation in regulating insect herbivore numbers.     

Flight or fight: flexible antipredatory strategies in porcelain crabs

Autotomy, the voluntary shedding of limbs or other body partsin the face of predation, is a highly effective escape mechanismthat has evolved independently in a variety of taxa. Crabs areunusual in that the limb that is typically sacrificed duringautotomy, the anterior clawed cheliped, can also be used toward off attack. During an encounter with a predator, an individualmust thus decide between two mutually exclusive strategies:flight or fight. We used experimental predation encounters withtwo species of porcelain crabs (genus Petrolisthes) to examinethe factors that influence the decision to flee versus fightand to determine the degree to which this decision is context-dependent.We found that autotomy was highly conditional. The characteristicsthat best predicted autotomy—smaller body size or femalegender—also correlated with a lower escape rate by thealternative escape tactic, struggling and pinching the predator.Variation among individuals in the benefit of autotomy (relativeto alternative tactics) appears to drive variation in propensityto autotomize. Porcelain crabs thus demonstrate adaptive flexibility,employing the costly strategy of autotomizing a limb as a lastresort, only when their chance at success by struggling is low.     

Tolerance of Gentianella campestris in relation to damage intensity: an interplay between apical dominance and herbivory

Meristem allocation models suggest that the patterns of compensatory regrowth responses following grazing vary, depending on (i) the number of latent meristems that escape from being damaged, and (ii) the activation sensitivity of the meristems in relation to the degree of damage. We examined the shape of compensatory responses in two late-flowering populations (59°20′N and 65°45′N) of the field gentian. Plants of equal initial sizes were randomly assigned to four treatment groups with 0, 10, 50 and 75% removal of the main stalk. The plants were clipped before flowering, and their performance was studied at the end of the growing season. The northern population showed a linear decrease in shoot biomass and fecundity with increasing biomass removal, while the response in the southern population was quadratic with maximum performance at the damage level of 50% clipping. This nonlinear shape depended upon the activation sensitivity of dormant meristems in relation to their position along the main stem. The highest plant performance was achieved by inflicting intermediate damage which induced regrowth from basally located meristems. In contrast, the topmost branches took over the dominance role of the main stem after minor apical damage (10% clipping). Consequently, the breakage of apical dominance is a necessary precondition of vigorous regrowth in this species. However, compensation in the field gentian is unlikely to be a mere incidental by-product of apical dominance. The ability to regrow from basally located meristems that escape from being damaged by grazing may well be a sign of adaptation to moderate levels of shoot damage. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two different strategies for light utilization in photosynthesis in relation to growth and cold acclimation

Seedlings of Lodgepole pine (Pinus contorta L.) and winter wheat (Triticum aestivum L. cv. Monopol) were cold acclimated under controlled conditions to induce frost hardiness. Lodgepole pine responded to cold acclimation by partial inhibition of photosynthesis with an associated partial loss of photosystem II reaction centres, and a reduction in needle chlorophyll content. This was accompanied by a low daily carbon gain, and the development of a high and sustained capacity for non‐photochemical quenching of absorbed light. This sustained dissipation of absorbed light as heat correlated with an increased de‐epoxidation of the xanthophyll cycle pigments forming the quenching forms antheraxanthin and zeaxanthin. In addition, the PsbS protein known to bind chlorophyll and the xanthophyll cycle pigments increased strongly during cold acclimation of pine. In contrast, winter wheat maintained high photosynthetic rates, showed no loss of chlorophyll content per leaf area, and exhibited a high daily carbon gain and a minimal non‐photochemical quenching after cold acclimation. In accordance, cold acclimation of wheat neither increased the de‐epoxidation of the xanthophylls nor the content of the PsbS protein. These different responses of photosynthesis to cold acclimation are correlated with pine, reducing its need for assimilates when entering dormancy associated with termination of primary growth, whereas winter wheat maintains a high need for assimilates as it continues to grow and develop throughout the cold‐acclimation period. It appears that without evolving a sustained ability for controlled dissipation of absorbed light as heat throughout the winter, winter green conifers would not have managed to adapt and establish themselves so successfully in the cold climatic zones of the northern hemisphere.     

Ambient temperature signaling in plants: An emerging field in the regulation of flowering time

Plants show remarkable developmental plasticity to survive in a continually changing environment. One example is their capability to adjust flowering time in response to environmental changes. Ambient growth temperature, which is strongly affected by global temperature changes, has a profound effect on flowering time. However, those effects have been largely ignored in research. Recent molecular genetic studies ofArabidopsis as a model system have implicated several genes, and have identified a molecular mechanism underlying the responses of plants to changes in ambient temperature. Here, we describe recent discoveries related to ambient temperature signaling and the control of flowering time inArabidopsis. We also discuss current perspectives on how plants sense and respond to such changes.     

UV damage to plant life in a photobiologically dynamic environment: the case of marine phytoplankton

The effect of UV-B radiation on growth of marine phytoplankton was investigated in relation to DNA damage induced by a range of biologically effective doses (BEDs). Emiliania huxleyi (Prymnesiophyceae) was chosen as a model organism of the ocean's phytoplankton because of its importance in global biogeochemical cycling of carbon and sulphur, elements that influence the world's climate as components of the trace gases carbon dioxide (CO₂) and dimethylsulfide (DMS). A marine diatom, Cyclotella, was studied for its capacity to repair the DNA damage, quantified as thymine dimers by the application of a monoclonal antibody against these photoproducts. DNA repair was shown to be complete after just a few hours of exposure to visible light; the repair rate increased with PAR intensity. E. huxleyi appeared to be most sensitive to UV-B radiation: growth was already affected above a dose of 100 J m^-2 d^-1 (biologically effective radiation, weighted with Setlow's DNA action spectrum), probably through effects on the cell cycle related to damage to nuclear DNA: mean specific growth rates were inversely correlated with thymine dimer contents in cells. Near the ocean's surface UV-B radiation conditions that induce the changes observed by us in cultures can be expected during the growing season of phytoplankton, not only in the tropics but also at higher latitudes. Nevertheles, blooms of species such as E. huxleyi are often excessive in the field. It is suggested that exposure duration of cells near the surface of the ocean can be shorter than our artificial 3 h in the laboratory due to vertical mixing, a phenomenon that is typical for the ocean's upper 50–100 m. When mixing reaches depths greater than the layer where most UV-B is attenuated, negative effects on cells through UV-A-induced inhibition of photosynthesis may prevail over DNA damage, the action spectrum of which has been shown to be limited to the UV-B part of the spectrum. Moreover, the radiation wavelengths that induce DNA damage repair (UV-A and visible) are attenuated vertically much less than UV-B. The photobiological situation in the upper ocean is much more complicated than on land, and effects of UV radiation on plankton biota can only be modelled realistically here when both the spectrally differential attenuation in the UV and visual part of the spectrum and the rate of vertical mixing are taken into account. Action spectra of both damage and repair of DNA and of photosynthesis inhibition of representative microalgal species are the second conditio sine qua non if we want to predict the effect of stratospheric ozone depletion on marine phytoplankton performance.     

Submerged plant survival strategies in relation to management and environmental pressures in drainage channel habitats

The abundance of submerged weeds, in relation to management regime and environmental factors, was surveyed during 1992 and 1993 in drainage channels located in four geographically-distinct areas of Britain. The aim of the study was to ascertain, using a multivariate approach, the degree to which species survival strategy and vegetation could be related to disturbance and stress pressures on plant survival. Indices of disturbance and stress were constructed from combined environmental data for each site. A species ordination using Canonical Correspondence Analysis showed that the combined disturbance variable explained more of the variability that did stress. Two main groups of species could be distinguished. The larger group scored low on the disturbance gradient and these species, with different tolerances to stress (especially light-limitation), appeared to be those better-adapted to habitats with low disturbance (e.g. Potamogeton pectinatus and Potamogeton lucens). The smaller group comprised species which tended to occur in sites with higher disturbance (e.g. regular cutting) such as Callitriche stagnalis. Using the terminology of strategy theory, most of the dominant species could be classed as competitive/disturbance tolerators (CD) or variants of this established-phase strategy. The limitations are discussed of applying the strategy approach at species level in a defined habitat-type which shows a high degree of uniformity between sites, such as artificial drainage channels.     

Oviposition and feeding preferences of a flower-feeding weevil,Coelocephalapion aculeatum, in relation to conspecific damage to its host-plant

The oviposition and feeding preferences ofCoelocephalapion aculeatum Fall (Coleoptera: Apionidae), a host specific florivore ofMimosa pigra L. (Mimosaceae), were studied in relation to conspecific damage to its hostplant. Adults ofC. aculeatum cease ovipositing in inflorescences when the egg load reaches a number consistent with the larval carrying capacity of the inflorescence. The basis for this oviposition deterrence was examined by offering inflorescences damaged by adult feeding alone, larval feeding alone and a combination of adult feeding and oviposition. Adults preferred to oviposit on inflorescences which are not damaged by either adult feeding, larval feeding, or oviposition. No evidence for the existence of an oviposition deterring pheromone (ODP) was found. I suggest that the ability of a single host inflorescence to support the development of many larvae causes selection for the use of these oviposition deterring cues which can convey more quantitative information about the level of previous infestation than can ODPs. Adults fed a similar amount on damaged compared to undamaged inflorescences. These results assisted in the design of host range testing trials and allows predictions to be made about the effectiveness of this insect as a biological control agent.     

DNA damage by superoxide-generating systems in relation to the mechanism of action of the anti-tumour antibiotic adriamycin

1. A mixture of NADPH and ferrodoxin reductase is a convenient way of reducing adriamycin in vitro. Under aerobic conditions the adriamycin semiquinone reacts rapidly with O₂ and superoxide radical is produced. 2. Superoxide generated either by adriamycin:ferredoxin reductase or by hypoxanthine: xanthine oxidase can promote the formation of hydroxyl radicals in the presence of soluble iron chelates. 3. Hydroxyl radicals produced by a hypoxanthine:xanthine oxidase system in the presence of an iron chelate cause extensive fragmentation in double-stranded DNA. Protection is offered by catalase, superoxide dismutase or desferrioxamine. 4. Addition of double-stranded DNA to a mixture of adriamycin, ferredoxin reductase, NADPH and iron chelate inhibits formation of both superoxide and hydroxyl radicals. This is not due to direct inhibition of ferredoxin reductase and single-stranded DNA has a much weaker inhibitory effect. It is concluded that adriamycin intercalated into DNA cannot be reduced.     

No evidence for increased oxidative damage to lipids, proteins, or DNA in Huntington's disease

It has been proposed that mitochondrial dysfunction and excitotoxic mechanisms lead to oxidative damage in the brain of Huntington;s disease patients. We sought evidence that increased oxidative damage occurs by examining postmortem brain material from patients who had died with clinically and pathologically diagnosed Huntington's disease. Oxidative damage was measured using methods that have already demonstrated the presence of increased oxidative damage in Parkinson's disease, Alzheimer's disease, and senile dementia of the Lewy body type. No alterations in the levels of lipid peroxidation (as measured by lipid peroxides and thiobarbituric acid-malondialdehyde adducts) were found in the caudate nucleus, putamen, or frontal cortex of patients with Huntington's disease compared with normal controls. Similarly, there were no elevations in the levels of 8-hydroxyguanine or of a wide range of other markers of oxidative DNA damage. Levels of protein carbonyls in these tissues were also unaltered. Our data suggest that oxidative stress is not a major component of the degenerative processes occurring in Huntington's disease, or at least not to the extent that occurs in other neurodegenerative disorders.     

Leaf miners on Ochna ciliata (Ochnaceae) growing on Aldabra Atoll: patterns of herbivory in relation to goat browsing and exposure to the sun

Abstract.

• 1 The patterns of herbivory by leaf miners on the shrub Ochna ciliata growing on Aldabra Atoll were studied in relation to browsing by feral goats and exposure to the tropical sun versus shading.
• 2 Damage due to both tunnelling (Diptera) and ‘blotch-making’ (Lepidoptera) leaf miners was statistically greater on trees that had been browsed and on those that received more extensive exposure to the sun.
• 3 Effects of exposure and browsing were additive, as evidenced by the lack of statistical interaction between these factors.
• 4 There was a significant negative correlation between the numbers of tunnelling and blotch-making miners in trees that had been browsed, but not in unbrowsed trees.
• 5 Three geographical areas were sampled to test whether the effects of browsing on leaf mining were invariant or influenced by as yet unknown environmental factors. A statistically significant interaction between browsing and geographical region indicated that, while browsed trees had significantly more mines whatever the habitat, the extent of this effect differed across the habitats.

     

Changes in the light sensitivity of buried Polygonum aviculare seeds in relation to cold-induced dormancy loss: development of a predictive model

The effect of cold (stratification) temperature on changes in the sensitivity of Polygonum aviculare seeds to light was investigated. Seeds buried in pots were stored under stratification temperatures (1.6, 7 and 12 degrees C) for 137 d. Seeds exhumed at regular intervals during storage were exposed to different light treatments. Germination responses obtained for seeds exposed to different light treatments and stratification temperatures were used to develop a model to predict the sensitivity of buried seeds to light. Seed sensitivity to light increased as dormancy loss progressed, showing the successive acquisition of low-fluence responses (LFR), very low-fluence responses (VLFR), and the loss of the light requirement for germination for a fraction of the seed population. These changes were inversely correlated to stratification temperature, allowing the use of a thermal time index to relate observed changes in seed light sensitivity to stratification temperature. The rate of increase in sensitivity of P. aviculare seeds to light during stratification is inversely correlated to soil temperature, and these changes in light sensitivity could be predicted in relation to temperature using thermal-time models.     

Intra- and interplant leaf sesquiterpene variability in Copaifera langsdorfii: relation to microlepidopteran herbivory

Oecophorid herbivory in Copaifera langsdorfii leaves along with sesquiterpene composition, concentration of most of the individual sesquiterpenes and total yield did not significantly differ between the lower and upper portions of tree canopies. Although sesquiterpene variation in leaves collected throughout individual tree canopies was less than variation among trees, leaves which were eaten by oecophorid larvae had slightly lower yields than those unattacked. Individual C. langsdorfii trees within the population were significantly different from one another in sesquiterpene yield, oecophorid herbivory and in the concentration of seven out of the 11 sesquiterpene compounds. Leaf sesquiterpenes appear to be more important in inhibiting herbivory by Stenoma aff. assignata than leaf moisture and nitrogen content and toughness.     

Orientation in high-flying migrant insects in relation to flows: mechanisms and strategies

High-flying insect migrants have been shown to display sophisticated flight orientations that can, for example, maximize distance travelled by exploiting tailwinds, and reduce drift from seasonally optimal directions. Here, we provide a comprehensive overview of the theoretical and empirical evidence for the mechanisms underlying the selection and maintenance of the observed flight headings, and the detection of wind direction and speed, for insects flying hundreds of metres above the ground. Different mechanisms may be used—visual perception of the apparent ground movement or mechanosensory cues maintained by intrinsic features of the wind—depending on circumstances (e.g. day or night migrations). In addition to putative turbulence-induced velocity, acceleration and temperature cues, we present a new mathematical analysis which shows that ‘jerks’ (the time-derivative of accelerations) can provide indicators of wind direction at altitude. The adaptive benefits of the different orientation strategies are briefly discussed, and we place these new findings for insects within a wider context by comparisons with the latest research on other flying and swimming organisms.This article is part of the themed issue ‘Moving in a moving medium: new perspectives on flight’.     

DNA damage in non-proliferating cells subjected to ionizing irradiation at high or low dose rates

Ionizing radiation damage to the genome of a non-cycling mammalian cell is analyzed using continuous time Markov chains. Immediate damage induced by the radiation is modeled as a batch Poisson arrival process of DNA double strand breaks (DSBs). Different kinds of radiation, for example gamma rays or alpha particles, have different batch probabilities. Enzymatic modulation of the immediate damage is modeled as a Markov process similar to the processes described by the master equation of stochastic chemical kinetics. An illustrative example is the restitution/complete exchange model, which postulates that radiation induced DSBs can subsequently either undergo enzymatically mediated repair (restitution) or can participate pairwise in chromosome exchanges, some of which make irremediable lesions such as dicentric chromosome aberrations. One may have rapid irradiation followed by enzymatic DSB processing or have prolonged irradiation with both DSB arrival and enzymatic DSB processing continuing throughout the irradiation period. A complete solution of the Markov chain is known for the case that the exchange rate constant is negligible so that no irremediable chromosome lesions are produced and DSBs are the only damage to the genome. Using PDEs for generating functions, a perturbation calculation is made assuming the exchange rate constant is small compared to the repair rate constant. Some non-perturbative results applicable to very prolonged irradiation are also obtained using matrix methods: Perron-Frobenius theory, variational methods and numerical approximations of eigenvalues. Applications to experimental results on expected values, variances and statistical distributions of DNA lesions are briefly outlined.Continuous time Markov chain models are the most systematic of those current radiation damage models which treat DSB-DSB interactions within the cell nucleus as homogeneous (e.g. ignore diffusion limitations). They contain most other homogeneous models as special cases, limiting cases or approximations. However, applying the continuous time Markov chain models to studying spatial dependence of DSB interactions, which is generally believed to be very important in some situations, presents difficulties.     

Shoot development and extension of Quercus serrata saplings in response to insect damage and nutrient conditions

BACKGROUND AND AIMS: Plants have the ability to compensate for damage caused by herbivores. This is important to plant growth, because a plant cannot always avoid damage, even if it has developed defence mechanisms against herbivores. In previous work, we elucidated the herbivory-induced compensatory response of Quercus (at both the individual shoot and whole sapling levels) in both low- and high-nutrient conditions throughout one growing season. In this study, we determine how the compensatory growth of Quercus serrata saplings is achieved at different nutrient levels. METHODS: Quercus serrata saplings were grown under controlled conditions. Length, number of leaves and percentage of leaf area lost on all extension units (EUs) were measured. KEY RESULTS: Both the probability of flushing and the length of subsequent EUs significantly increased with an increase in the length of the parent EU. The probability of flushing increased with an increase in leaf damage of the parent EU, but the length of subsequent EUs decreased. This indicates that EU growth is fundamentally regulated at the individual EU level. The probabilities of a second and third flush were significantly higher in plants in high-nutrient soil than those in low-nutrient soil. The subsequent EUs of damaged saplings were also significantly longer at high-nutrient conditions. CONCLUSIONS: An increase in the probability of flushes in response to herbivore damage is important for damaged saplings to produce new EUs; further, shortening the length of EUs helps to effectively reproduce foliage lost by herbivory. The probability of flushing also varied according to soil nutrient levels, suggesting that the compensatory growth of individual EUs in response to local damage levels is affected by the nutrients available to the whole sapling.