Physiological responses of resistant and susceptible barley, Hordeum vulgare to the Russian wheat aphid, Diurpahis noxia (Mordvilko)

Knowledge of the physiological responses of barley, Hordeum vulgare L., to the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae) is critical to understanding the defense response of barley to aphid injury and identifying resistance mechanisms. This study documented the impact of D. noxia feeding on resistant (‘Sidney’) and susceptible (‘Otis’) barley through chlorophyll fluorescence measurements, chlorophyll content, and carbon assimilation (A–C_i) curves recorded at 1, 3, 6, 10, and 13 days after aphid introduction. All chlorophyll fluorescence parameters evaluated were similar between aphid-infested and control plants for both cultivars. A–C_i curves showed that D. noxia feeding negatively impacts the photosynthetic capacity in both cultivars, but this effect was greater in the susceptible plants. From the A–C_i curves, it is apparent that compensation occurs in resistant barley by day 10, but by the conclusion of the experiment, aphid populations reached levels that overwhelmed the resistant barley seedlings. Differences observed in carbon assimilation curves between control and infested plants show that D. noxia feeding impacts the dark reaction, specifically rubisco activity and RuBP regeneration. It is likely that declines in the photochemical efficiency and chlorophyll content of the plants may be a secondary effect and not the primary trigger of declines in host plant function.     

Differential induction of apoplastic peroxidase and chitinase activities in susceptible and resistant wheat cultivars by Russian wheat aphid infestation

The intercellular peroxidase and chitinase activities of three wheat cultivars [Triticum aestivum L. cvs `Tugela DN', `Molopo DN' (Gariep) and `Betta DN'] containing the Dn-1 gene for resistance to the Russian wheat aphid (RWA) Diuraphis noxia (Mordvilko) and the corresponding near-isogenic susceptible cultivars (`Tugela', `Molopo' and `Betta') were studied under conditions of infestation and non-infestation. The aim was to gain information on the mechanism of resistance. The resistance response was induced by RWA infestation. Infestation rapidly induced the activities of both enzymes selectively in resistant wheat to levels of magnitudes higher than those in susceptible wheat. The genetic background in which the Dn-1 resistance gene is bred played a role and the level of activity corresponded to the level of resistance. Immunologic studies confirmed that the induction of enzyme activities was due to the induction of higher protein levels. These results indicate that peroxidase and chitinase may have a role in insect resistance. Received: 20 June 1997 / Revision received: 9 April 1998 / Accepted: 5 June 1998     

Interactions of Russian wheat aphid, a hymenopterous parasitoid and resistant and susceptible slender wheatgrasses

Interactions among three trophic levels of resistant and susceptible slenderwheat grasses, Elymus trachycaulum (Link) Goule ex Shinners ex. H.F. Lewis, Russian wheat aphid, Diuraphis noxia (Mordvilko), and a hymenopterous parasitoid were studied in the laboratory and greenhouse. These relationships were compared with a commercial susceptible wheat Triticum aestivum L. variety. Aphids reared on the resistant entries showed significantly lower weights and numbers. Significant reduction of parasitoid mummy weight and adult size was positively correlated with the effects on the aphids. Resistant entries also induced a longer prereproductive period for both the aphids and parasitoids. Numbers of aphids and aphid damage were significantly modified by the addition of parasitoids. Parasitism was higher on plants that did not have leaf rolling. These findings may indicate that antibiosis resistance studied here is not the most desirable because it decreases natural enemy vitality.     

Species diversity, abundance, and phenology of aphid natural enemies on spring wheats resistant and susceptible to Russian wheat aphid

The species composition, relativeabundance, and seasonal dynamics of selectednatural enemies of cereal aphids were monitoredin spring wheat fields in Moscow, Idaho in 1997and 1998. Trials also examined the potentialimpact of resistance to Russian wheat aphid(RWA), Diuraphis noxia (Mordvilko)(Homoptera: Aphididae) in wheat, on aphidbiological control agents. Natural enemypopulations were monitored on two springwheats: D. noxia susceptible variety`Centennial' and resistant genotype `IDO488'. Field plots were artificially infested withadult D. noxia, and sampled for cerealaphids and parasitoids weekly. Coccinellidpredators were monitored once in 1997 and twicein 1998. The coccinellids Hippodamiaconvergens Guerin, Coccinellaseptempunctata L., C. transversoguttataBrown and C. trifasciata Mulsant weredetected. No significant differences in adultor immature coccinellid densities were observedbetween the D. noxia resistant andsusceptible genotypes. During both years, themost abundant primary hymenopteran parasitoidswere Diaeretiella rapae (M'Intosh), Aphidius ervi Haliday, A. avenaphis(Fitch), and Lysiphlebus testaceipes(Cresson), Aphelinus varipes (Foerster),Aphidius colemani Viereck, Aphidiuspicipes (Nees), Aphidius sp., Monoctonus washingtonensis Pike & Stary, Praon gallicum Stary, Praon occidentaleBaker, and Praon sp. were also detected. Numbers of both D. noxia and D.rapae were significantly greater on Centennialthan on IDO488 in both years. When all speciesof cereal aphids and parasitoids areconsidered, the total percentage parasitism wasnot significantly different between thegenotypes. There was no interaction betweenD. noxia resistance and the populationdensity of the predators or parasitoidsmonitored. These results suggest that the D. noxia resistant line had no adverse impacton natural enemies under the conditions ofthese field experiments.     

Physiological responses of resistant and susceptible reproductive stage soybean to soybean aphid (Aphis glycines Matsumura) feeding

We examined the physiological responses of four soybean genotypes (KS4202, K-1639-2, ‘Jackson,’ ‘Asgrow 2703’) to soybean aphid (Aphis glycines Matsumura) feeding in reproductive stage soybeans (R1, beginning bloom). Photosynthetic capacity was evaluated by taking survey measurements at 7, 17, 24, and 28 days after aphid introduction and by measuring assimilation/internal CO₂ (AC_i) curves at 29 days after aphid introduction. There were no significant differences in survey measurements between the control and infested KS4202, K-1639-2, Jackson, and Asgrow 2703 plants at 7, 17, 24, and 28 days after aphid introduction. At 29 days after aphid introduction, Asgrow 2703 plants showed a significant reduction in photosynthetic capacity compared to its control plants, while infested KS4202 plants had photosynthetic rates similar to control plants, suggesting the plant’s ability to compensate for aphid feeding. Differences in gas-exchange parameters, specifically J_max and CE, between control and infested Asgrow 2703 plants showed that soybean aphid feeding negatively impacts the carbon-linked/dark reactions, specifically rubisco activity and RuBP regeneration. This research also investigated the role of peroxidases in the defense response of soybeans to the soybean aphid. Enzyme kinetics studies documented the up-regulation of peroxidase activity for both Asgrow 2703 and KS4202 aphid-infested plants compared to their respective uninfested control plants at 24 and 28 days after aphid introduction. Peroxidase expression profiles identified differences in the isozyme profiles of aphid-infested and control plants for Asgrow 2703 and KS4202. Differences between physiological responses of infested KS4202 and Asgrow 2703, particularly temporal changes in photosynthesis activity, imply that KS4202 tolerates some impacts of soybean aphid feeding on photosynthetic integrity.     

Reproduction and development of Russian wheat aphid biotype 2 on crested wheatgrass, intermediate wheatgrass, and susceptible and resistant wheat

The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is an economically important pest of small grains. Since its introduction into North America in 2003, Russian wheat aphid Biotype 2 has been found to be virulent to all commercially available winter wheat, Triticum aestivum L., cultivars. Our goal was to examine differences in Russian wheat aphid reproduction and development on a variety of plant hosts to gain information about 1) potential alternate host refuges, 2) selective host pressures on Russian wheat aphid genetic variation, and 3) general population dynamics of Russian wheat aphid Biotype 2. We studied host quality of two wheatgrasses (crested wheatgrass, Agropyron cristatum [L.] Gaertn., and intermediate wheatgrass, Agropyron intermedium [Host] Beauvoir) and two types of winter wheat (T. aestivum, one Biotype 2 susceptible wheat, 'Custer' and one biotype 2 resistant wheat, STARS02RWA2414-11). The susceptible wheat had the highest intrinsic rate of increase, greatest longevity and greatest fecundity of the four host studied. Crested wheatgrass and the resistant wheat showed similar growth rates. Intermediate wheatgrass had the lowest intrinsic rate of increase and lowest fecundity of all tested hosts.     

Seasonal dynamics of cereal aphids on Russian wheat aphid (Homoptera: Aphididae) susceptible and resistant wheats

Field experiments were conducted in 1997 and 1998 to evaluate the impact of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), on the cereal aphid complex in wheat. Two spring wheats were planted: the variety "Centennial" (Russian wheat aphid susceptible) and the advanced line IDO488 (Russian wheat aphid resistant). IDO488 incorporates the resistance found in PI 294994 into a Centennial background. Field plots were artificially infested with adult D. noxia and sampled weekly. The most abundant aphid species in 1997 were Metopolophium dirhodum (Walker), Sitobion avenae (F.), D. noxia, and Rhopalosiphum padi (L.). In 1998, the order of abundance was M. dirhodum, R. padi, S. avenae, and D. noxia. The resistant genotype had significantly fewer D. noxia than the susceptible one during both years. However, plant genotype had no significant effect on the other aphid species in either year. Both the initial density of D. noxia and plant growth stage, had a significant effect on D. noxia population development, but had no effect on the other aphid species. There was no interaction between D. noxia resistance and the population density of the other aphid species observed.     

Glycoproteins from Russian wheat aphid infested wheat induce defence responses

Elicitors are molecules which can induce the activation of plant defence responses. Elicitor activity of intercellular wash fluid from Russian wheat aphid, Diuraphis noxia (Mordvilko) infested resistant (cv Tugela DN), and susceptible (cv Tugela), wheat (Triticum aestivum L.), was investigated. Known Russian wheat aphid resistance related responses such as peroxidase and beta-1,3-glucanase activities were used as parameters of elicitor activity. The intercellular wash fluid from infested resistant plants contains high elicitor activity while that from infested susceptible plants contains no or very little elicitor activity. After applying C-18 reverse phase and concanavalin A Sepharose chromatography, elicitor active glycoproteins were isolated from the intercellular wash fluid of Russian wheat aphid infested resistant wheat. The elicitor-active glycoproteins separated into three polypeptides during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isolated glycoproteins elicited peroxidase activity to higher levels in resistant than in susceptible cultivars. It was evident that the glycoproteins were probably a general elicitor of plant origin. Information gained from these studies is valuable for the development of plant activators to enhance the defence responses of plants.     

Light activation of Russian wheat aphid-elicited physiological responses in susceptible wheat

The impact of light and its role in Russian wheat aphid, Diuraphis noxia (Mordvilko), damage symptom formation, and photosynthetic capacity in 'Arapahoe' wheat (Triticum aestivum L.) were examined. After 72 h under continuous dark or continuous light regimes, the number of aphids (nymphs), leaf rolling and chlorosis ratings, fresh leaf weight, and chlorophyll contents were recorded. Photosynthetic rates, chlorophyll a, kinetics and chlorophyll extractions also were determined. Aphid infestation caused significant reductions in plant height, fresh weight, gas exchange, and chlorophyll fluorescence only under continuous light. Under the 72 h continuous dark regime, aphid infestation did not cause either damage symptom formation or reduction in plant growth or metabolism (photosynthesis). Furthermore, significantly more D. noxia nymphs were produced under continuous light condition than continuous dark. Our results demonstrate that the development of D. noxia feeding damage symptoms (i.e., leaf rolling and chlorotic streaks) on susceptible wheat seedlings is a light-activated process, even though the elicitor of the plant damage symptoms is aphid feeding.     

Fractionated extracts of Russian wheat aphid eliciting defense responses in wheat

It is hypothesized that the interaction between aphids and plants follows a gene-for-gene model. The recent appearance of several new Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), biotypes in the United States and the differential response of wheat, Triticum aestivum L., genotypes containing different resistance genes also suggest a gene-for-gene interaction. However, aphid elicitors remain unknown. This study was conducted to identify fractionated Russian wheat aphid extracts capable of eliciting differential responses between resistant and susceptible wheat genotypes. We extracted whole soluble compounds and separated proteins and metabolites from two Russian wheat aphid biotypes (1 and 2), injected these extracts into seedlings of susceptible wheat Gamtoos (dn7) and resistant 94M370 (Dn7), and determined phenotypic and biochemical plant responses. Injections of whole extract or protein extract from both biotypes induced the typical susceptible symptom, leaf rolling, in the susceptible cultivar, but not in the resistant cultivar. Furthermore, multiple injections with protein extract from biotype 2 induced the development of chlorosis, head trapping, and stunting in susceptible wheat. Injection with metabolite, buffer, or chitin, did not produce any susceptible symptoms in either genotype. The protein extract from the two biotypes also induced significantly higher activities of three defense-response enzymes (catalase, peroxidase, and beta-glucanase) in 94M370 than in Gamtoos. These results indicate that a protein elicitor from the Russian wheat aphid is recognized by a plant receptor, and the recognition is mediated by the Dn7-gene product. The increased activities of defense-response enzymes in resistant plants after injection with the protein fraction suggest that defense response genes are induced after recognition of aphid elicitors by the plant.     

Plant damage and yield response to the Russian wheat aphid (Homoptera: Aphididae) on susceptible and resistant winter wheats in Colorado

Plant damage and yield response to the Russian wheat aphid, Diuraphis noxia (Mordvilko), were evaluated on a susceptible (TAM 107) and a resistant (RWA E1) winter wheat, Triticum aestivum L., in three Colorado locations in the 1993 and 1994 crop years. Russian wheat aphid was more abundant on TAM 107 than on RWA E1. Russian wheat aphid days per tiller were greater at the higher infestation levels. Yield losses as a result of Russian wheat aphid infestation occurred most of the time with TAM 107 but rarely with RWA E1. Seed densities were reduced at higher infestation levels in TAM 107 at two locations. Russian wheat aphids per tiller had a negative relationship to yield in TAM 107 but not in RWA E1. In TAM 107 yield decreased as aphid densities increased, but yield remained constant regardless of initial aphid abundance on RWA E1 in all environments. Seed densities were reduced at higher infestation levels in TAM 107 at two locations. The resistance conferred by the Dn4 gene seems to be an effective management approach across a range of field conditions.     

Parasitism of soybean aphid by Aphelinus species on soybean susceptible versus resistant to the aphid

The soybean aphid, Aphis glycines, is native to Asia, but during the last decade it has invaded North America, where it has spread to most soybean growing regions and become the most important insect pest of soybean. Current control of soybean aphid relies primarily on insecticides, but alternatives to insecticidal control are being explored, especially host plant resistance and biological control, which may interact positively or negatively. Research on host plant resistance to the soybean aphid has revealed six genes that affect resistance. We measured the impact of the two most studied resistance loci, Rag1 and Rag2, on two parasitoid species: Aphelinus glycinis, a recently described species from Asia, which is being introduced into the USA to control the soybean aphid, and Aphelinus certus, also from Asia but accidentally introduced into the USA. Resistance did not affect oviposition by either parasitoid species. However, resistance did reduce successful parasitism by A. glycinis, with each resistance allele causing a two-fold reduction in number of mummified aphids. The resistance alleles did not affect adult emergence, sex ratio, or the size of A. glycinis. For A. certus, the Rag1 resistance allele had no effect on parasitism, while the Rag2 resistance allele reduced parasitism four-fold. On the other hand, the Rag1 resistance allele increased the frequency of males among progeny and decreased female size of A. certus. Despite the reduction in parasitism, these parasitoids are nonetheless able to parasitize the soybean aphid on resistant soybean, which means that they should still contribute to the management of soybean aphid on resistant varieties.     

Physiological responses of resistant and susceptible plants to diclofop-methyl and its antagonism by 2,4-D and antioxidants

Diclofop-methyl (DM) sprayed onto 6–8-week-old plants of leafy spurge ( Euphorbia esula L.) caused senescence and abscission of older leaves, while the young leaves and apex remained attached. The phytotoxicity of DM was reversed by the antioxidant, α -tocopherol (vitamin E), in leafy spurge and DM-susceptible oat ( Avena sativa L. cv. Gary). DM and 2,4-dichlorophenoxyacetic acid (2,4-D) increased ethylene evolution in mature leaves of leafy spurge. Vitamin E reduced the DM-induced ethylene by ampproximately 50%, but had no effect on the 2,4-D-induced ethylene. DM did not increase ethylene in DM-resistant pea or tobacco, but 2,4-D induced a 3-fold increase in ethylene evolution over controls in DM-resistant tobacco. 2,4-D amppears to act at a site different from that of DM in the pathway of ethylene formation. Ethylene evolution increased in DM-treated susceptible biotypes of annual ryegrass ( Lolium rigidum L.) and wild oat ( Avena fatua L.), but not in resistant biotypes of these species. DM reduced root and shoot formation and dry weight in hypocotyl segments of etiolated leafy spurge seedlings grown in vitro. Organogenesis and dry weights were increased by the combination of DM+antioxidants. Vitamin E was a more effective antioxidant than ascorbic acid. These results sumpport the hypothesis that DM induces oxidative stress in susceptible plant tissues and that antioxidants reduce the damaging action of the phytotoxic free radicals.     

The reactive oxygen species are involved in resistance responses of wheat to the Russian wheat aphid

The effect of Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), infestation on the hydrogen peroxide (H(2)O(2)) content and NADPH oxidase (EC 1.6.3.1) activity was studied in the resistant (cv. Tugela DN) and near-isogenic susceptible (cv. Tugela) wheat (Triticum aestivum L.). The objective of this study was to investigate the involvement of the reactive oxygen species (ROS) during the resistance responses against the RWA. Infestation significantly induced an early accumulation of the H(2)O(2) and increase of NADPH oxidase activity to higher levels in the resistant than susceptible plants. Results of inhibitory studies using diphenylene iodonium (DPI), a suicide inhibitor of NADPH oxidase, strongly suggested a possible signalling role for H(2)O(2) during RWA resistance response by activation of downstream defence enzymes [intercellular peroxidase (EC 1.11.1.7) and beta-1,3-glucanase (EC 3.2.1.39)].     

Test of semiochemicals and a resistant wheat variety for Russian wheat aphid management in South Africa

Abstract:  Aphid behaviour-modifying semiochemicals were tested against Russian wheat aphid Diuraphis noxia in South African wheat. Volatile substances from plant essential oils, methyl salicylate, 1,8-cineole and menthol were tested in the laboratory and field in combination with the D. noxia -susceptible wheat variety Betta and the resistant variety Elands. All three substances were repellent to D. noxia in olfactometric tests. Diuraphis noxia settled less on Elands plants that had been exposed to the volatiles, whereas the effect of the volatiles on D. noxia settling on Betta was less obvious. A slow-release pellet formulation was used to apply semiochemicals in wheat in replicated plot field trials in 2004 and 2005. In 2004, semiochemicals reduced aphid populations in Elands, but led to increased aphid populations in Betta. Further, the impact of the chemicals on aphid numbers and grain quality (thousand grain weight) varied according to plant variety, indicating an interaction between semiochemicals and plant resistance/variety.     

Reproductive rates of Russian wheat aphid (Hemiptera: Aphididae) biotypes 1 and 2 on a susceptible and a resistant wheat at three temperature regimes

The reproductive rates of Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), Biotype 1 (RWA 1) and Biotype 2 (RWA 2) were compared in the laboratory at three temperature regimes on a Russian wheat aphid resistant cultivar ('Prairie Red') and a susceptible cultivar ('TAM 107'). The objective of this study was to expose RWA 1 and RWA 2 to three temperature regimes and two levels of resistance to find whether there were reproductive differences that may occur within each biotype as well as differences in reproduction between biotypes. In addition, temperature effects of the Dn4 gene on biotype reproduction were noted. Differences in reproductive rates between the two biotypes seem to be driven by temperature. For both biotypes, longevity and reproductive rate parameters, except for intrinsic rate of increase, were lower at the 24-29 degree C temperature regime than the 13-18 degree C and 18-24 degree C temperature regimes. The intrinsic rate of increase was higher for both biotypes at the 18-24 degree C and 24-29 degree C temperature regimes than at the 13-18 degree C temperature regime. Reproductive rates between biotypes were similar at the two higher temperature regimes, but the fecundity for RWA 1 was less than RWA 2 at the 13-18 degree C temperature. The change in fecundity rates between RWA 1 and RWA 2 at lower temperatures could have ecological and geographical implications for RWA 2.     

Stylet penetration by two aphid species on susceptible and resistant lettuce

The probing behavior of two aphid species, Myzus persicae (Sulz.) and Nasonovia ribisnigri (Mosley), was electronically monitored on susceptible and resistant lettuce lines using a DC amplifier. A waveform pattern associated with extracellular stylet pathway activities, pattern C, occurred for longer periods when either aphid species probed resistant plants. This pattern is usually regularly interrupted by drops in electrical potential lasting a few seconds, reflecting cell membrane punctures followed by rapid withdrawal of the stylet tips. For M. persicae on resistant lettuce a large increase in pattern C without these potential drops accounted for the increased duration of this pattern. For N. ribisnigri the increase in pathway activity on resistant plants was due to an increase in the more typical pattern C with potential drops, as well as to an increased duration of pattern F, associated with a curious type of stylet penetration within cell walls. Both aphids made more but shorter probes on resistant than on susceptible plants, and these probes led less frequently to periods of sieve element contact and ingestion. The effects of resistance appear to involve both mesophyll and phloem factors. The underlying mechanisms, however, remain unclear. The results indicate which stylet penetration activities or waveform patterns are of interest for further investigation of resistance mechanisms.

---

Résumé Le comportement de sondage de variétés sensibles et résistantes de laitues par deux espèces de pucerons, Myzus persicae (Sulz.) et Nasonovia ribisnigri (Mosley), a été enregistré électroniquement en utilisant un amplificateur DC. Les deux espèces de pucerons ont présenté une amplification de l'onde de type C associée au cheminement extracellulaire des stylets lors du sondage des variétés résistantes. Cette onde est normalement interrompue par des chutes de quelques secondes du potentiel électrique, traduisant des ponctions de la membrane cellulaire suivies par un rapide retrait de l'extrémité des stylets. La forte augmentation de l'onde C mais sans chutes de potentiel rend compte de la plus longue durée de cette onde chez M. persicae sur laitue résistante. L'accroissement du cheminement des stylets chez N. ribisnigri sur plantes résistantes est dû à une augmentation de l'onde C typique avec chutes de potentiel, ainsi qu'à une prolongation de l'onde F liée à la pénétration des stylets dans les parois cellulaires. Les deux espèces font des sondages plus brefs et plus nombreux sur variétés résistantes, et ces sondages entraînent des contacts moins fréquents avec les éléments criblés et débouchent moins souvent sur de l'ingestion. Les effects de la résistance semblent impliquer des facteurs liés à la fois au mésophylle et au phloème. Les mécanismes sous-jacents, cependant, ne sont pas encore clairs. Ces résultats Montrent que l'examen des ondes liées à la pénétration des stylets est important pour des études ultérieures sur les mécanismes de résistance.

     

Characterisation of bird cherry-oat aphid (Rhopalosiphum padi L.) behaviour and aphid host preference in relation to partially resistant and susceptible wheat landraces

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a major pest of wheat (Triticum aestivum L.) and can cause up to 30% yield losses. Heritable plant resistance to aphids is both an economically and ecologically sound method for managing aphids. Here we report how the behaviour and performance of R. padi differs on two resistant, one susceptible wheat landrace and a susceptible elite wheat variety. Feeding behaviour differed among the genotypes, with aphids on resistant lines spending longer in the pathway phase and less time phloem feeding. These behaviours suggest that both inter- and intracellular factors encountered during pathway and phloem feeding phases could be linked to the observed aphid resistance. Locomotion and antennal positioning choice tests also revealed a clear preference for susceptible lines. Although feeding studies revealed differences in the first probe indicating that the resistance factors might also be located in the peripheral layers of the plant tissue, scanning electron microscopy revealed no difference in trichrome length and density on the surface of leaves. Aphids are phloem feeders and limiting the nutrient uptake by the aphids may negatively affect their growth and development as shown here in lower weight and survival of nymphs on resistant genotypes and decreased reproductive potential, with lowest mean numbers of nymphs produced by aphids on W064 (54.8) compared to Solstice (71.9). The results indicate that resistant lines markedly alter the behaviour, reproduction and development potential of R. padi and possess both antixenosis and antibiosis type of resistance.     

Physiological responses of resistant and susceptible buffalograsses to Blissus occiduus (Hemiptera: Blissidae) feeding

The impact of Blissus occiduus Barber feeding on resistant ('Prestige') and susceptible ('378') buffalograsses, Buchlo? dactyloides (Nuttall) Engelmann, was evaluated through measurement of carbon exchange rate, light and carbon assimilation (A-C(i)) curves, chlorophyll a fluorescence, and nonstructural carbohydrates. No significant differences in carbon exchange rates were observed between infested and control plants for 378 at 5 and 10 d after infestation; however, at 20 d after chinch bug introduction, significant differences in carbon exchange rates between infested and control 378 plants were detected. Carbon exchange rates were similar between infested and control Prestige plants at 5,10, and 20 d after infestation, suggesting that resistant plants can allocate energy for recovery from chinch bug injury. Significant differences in the photochemical efficiency of photosystem II (PSII) and the apparent photosynthetic electron transport ratio were observed between infested and control 378 plants, whereas, no significant differences in the photochemical efficiency of PSII and the electron transport ratio were detected between control and infested Prestige plants. Blissus occiduus-infested 378 and Prestige plants consistently had similar or higher levels of nonstructural carbohydrates compared with their respective control plants. These data suggest that both resistant and susceptible buffalograsses increase levels of nonstructural carbohydrates in response to B. occiduus feeding. This research also suggests compensatory photosynthesis takes place in Prestige but not in 378.     

Microsatellite markers linked to six Russian wheat aphid resistance genes in wheat

The Russian wheat aphid (RWA), Diuraphis noxia Mordvilko, is a serious economic pest of wheat and barley in North America, South America, and South Africa. Using aphid-resistant cultivars has proven to be a viable tactic for RWA management. Several dominant resistance genes have been identified in wheat, Triticum aestivum, including Dn1 in PI 137739, Dn2 in PI 262660, and at least three resistance genes (Dn5+) in PI 294994. The identification of RWA-resistant genes and the development of resistant cultivars may be accelerated through the use of molecular markers. DNA of wheat from near-isogenic lines and segregating F₂ populations was amplified with microsatellite primers via PCR. Results revealed that the locus for wheat microsatellite GWM111 (Xgwm111), located on wheat chromosome 7DS (short arm), is tightly linked to Dn1, Dn2 and Dn5, as well as Dnx in PI 220127. Segregation data indicate RWA resistance in wheat PI 220127 is also conferred by a single dominant resistance gene (Dnx). These results confirm that Dn1, Dn2 and Dn5 are tightly linked to each other, and provide new information about their location, being 7DS, near the centromere, instead of as previously reported on 7DL. Xgwm635 (near the distal end of 7DS) clearly marked the location of the previously suggested resistance gene in PI 294994, here designated as Dn8. Xgwm642 (located on 1DL) marked and identified another new gene Dn9, which is located in a defense gene-rich region of wheat chromosome 1DL. The locations of markers and the linked genes were confirmed by di-telosomic and nulli-tetrasomic analyses. Genetic linkage maps of the above RWA resistance genes and markers have been constructed for wheat chromosomes 1D and 7D. These markers will be useful in marker-assisted breeding for RWA-resistant wheat. Received: 17 May 2000 / Accepted: 13 June 2000