Role of acquisition and inoculation time in the expression of clonal variation for BYDV-PAV transmission in the aphid species Rhopalosiphum padi

Vector efficiency of 20 Rhopalosiphum padi clones, originating from Europe, North America and North Africa and exhibiting different types of life cycle, was evaluated by transmitting a French BYDV-PAV isolate to barley plants under five different acquisition/inoculation sequences (AAP/IAP). Differences between clones in transmission efficiency were found only when a short AAP was followed by a long IAP (6 h/120 h) and, to some extent, when a long AAP (48 h) was followed by a short IAP (6 h), but no differences were found when the conditions for virus transmission were optimal, i.e. long AAP followed by long IAP (48 h/120 h). There were no differences in transmission rates by clones of different geographical origins and with different life cycles. As a consequence, clonal variation is probably of little importance in the vector aspects of the epidemiology of PAV serotypes transmitted by R. padi , but the availability of a range of clones exhibiting transmission differences under limiting AAP or IAP conditions could be of interest for studies of virus–vector relationships.     

Transmission of grapevine leafroll-associated closteroviruses by Pseudococcus longispinus and P. calceolariae

The transmission of two closteroviruses associated with grapevine leafroll, GLRaV-1 and GLRaV-3, from grapevine to grapevine by the mealybugs, Pseudococcus longispinus and P. calceolariae (Homoptera: Pseudococcidae) was studied. Controlled transmission experiments using the first and third instars of each insect were conducted twice during the 1993–94 growing season to investigate the consequence of virus accumulation within the donor vine leaf tissue on the incidence of virus transmission to healthy recipient vines. Transmission of GLRaV-1 and GLRaV-3 was determined by ELISA testing recipient vines in July 1994 and March 1995. GLRaV-3 was transmitted to recipient vines by P. longispinus and P. calceolariae first instars only. An increase in virus titre within the season did not significantly alter the transmission rate of GLRaV-3 by either P. longispinus or P. calceolariae first instars. P. longispinus and P. calceolariae failed to transmit GLRaV-1 to recipient vines.     

The effect of component number on Rhynchosporium secalis infection and yield in mixtures of winter barley cultivars

Mixtures of winter barley cultivars containing up to six components were grown over three years with and without fungicide treatment. Yield increases were recorded for mixtures compared with the mean of their monoculture components and there was a significant trend towards greater benefit from increased number of components. These benefits were partially attributable to a corresponding increase in control of Rhynchosporium secalis as component number increased. The potential for exploitation of mixtures in cereals for control of splash-dispersed pathogens is discussed.     

Weed communities of winter wheat as influenced by input level and rotation

The influence of cropping systems constituted by the combination of three input levels (low, intermediate and high) and four 2-year rotations between a spring summer crop and winter wheat ( Triticum aestivum L.) on the density and composition of the winter wheat weed flora was evaluated from 1989 to 1992. Spring summer crops were maize ( Zea mays L.). soyabean ( Glycine max (L.) Merr.). sugar beet ( Beta vulgaris L.) and sunflower ( Helianthus and L.) Total weed density at the end of the wheat tillering stage ranged from 2.5 to 128.7 plants m^-2 and generally decreased with cropping intensity, although to a differing extent, depending upon rotation. Differences among weed communities of the 12 cropping systems were mostly dependent upon input level. The effect of the preceding crop on the weed flora composition of winter wheat was evident only with reduced herbicide use (low-input systems). Weed species could be ascribed to four categories: (a) species sporadically associated with cropping systems (most of the weeds); (b) species associated with a specific input level (e.g. Papaver rhoeas L. to low and intermediate input levels and Veronica persica Poiret to the high input ievel): (c) species associated with a specific rotation (e.g. Anm majus L. to sugar beet winter wheat): and (d) species associated with specitie plots that become important every other year (e.g. Convolvulus arvensis L. and Stellaria media (L.) Vill.).     

Modelling crop:weed interactions in wheat with ALMANAC

ALMANAC is a dynamic model for plant growth, water balance and soil nitrogen dynamics that can simulate on a daily basis two or more competing species. The simulation of competition for light is based on Beer's law, allowing a different extinction coefficient ( k ) for each species. Light is partitioned between species based on k -values, leaf area index and plant heights. Total hiomass is simulated with radiation use efficiency and grain yield with a harvest index approach, sensitive to water stress. The model simulates competition for water and nutrients based on each species current rooting zone and demand by each species. The effect of crop management on the competition issue can he simulated. The model was evaluated in Dijon (France) using 4 years of experimental data on wheat:oat mixtures, differing in oat ( Avena sativa L.) densities, the period of oat emergence, the date of weed suppression by herbicides and the wheat ( Triticun aestivum L.) genotype. Additional data on oilseed rape ( Brassica nupus L.) and vetch ( Vilcia sauiva L.) competition in spring wheat were also used. The wheat grain yield was reasonably simulated with a root mean square error (RMSE) of 0.10-0.35 t ha^-1. Corresponding values for oats were 0.10-0.55 t ha^-1. The competitiveness of oats, oilseed rape and vetch was correctly simulated. The model appears as a reasonable tool for estimating damage thresholds in integrated weed control programmes.     

Abscisic acid is a potent inhibitor of growth and sporidial formation inneovossia indica cultures: Dual mode of actionvia loss of polyamines and cellular turgidity

The inhibition of polyamine biosynthesis inNeovossia indica (Mitra) Mundkur by D,L— α-difluoromethylornithine (DFMO) caused an effective reduction of mycelial growth and sporidial production underin vitro conditions, which was reversed by ornithine application. Abscisic acid proved to be similarly effective, not only in inhibiting mycelial growth and sporidial formation but also the germination of teliospores, which constitute the primary inoculum of the pathogen. ABA-mediated inhibition resulted in decreased polyamine levels and loss of cellular turgidity of mycelial cultures. Scanning electron microscopy of ABA-treated cultures revealed extremely shrunken hyphae, in marked contrast to the turgid controls. It is suggested that the manipulation of ABA levels and/or tissue sensitivity in wheat could be a strategy to combat ‘Karnal’ bunt, the disease caused byN. indica.     

Ecotonal changes and altered tree spatial patterns in lower mixed-conifer forests, Grand Canyon National Park, Arizona, U.S.A.

This research analyzes patch development and determines tree spatial patterns along the lower mixed-conifer ecotone on the North Rim of Grand Canyon National Park in Arizona (U.S.A.). Patterns of patch development were interpreted from spatial analyses, based on tree age and size, and past records of disturbance and climate. Five plots in the ecotone between mixed conifer forests and monospecific stands of ponderosa pine (Pinus ponderosa) were studied for patterns of patch development. The methods used include: (1) size-structure analyses, to compare species patch development; (2) dendrochronological dating of tree establishment; (3) tree ring master chronology, to determine periods of suppressed growth, compared to a Palmer Drought Severity Index; and (4) spatial analyses by species composition, size and age, with univariate and bivariate analyses of spatial association and spatial autocorrelation. We found an increased density of shade-tolerant and fire-intolerant species namely clusters of pole-sized white fir, and fewer large ponderosa pine. This revised version was published online in May 2005 with corrections to the Cover Date. This revised version was published online in July 2006 with corrections to the Cover Date.     

新一代植酸酶的经济效益

现已证明，在酵母中产生的一种新型的6-植酸酶比其他在市场上供应的植酸酶的效率至少高20％。因为6-植酸酶与其他商品化的植酸酶相比，可进一步减少无机磷源的使用，从而降低生产成本以及磷的污染。