The effect of semi-natural habitats on aphids and their natural enemies across spatial and temporal scales

Semi-natural habitats in agricultural landscapes are generally assumed to enhance the biological control of insect pests based on native beneficial insects, by providing alternative prey and hosts, resources and refuges for overwintering. We hypothesized that natural enemies of winter wheat aphids should arrive sooner in fields near semi-natural habitats. We compared aphid, hoverfly (larvae and eggs) and parasitized aphid (mummies) abundances in 54 winter wheat fields located in southern France from 2003 to 2007. Six surveys were recorded each spring and were split into the early period (defined as the period before the peak of aphid growth) and the late period (after the peak). The wheat fields differed by their surrounding landscape composition measured as the proportion of semi-natural habitats (woods, hedges and grasslands), at three different spatial scales: 200 m, 500 m, and 1200 m. Despite great variability in abundance data between years, the abundance of hoverflies appeared more sensitive to landscape composition than aphid abundance was. Early abundance for both aphids and hoverflies was positively related to wood cover, but not late abundance in spring. The abundance of hoverflies was positively related to hedge and grassland cover at all spatial scales and both periods considered. Aphid parasitism was higher near hedges at the small spatial scale late in the spring. Our results confirmed that higher proportions of semi-natural habitats in agricultural landscapes enhance the biological control of pests, but this effect depends on the spatial scale, the time period in the spring and the natural enemies considered.     

Ultraviolet-B-induced DNA damage and photorepair in the cyanobacterium Anabaena variabilis PCC 7937

The impact of simulated solar radiation on DNA and the mitigation of DNA-damaging effects by photoreactivation was studied in a cyanobacterium Anabaena variabilis PCC 7937. Cultures were irradiated under 295, 320 and 395 nm cut-off filters as well as seven other filters such as WG 280, WG 295, WG 305, WG 320, WG 335, WG 345 and GG 400. Growth of the test organism was found to be affected mostly under UV-B radiation as compared to PAR and PAR + UV-A radiations. Amplification of 16s rDNA and RAPD profile was significantly affected following exposure of genomic DNA to UV-B radiation. The formation of T<>T CPDs was recorded only in the cultures irradiated with UV-B radiation (i.e., under 295 nm as well as under WG 280, WG 295 and WG 305 nm cut-off filters), but maximum yield was found under 280 nm cut-off filter. Furthermore, the considerable induction of thymine dimers was observed with increasing UV-irradiation times. Fluorometric analysis of DNA unwinding (FADU) assay for UV-induced DNA strand breaks exhibited the maximum loss in the percentage of dsDNA under UV-B radiation followed by UV-A and PAR in comparison to the light control samples. We observed that T<>T CPD repair is light-dependent, since these lesions were more efficiently removed upon exposure to visible light than in the darkness. Blue radiation was found to be the most effective in photoreactivation than any other wavebands of light. Furthermore, the rate of photoreactivation was measured under varying temperatures (10, 20 and 30 °C); the repair rate was found to be the maximum at 20 °C under white fluorescent light. Our results indicate that photoreactivation play an important role in survival of the organism under natural conditions in spite of being exposed to the UV-B component present in the solar drops.     

The cost of being able to fly in the milkweed-oleander aphid,Aphis nerii (Homoptera: Aphididae)

Summary In the wing dimorphic milkweed-oleander aphid,Aphis nerii, winged aphids begin reproducing about 1.5 days after wingless aphids. The longer maturation period is primarily due to slower development since even adult eclosion by winged aphids takes place after wingless aphids begin reproducing. The delay is not due to a post-eclosion, pre-reproductive flight since, beginning with the fourth instar, larval winged aphids were reared at a density of one per plant and the vast majority were not stimulated to fly under such low-density conditions. Thus, the ability to fly incurs a fitness cost in terms of delayed reproduction, irrespective of whether flight actually occurs. We did not observe a difference between morphs for lifetime fecundity, even though wingless aphids have larger abdomens than winged aphids and for both morphs there is a significant correlation between abdomen width and fecundity. Offspring produced by wingless aphids over the first four days of reproduction are larger than those produced by winged aphids, and the size difference at birth is maintained into adulthood. However, there are no differences in life history traits between these offspring, including maturation period and lifetime fecundity. Thus, reduced body size does not increase the cost of being able to fly, at least under the conditions of these experiments. The cost of being able to fly in this species should favor reduced production of winged individuals in populations that exploit more permanent host plants.     

Diurnal pattern of aphid feeding and its effect on cotton leaf physiology

Cotton aphid (Aphis gossypii G.) populations seemed to fluctuate over the past years in cotton (Gossypium hirsutum L.) perhaps as a result of excessive use of insecticides for controlling more problematic pests. Contradictory plant responses have been observed depending upon the aphid/plant system, and it is unclear if cotton aphids, abiotic stress or both are responsible for cotton yield reduction in aphid-infested fields. Our objectives were to investigate the diurnal changes in the physiology of cotton leaves following aphid herbivory, and the diurnal pattern of aphid feeding. The experiment was conducted in a growth chamber using the cotton cultivar ‘Stoneville 474’. Leaves of the same age and size were infested with wingless adults plus nymphs. Cotton aphids were allowed to increase in numbers without restriction for 9 days, after which the amounts of carbohydrates in aphid-honeydew, and the number of honeydew droplets excreted per aphid were measured. Photosynthetic rates, dark respiration rates and foliar non-structural carbohydrates were measured. The amount of individual carbohydrates found in the honeydew was significantly different with time. The total amount of carbohydrates excreted per aphid within a 24-h period averaged 2.5 μg. The number of honeydew droplets excreted per aphid varied significantly from time to time period. Cotton aphids did not significantly alter photosynthesis or respiration rates or non-structural carbohydrates on leaves. Aphid populations of approximately 300 per leaf on the 9th day of infestation did not appear to significantly alter the physiology of cotton leaves.     

An LED-based UV-B irradiation system for tiny organisms: System description and demonstration experiment to determine the hatchability of eggs from four Tetranychus spider mite species from Okinawa

We developed a computer-based system for controlling the photoperiod and irradiance of UV-B and white light from a 5 × 5 light-emitting diode (LED) matrix (100 × 100 mm). In this system, the LED matrix was installed in each of four irradiation boxes and controlled by pulse-width modulators so that each box can independently emit UV-B and white light at irradiances of up to 1.5 and 4.0 W m⁻², respectively, or a combination of both light types. We used this system to examine the hatchabilities of the eggs of four Tetranychus spider mite species (T. urticae, T. kanzawai, T. piercei and T. okinawanus) collected from Okinawa Island under UV-B irradiation alone or simultaneous irradiation with white light for 12 h d⁻¹ at 25 °C. Although no eggs of any species hatched under the UV-B irradiation, even when the irradiance was as low as 0.02 W m⁻², the hatchabilities increased to >90% under simultaneous irradiation with 4.0 W m⁻² white light. At 0.06 W m⁻² UV-B, T. okinawanus eggs hatched (15% hatchability) under simultaneous irradiation with white light, whereas other species showed hatchabilities <1%. These results suggest that photolyases activated by white light may reduce UV-B–induced DNA damage in spider mite eggs and that the greater UV-B tolerance of T. okinawanus may explain its dominance on plants in seashore environments, which have a higher risk of exposure to reflected UV-B even on the undersurface of leaves. Our system will be useful for further examination of photophysiological responses of tiny organisms because of its ability to precisely control radiation conditions.     

Increased feeding damage under elevated Co2: The case of the Russian wheat aphid

We report the results of a baseline study on the effects of Russian wheat aphid infestation on barley lines grown under ambient and elevated (450 and 550 μmol mol^− 1) CO₂ concentrations [CO₂]. Elevated CO₂ impacted on plant biomass, C:N ratios and leaf nitrogen concentrations. Visible manifestation of aphid feeding related damage was assessed by examining resultant chlorosis and leaf roll under ambient and two elevated [CO₂] levels using a control and three resistant barley host combinations. Elevated [CO₂] had a significant positive effect on the growth of the four barley lines that were not infested by the aphids. However under the same conditions aphid feeding under elevated CO₂ conditions caused very high biomass loss, which was more noticeable in experiments involving non-resistant PUMA than in the resistant barley lines. The results of this study demonstrate that CO₂ enrichment substantially increases aphid populations of RWASA1 and RWASA2 on the four barley lines investigated. Furthermore, aphid populations were higher on non-resistant PUMA than the three resistant lines and the RWASA2 biotype out-performed RWASA1 in each case. Under elevated [CO₂], aphid feeding, resulted in a significant increase in the leaf C:N ratios (as a percentage change) in most treatments, compared to levels recorded on uninfested plants. The resistant lines also showed a significant reduction in leaf nitrogen (~ 40% for PUMA and not less than 30% for the resistant STARS lines tested). C:N ratio changes and N loss correlated to [CO₂] and aphid biotype. By 28 days of infestation, most of the non-resistant PUMA line in particular showed significant irrecoverable levels of leaf chlorosis. At level 9 rating on the chlorosis scale (i.e. plant death when recovery was not possible), experiments were terminated. As aphid success is unlikely to be the sole product of [CO₂], but also of other limiting nutrients such as N, it may be worth further investigating the effect of plant quality and ultimately plant nutrition on the population growth of aphids.     

Morphological and physiological responses of nine southern U.S. rice cultivars differing in their tolerance to enhanced ultraviolet-B radiation

The impact of climatic change on crop production is a major global concern. One of the climatic factors, ultraviolet-B radiation (UV-B; 280–320 nm), which is increasing as a result of depletion of the global stratospheric ozone layer, can alter crop productivity. As the initial step in development of UV-B tolerant rice cultivars for the southern U.S., in this study we screened popular southern U.S. rice cultivars for variation in tolerance to elevated UV-B radiation with respect to morphological, phenological and physiological parameters. Plants grown in the greenhouse at the Texas AgriLife Research and Extension Center in Beaumont, Texas, U.S. were exposed to 0, 8 or 16 kJ m⁻² day⁻¹ UV-B radiation for 90 days. Our results showed differences among southern US rice cultivars in response to UV-B treatments with respect to leaf photosynthetic rate (P_n), leaf phenolic concentration, pollen germination (PG), spikelet fertility (SF), leaf number, leaf area, and yield. For most of the cultivars, plants exposed to enhanced UV-B radiation showed decreased P_n, PG, SF and yield and increased spikelet abortion and leaf phenolic concentration compared to the plants grown in a UV-B-free environment. In this study, cultivar ‘Clearfield XL729’ performed better than the other cultivars under enhanced UV-B radiation.     

Partial protection of photosynthetic apparatus from UV-B-induced damage by UV-A radiation

Alteration in the photosynthetic apparatus of clusterbean (Cyamopsis tetraganoloba) cotyledons owing to UV-B irradiation in the absence or presence of UV-A radiation (UV-A + UV-B) during steady phase of its growth has been studied. UV-B radiation induces a decline in the photosynthetic pigments content and O₂ evolution along with a modification in the absorption spectra of chloroplasts. UV-A + UV-B irradiation moderately reverses these changes. The partial restoration of F_V/F_M value and other fluorescence transient parameters in UV-A + UV-B treated sample compared to that of UV-B treated one suggest that UV-A helps in developing a protective pathway against UV-B-induced impairment. UV-B-mediated alteration in S state transition of Mn cluster associated with oxygen evolving complex, as appeared from TL glow curves, is retrieved by UV-A radiation and Car is considered to negotiate against UV-B-induced damage of photosynthetic apparatus.     

Effects of wing polyphenism, aphid genotype and host plant chemistry on energy metabolism of the grain aphid, Sitobion avenae

Wing dimorphism has been proposed as a strategy to face trade-offs between flight capability and fecundity. In aphids, individuals with functional wings have slower development and lower fecundity compared with wingless individuals. However, differential maintenance costs between winged and wingless aphids have not been deeply investigated. In the current study, we studied the combined effect of wing dimorphism with the effects of aphid genotypes and of wheat hosts having different levels of chemical defences (hydroxamic acids, Hx) on adult body mass and standard metabolic rates (SMR) of winged and wingless morphs of the grain aphid, Sitobion avenae. We found that wingless aphids had higher body mass than winged aphids and that body mass also increased towards host with high Hx levels. Furthermore, winged aphids showed a plastic SMR in terms of Hx levels, whereas wingless aphids displayed a rigid reaction norm (significant interaction between morph condition and wheat host). These findings suggest that winged aphids have reduced adult size compared to wingless aphids, likely due to costs associated to the development of flight structure in early-life stages. These costs contrast with the absence of detectable metabolic costs related to fuelling and maintenance of the flight apparatus in adults.     

The interactive effect of water deficit and UV-B radiation on salicylic acid accumulation in barley roots and leaves

The aim of the paper was to determine the effect of water deficit (WD) and UV-B radiation acting individually and in combination on salicylic acid (SA) accumulation as well as on the activity of phenylalanine ammonia-lyase (PAL) and benzoic acid hydroxylase (BA2H) that control its biosynthetic route from phenylalanine. An additional aim was to test whether the interaction of these stresses limits the negative effect of a single stress on tissue hydration and membrane injury. Two-week-old seedlings were subjected to water deficit (WD), UV-B irradiation (UV-B) and three different combinations of WD and UV-B: (I) WD and UV-B applied at the same time, (II) UV-B applied before WD, and (III) WD applied before UV-B. Water deficit was imposed by immersing the root system in aerated nutrient solution with polyethylene glycol (PEG 6000) of water potential – 0.5 MPa. UV-B dosage was 24 kJ m⁻² day⁻¹ (0.84 W m⁻²) at the canopy level. UV-B and WD imposed individually and jointly, caused, in a time-dependent manner, an increase in SA content in both organs. Increased levels of SA in WD stressed plants were accompanied by an increase in the activity of PAL and BA2H. However, in plants exposed to UV-B were accompanied only by an increase in the activity of BA2H. Under WD conditions, an earlier increase of SA content was observed in roots than in leaves, which may indicate the involvement of SA in the signal transduction between roots and leaves. In plants exposed to sequential action of WD and UV-B, regardless of the order of its imposition, the effect of each single factor on SA accumulation in leaves was strengthened. WD had a greater effect on water loss and membrane injury than UV-B radiation. In plants exposed to WD after pre-treatment with UV-B radiation, a cross-tolerance mechanism was observed. Leaves of these plants did not show increased lipid peroxidation, measured in terms of malondialdehyde content, and a decrease in water content. This protective action was probably caused by the increase of the SA level in leaves of the UV-B treated plants prior to WD imposition.     

Metabolism of terpenes in the response of grape (Vitis vinifera L.) leaf tissues to UV-B radiation

This study investigated the terpene profiles as determined by GC–EIMS analysis of in vitro cultured plants of Vitis vinifera exposed to a “field-like” dose of UV-B (4.75 kJ m⁻² d⁻¹) administered at two different fluence rates (low, 16 h at 8.25 μW cm⁻², and high 4 h at 33 μW cm⁻²). Low UV-B treatment increased levels of the membrane-related triterpenes sitosterol, stigmasterol and lupeol, more notable in young leaves, suggesting elicitation of a mechanism for grapevine acclimation. By contrast, accumulation of compounds with antioxidant properties, diterpenes α and γ tocopherol and phytol, the sesquiterpene E-nerolidol and the monoterpenes carene, α-pinene and terpinolene had maximum accumulation under high UV-B, which was accentuated in mature leaves. Also the levels of the sesquiterpenic stress-related hormone abscisic acid (ABA) increased under high UV-B, although 24 h post irradiation ABA concentrations decreased. Such increments of antioxidant terpenes along with ABA suggest elicitation of mechanism of defense. The adaptative responses induced by relatively low UV-B irradiations as suggested by synthesis of terpenes related with membrane stability correlated with augments in terpene synthase activity.     

Principal component analysis of intraspecific responses of tartary buckwheat to UV-B radiation under field conditions

Fifteen populations of tartary buckwheat (Fagopyrum tataricum Gaertn.) occurring in habitats with different natural UV-B levels were sampled, and the plants were exposed to enhanced UV-B radiation under field conditions simulating 25% depletion of the stratospheric ozone layer. The experimental design was a 2 × 15 factorial, with two levels of UV-B radiation (ambient and enhanced UV-B radiation) and plants from 15 populations. The responses of plants in growth, morphology, productivity and in the composition of photosynthetic pigments were measured. The results demonstrated that there were significant differences among populations in responses to UV-B radiation: some populations exhibited a positive effect while others were negatively affected. The UV-B effects on plant traits were correlated with the constitutive values. A principal component analysis (PCA) was used to evaluate the overall sensitivity of responses to UV-B radiation. Our results suggest that the sensitivity of plants to UV-B radiation is not only associated with the ambient UV-B level in natural habitats but also with the relative growth rate and other factors.     

Effects of low UV-B doses on the accumulation of UV-B absorbing compounds and total phenolics and carbohydrate metabolism in the peel of harvested lemons

This paper examined the peel (albedo and flavedo) of postharvest lemon fruits after UV-B exposure in order to analyze relationships between soluble carbohydrate metabolism and secondary metabolite accumulation. Lemons (Citrus limon, cv. Limoneira 8A) were harvested in winter months (June to August), treated with 0.43 W m⁻² (22 kJ m⁻² d⁻¹ UV-BBE) of UV-B radiation during 0 (control), 0.5, 1.0, 2.0, 3.0, and 5.0 min, and then stored at 25 °C for 24 h. Peel samples from irradiated areas were obtained with a razor blade and frozen in liquid nitrogen until use for measurements. Data obtained showed that 2 and 3 min of UV-B exposure effectively increased the level of UV-B absorbing compounds and total phenolics in flavedo without causing visual alterations of the peel colour as compared with non-irradiated lemons. By contrast, there were no significant changes in albedo secondary metabolite accumulation. The amount of secondary metabolites was depending upon UV-B time–dose. Exposure over 3.0 min did not further improve the accumulation of UV-B absorbing and phenolic compounds. Soluble sugars (sucrose, glucose and fructose) also accumulated in the lemon peel after UV-B exposure, but the distribution patterns were different. After 3 min time–dose, sucrose and hexoses increased in flavedo, whereas in albedo only increased the sucrose and glucose. This effect was related to UVB-induced changes in the activity of sucrose-hydrolyzing and sucrose-synthesizing enzymes: soluble and cell-bound invertase, sucrose synthase (SS) and sucrose phosphate synthase (SPS). Data indicate that lemon peel retains the capacity to modify the enzyme activity of sucrose metabolism in response to UV-B exposure. Our results also suggest that the exposure of postharvest lemons to low supplemental UV-B doses produces changes in the carbon allocation of peel tissues including synthesis, but probably not only limited to them, of UV-B absorbing and phenolic compounds.     

The role of proline-associated pentose phosphate pathway in cool-season turfgrasses after UV-B exposure

Physiological adjustments of cool-season turfgrasses were investigated to determine the role of proline-associated pentose phosphate pathway for phenolic biosynthesis and stimulation of antioxidant response system following UV-B exposure. Creeping bentgrass, Kentucky bluegrass, tall fescue, and perennial ryegrass plugs were subjected to artificial UV-B exposure (biologically effective UVB_BE radiation 8 kJ m⁻² d⁻¹.) for one week with 10-h photoperiod. Significant accumulation of phenolics and stimulation of antioxidant enzyme activity was observed in all investigated cool-season turfgrasses after UV-B exposure and this induction corroborated with higher glucose-6-phosphate dehydrogenase activity and high accumulation of proline. Guaiacol peroxidase activity also increased in all investigated cool-season turfgrass species after UV-B exposure. In this study, the shift of carbon flux from glycolysis to pentose phosphate pathway following UV-B exposure and as a result of that, the higher accumulation of phenolics and stimulation of antioxidant response system provides an insight to understand a probable defense mechanism of cool-season turfgrasses against UV stress.     

Changes in oxidative stress defense system in wheat (Triticum aestivum L.) and mung bean (Vigna radiata L.) cultivars grown with and without mineral nutrients and irradiated by supplemental ultraviolet-B

Field study was conducted to evaluate the inter- and intra-specific variations in sensitivity of two cultivars each of wheat (Triticum aestivum L. cv. HD 2329 and HUW 234) and mung bean (Vigna radiata L. cv. Malviya Jyoti and Malviya Janpriya) to supplemental levels of UV-B irradiation (sUV-B, 280–315 nm) with and without recommended levels of mineral nutrients. Results showed decrease in photosynthetic pigments and biomass of all the four cultivars due to sUV-B radiation. Antioxidative defense system was activated in all the cultivars after irradiation with sUV-B. SOD, peroxidase and total thiol contents increased, while catalase activity and ascorbic acid contents decreased under sUV-B irradiation. On the basis of biomass, UV-B sensitivity can be arranged in decreasing order as: Malviya Janpriya < Malviya Jyoti < HD 2329 < HUW 234. Application of mineral nutrients (N, P and K) showed significant positive response in all cultivars by ameliorating the negative impact of sUV-B.     

Photooxidation and antioxidant responses in the earthworm Amynthas gracilis exposed to environmental levels of ultraviolet B radiation

Ultraviolet (UV) radiation leads to photooxidation in various organisms. Our previous study demonstrated that ultraviolet B (UV-B) radiation is lethal for particular species of earthworms, but the mechanisms responsible for the lethality are unclear. In our current study, we investigated that ultraviolet light causes photooxidative damage and reduces antioxidant responses in the earthworm Amynthas gracilis. Intact earthworms and skin/muscle tissue extracts were exposed to UV-B radiation for in vivo and in vitro studies. Both in vitro and in vivo results showed that the products of photooxidative damage, MDA and H₂O₂, increased after UV-B exposure. Glutathione peroxidase (GPx) and catalase were inhibited immediately after exposure to high doses (3000 J/m²) of UV-B radiation in vivo. Catalase activity was increased following a low UV-B dose (500 J/m²) in vivo, but decreased in response to all dosage levels in vitro. These data indicate that a relationship exists between UV-B induced damage and photooxidation and also that catalase and GPx act as important antioxidants to prevent photooxidation. According to these data, A. gracilis exhibits high sensitivity to environmental levels of UV-B. Therefore, A. gracilis represents a sensitive and cost-effective model organism for investigations of UV-radiation damage and environmental UV stress.     

The cabbage aphid: a walking mustard oil bomb

The cabbage aphid, Brevicoryne brassicae, has developed a chemical defence system that exploits and mimics that of its host plants, involving sequestration of the major plant secondary metabolites (glucosinolates). Like its host plants, the aphid produces a myrosinase (beta-thioglucoside glucohydrolase) to catalyse the hydrolysis of glucosinolates, yielding biologically active products. Here, we demonstrate that aphid myrosinase expression in head/thoracic muscle starts during embryonic development and protein levels continue to accumulate after the nymphs are born. However, aphids are entirely dependent on the host plant for the glucosinolate substrate, which they store in the haemolymph. Uptake of a glucosinolate (sinigrin) was investigated when aphids fed on plants or an in vitro system and followed a different developmental pattern in winged and wingless aphid morphs. In nymphs of the wingless aphid morph, glucosinolate level continued to increase throughout the development to the adult stage, but the quantity in nymphs of the winged form peaked before eclosion (at day 7) and subsequently declined. Winged aphids excreted significantly higher amounts of glucosinolate in the honeydew when compared with wingless aphids, suggesting regulated transport across the gut. The higher level of sinigrin in wingless aphids had a significant negative impact on survival of a ladybird predator. Larvae of Adalia bipunctata were unable to survive when fed adult wingless aphids from a 1% sinigrin diet, but survived successfully when fed aphids from a glucosinolate-free diet (wingless or winged), or winged aphids from 1% sinigrin. The apparent lack of an effective chemical defence system in adult winged aphids possibly reflects their energetic investment in flight as an alternative predator avoidance mechanism.     

AhpC (alkyl hydroperoxide reductase) from Anabaena sp. PCC 7120 protects Escherichia coli from multiple abiotic stresses

Alkyl hydroperoxide reductase (AhpC) is known to detoxify peroxides and reactive sulfur species (RSS). However, the relationship between its expression and combating of abiotic stresses is still not clear. To investigate this relationship, the genes encoding the alkyl hydroperoxide reductase (ahpC) from Anabaena sp. PCC 7120 were introduced into E. coli using pGEX-5X-2 vector and their possible functions against heat, salt, carbofuron, cadmium, copper and UV-B were analyzed. The transformed E. coli cells registered significantly increase in growth than the control cells under temperature (47 °C), NaCl (6% w/v), carbofuron (0.025 mg ml^?1), CdCl₂ (4 mM), CuCl₂ (1 mM), and UV-B (10 min) exposure. Enhanced expression of ahpC gene as measured by semi-quantitative RT-PCR under aforementioned stresses at different time points demonstrated its role in offering tolerance against multiple abiotic stresses.     

Effect of seed pretreatment by magnetic field on the sensitivity of cucumber (Cucumis sativus) seedlings to ultraviolet-B radiation

The effects of seed pretreatment by magnetic field (MF) on the impacts of ultraviolet-B (UV-B) radiation were tested using cucumber (Cucumis sativus) seedlings in phytotron. Soaked cucumber seeds were placed in MF of various strengths (0, 0.2 and 0.45 T). After germination the seeds were sowed in homogeneous garden soil and grown, then cucumber seedlings were exposed to 0 (as control) and 3.5 kJ m⁻² UV-B irradiation, respectively. Some effects of UV-B radiation and MF-pretreatment as well as their combination were investigated. MF-pretreatment increased seed germination rate, seedling growth and development, although also increased lipid oxidation and ascorbic acid contents. On the other hand, our results provided evidence that seed MF-pretreatment increased the sensitivity of cucumber seedlings to UV-B radiation. The seedling growth and development were significantly decreased by the combination of UV-B irradiation and MF-pretreatment. This combination also increased oxidative pressure and decreased actual quantum yield of PS II. Leaf UV-B absorbing compound was increased by MF-pretreatment or UV-B irradiation, whereas their combination significantly decreased it. These results suggested that the harmful effects of combination were partially due to the inhibition of secondary metabolism.     

The performance of Brevicoryne brassicae on ornamental cabbages grown in CO2-enriched atmospheres

The effect of different atmospheric CO₂ concentrations on life table parameters and the biology of the cabbage aphid, Brevicoryne brassicae, when fed on two cultivars of ornamental cabbage, was studied in a greenhouse designed for CO₂ studies. Aphid performance was influenced by increasing atmospheric CO₂ levels, significantly affecting the intrinsic rate of increase (r_m), finite rate of increase (λ), mean generation time (T), doubling time (DT), and pre-reproductive period. The longest pre-reproductive period was observed for aphids grown at 380 ppm CO₂. The intrinsic rate of natural increase was highest for aphids at 1050 ppm CO₂, because of their faster development, high daily rate of progeny production, and higher survivorship. Future elevated CO₂ concentrations will enhance aphid population outbreaks and consequently increase the damage caused.