Identificación molecular del endosimbionte secundario Hamiltonella defensa en el pulgón amarillo de los cereales, Metopolophium dirhodum

255-258This is the first report of the association between the rose-grain aphid Metopolophium dirhodum, a potentially important cereal pest and the facultative symbiont Hamiltonella defensa. The infection with this gamma-proteobacterium was determined by PCR in laboratory-reared and field-collected specimens of an Argentinian population of the aphid. Partial bacterial 16S, IGS and 23S rRNA genes were sequenced and compared to other available Hamiltonella sequences by phylogenetic analysis. The present study provides new information on previously unknown M. dirhodum microbiota

Mutualism effectiveness and vertical transmission of symbiotic fungal endophytes in response to host genetic background

838-849Certain species of the Pooideae subfamily develop stress tolerance and herbivory resistance through symbiosis with vertically transmitted, asexual fungi. This symbiosis is specific, and genetic factors modulate the compatibility between partners. Although gene flow is clearly a fitness trait in allogamous grasses, because it injects hybrid vigor and raw material for evolution, it could reduce compatibility and thus mutualism effectiveness. To explore the importance of host genetic background in modulating the performance of symbiosis, Lolium multiflorum plants, infected and noninfected with Neotyphodium occultans, were crossed with genetically distant plants of isolines (susceptible and resistant to diclofop-methyl herbicide) bred from two cultivars and exposed to stress. The endophyte improved seedling survival in genotypes susceptible to herbicide, while it had a negative effect on one of the genetically resistant crosses. Mutualism provided resistance to herbivory independently of the host genotype, but this effect vanished under stress. While no endophyte effect was observed on host reproductive success, it was increased by interpopulation plant crosses. Neither gene flow nor herbicide had an important impact on endophyte transmission. Host fitness improvements attributable to gene flow do not appear to result in direct conflict with mutualism while this seems to be an important mechanism for the ecological and contemporary evolution of the symbiotum

Effect of different proportions of Tagetes minuta and Glycine max on Meloidogyne incognita

Se estudió la infestación de raíces por Meloidogyne incognita (Kofoid and White) Chitwood, en mezclas de plantas de Glycine max (L.) Merr. y Tagetes minuta L. en distintas proporciones, por medio de un experimento diseñado para probar cómo huéspedes en diferentes proporciones modifican la reproducción de la primera generación y la densidad de la segunda generación del nematodo. En ambos huéspedes se evaluó la densidad de agallas radicales, sitios de alimentación, reacciones histológicas y número de larvas de la segunda generación de M. incognita. La densidad de agallas fue mayor en los tratamientos donde las plantas de G. max se encontraban en más de 50%. En raíces de T. minuta se encontraron sitios de alimentación no funcionales y las larvas en estos sitios no alcanzaron la etapa reproductiva. La reducción de la densidad de las larvas en el suelo fue proporcional a las cantidades de T. minuta en la mezcla. Estos datos indican que T. minuta infectado por nematodos actúa como una "planta trampa" debido a la captura de algunos individuos, muchos de los cuales nunca alcanzan la madurez sexual, y por la reducción de la densidad de segunda generación de M. incognita.Root infestation by Meloidogyne incognita (Kofoid and White) Chitwood, in mixtures of Glycine max (L.) Merr. and Tagetes minuta L. was studied through an experiment designed to test how host different proportions influenced nematode reproduction and second generation density. Root-knot density, feeding sites, histological reactions and number of second generation M. incognita in both host species were evaluated. Gall density was higher in treatments where G. max plants were in more than 50%. In roots of T. minuta feeding sites they were found nonfunctional and larvae on these sites did not reach reproductive stage. Density reduction of soil larvae was proportional to the amounts of T. minuta in the mixture. These data indicate that T. minuta infected by nematodes acts as a “trap-plant”, capturing some individuals, many of which will never reach sexual maturity, and reducing second generation density

Effects of mowing frequency and nitrogen fertilization on the structure of a Festuca arundinacea Schreb

La estructura y la dinámica de las poblaciones vegetales de los sistemas pastoriles están principalmente controladas por la competencia intra e interespecífica y por la herbivoría, en el marco impuesto por el ambiente edáfico y el clima. Las defoliaciones producidas por los herbívoros impactan directamente sobre las poblaciones vegetales, pero también indirectamente, mediante el control de la intensidad de la competencia. En este trabajo evaluamos los efectos de la frecuencia de defoliaciones mecánicas y de la fertilización con nitrógeno sobre la estructura poblacional de Festuca arundinacea Schreb. sembrada en una pastura mixta y discutimos los efectos de estas prácticas agronómicas sobre la competencia intraespecífica. Sobre una pastura compuesta por F. arundinacea y Lotus glaber Mill. instalamos un experimento factorial que combina dos niveles de frecuencia de cortes (uno y cuatro cortes anuales) y dos de fertilización nitrogenada (0 y 50 kg N/ha/año) en cuatro bloques. Los cortes frecuentes resultaron en mayor densidad y menor jerarquización de tamaños y no modificaron la cobertura total en comparación con los cortes poco frecuentes. En cambio, la fertilización nitrogenada disminuyó la densidad, generó mayor jerarquización de tamaños y aumentó la cobertura total. El log10 del tamaño promedio estuvo negativamente asociado con el log10 de la densidad; la pendiente estimada de la relación lineal fue más cercana a –3/2 para las parcelas fertilizadas que para las no fertilizadas. El apiñamiento de individuos (medido en una escala de 0.25 m2) disminuyó significativamente con el aumento del tamaño individual. Nuestros resultados sugieren que la dinámica de F. arundinacea estuvo fuertemente controlada por la competencia intraespecífica en condiciones de baja frecuencia de disturbios y alta disponibilidad de nitrógeno. Además, indican que la adición de un recurso posiblemente limitante puede intensificar la competencia intraespecífica en lugar de relajarla.Structure and dynamics of plant populations in pastoral systems are mainly controlled by inter and intraspecific competition and herbivory within the constraints imposed by soil characteristics and climate. Defoliations caused by herbivores have a direct impact on plant populations, but also an indirect effect through changes in the intensity of competition. We evaluated the effects of mechanic defoliations and nitrogen fertilization on the structure of a Festuca arundinacea Schreb. population in a mixed pasture, and discussed the effects of these two agronomic tools on the intensity of intraspecific competition. A four-block factorial experiment combining two regimes of mowing frequency (1 and 4 mowings/year) and two levels of nitrogen fertilization (0 and 50 kg N/ha/year), was installed on a mixed pasture composed of F. arundinacea and Lotus glaber Mill. Data was obtained from eight subplots randomly arranged inside each plot. High frequency mowed plots had higher density and lower size asymmetry than the low frequency ones, whereas total cover did not differ between treatments. On the contrary, nitrogen fertilization diminished population density, increased size asymmetry and augmented total cover. Overall, there was a negative association between log10 mean size and log10 density; the estimated slope of the linear relation was closer to –3/2 in fertilized than in non-fertilized plots. Finally, crowding decreased significantly with plant size. These results suggest that the dynamics of F. arundinacea in the experimental plots were strongly controlled by intraspecific competition when nitrogen availability was high and mowing frequency was low. Lack of fertilization and frequent mows would limit individual growth and hence intensity of competition. Furthermore, the intensity of competition seems to have been more closely related to nitrogen availability than to mowing frequency. Additional data showed that the abundance of companion species was precluded in fertilized plots and in low frequency mowed plots, suggesting that F. arundinacea is a highly competitive species, which maximizes vegetative growth in productive, relatively undisturbed conditions. Overall, our results indicate that resource supply could intensify plant competition instead of relax it

Decision Support Systems for Weed Management

Editors: Guillermo R. Chantre, José L. González-Andújar.Weed management Decision Support Systems (DSS) are increasingly important computer-based tools for modern agriculture. Nowadays, extensive agriculture has become highly dependent on external inputs and both economic costs, as well the negative environmental impact of agricultural activities, demands knowledge-based technology for the optimization and protection of non-renewable resources. In this context, weed management strategies should aim to maximize economic profit by preserving and enhancing agricultural systems. Although previous contributions focusing on weed biology and weed management provide valuable insight on many aspects of weed species ecology and practical guides for weed control, no attempts have been made to highlight the forthcoming importance of DSS in weed management. This book is a first attempt to integrate 'concepts and practice' providing a novel guide to the state-of-art of DSS and the future prospects which hopefully would be of interest to higher-level students, academics and professionals in related areas

The Effects of Seed Size on Hybrids Formed between Oilseed Rape (Brassica napus) and Wild Brown Mustard (B. juncea)

Background : Seed size has significant implications in ecology, because of its effects on plant fitness. The hybrid seeds that result from crosses between crops and their wild relatives are often small, and the consequences of this have been poorly investigated. Here we report on plant performance of hybrid and its parental transgenic oilseed rape (Brassica napus) and wild B. juncea, all grown from seeds sorted into three seed-size categories.[br/] Methodology/Principal Findings : Three seed-size categories were sorted by seed diameter for transgenic B. napus, wild B. juncea and their transgenic and non-transgenic hybrids. The seeds were sown in a field at various plant densities. Globally, small-seeded plants had delayed flowering, lower biomass, fewer flowers and seeds, and a lower thousand-seed weight. The seed-size effect varied among plant types but was not affected by plant density. There was no negative effect of seed size in hybrids, but it was correlated with reduced growth for both parents.[br/] Conclusions : Our results imply that the risk of further gene flow would probably not be mitigated by the small size of transgenic hybrid seeds. No fitness cost was detected to be associated with the Bt-transgene in this study

Herbicide-Resistant Crops: Utilities and Limitations for Herbicide-Resistant Weed Management

Since 1996, genetically modified herbicide-resistant (HR) crops, particularly glyphosate-resistant (GR) crops, have transformed the tactics that corn, soybean, and cotton growers use to manage weeds. The use of GR crops continues to grow, but weeds are adapting to the common practice of using only glyphosate to control weeds. Growers using only a single mode of action to manage weeds need to change to a more diverse array of herbicidal, mechanical, and cultural practices to maintain the effectiveness of glyphosate. Unfortunately, the introduction of GR crops and the high initial efficacy of glyphosate often lead to a decline in the use of other herbicide options and less investment by industry to discover new herbicide active ingredients. With some exceptions, most growers can still manage their weed problems with currently available selective and HR crop-enabled herbicides. However, current crop management systems are in jeopardy given the pace at which weed populations are evolving glyphosate resistance. New HR crop technologies will expand the utility of currently available herbicides and enable new interim solutions for growers to manage HR weeds, but will not replace the long-term need to diversify weed management tactics and discover herbicides with new modes of action. This paper reviews the strengths and weaknesses of anticipated weed management options and the best management practices that growers need to implement in HR crops to maximize the long-term benefits of current technologies and reduce weed shifts to difficult-to-control and HR weeds