Influence of insects and fungal pathogens on individual and population parameters of <Emphasis Type="Italic">Cirsium arvense</Emphasis> in its native and introduced ranges

Introduced weeds are hypothesized to be invasive in their exotic ranges due to release from natural enemies. Cirsium arvense (Californian, Canada, or creeping thistle) is a weed of Eurasian origin that was inadvertently introduced to New Zealand (NZ), where it is presently one of the worst invasive weeds. We tested the ‘enemy release hypothesis’ (ERH) by establishing natural enemy exclusion plots in both the native (Europe) and introduced (NZ) ranges of C. arvense. We followed the development and fate of individually labelled shoots and recorded recruitment of new shoots into the population over two years. Natural enemy exclusion had minimal impact on shoot height and relative growth rate in either range. However, natural enemies did have a significant effect on shoot population growth and development in the native range, supporting the ERH. In year one, exclusion of insect herbivores increased mean population growth by 2.1–3.6 shoots m⁻², and in year two exclusion of pathogens increased mean population growth by 2.7–4.1 shoots m⁻². Exclusion of insect herbivores in the native range also increased the probability of shoots developing from the budding to the reproductive growth stage by 4.0× in the first year, and 13.4× in the second year; but exclusion of pathogens had no effect on shoot development in either year. In accordance with the ERH, exclusion of insect herbivores and pathogens did not benefit shoot development or population growth in the introduced range. In either range, we found no evidence for an additive benefit of dual exclusion of insects and pathogens, and in no case was there an interaction between insect and pathogen exclusion. This study further demonstrates the value of conducting manipulative experiments in the native and introduced ranges of an invasive plant to elucidate invasion mechanisms.     

Examining olfactory and visual cues governing host-specificity of a weed biological control candidate species to refine pre-release risk assessment

In weed biological control programs, pre-release host-specificity testing relies traditionally on no-choice and choice feeding, oviposition, and development tests. Rarely have they included detailed examination of behavioral responses to olfactory and visual cues of biological control candidates, although a better understanding of the mechanisms underlying host recognition may explain potential discrepancies between choice and no-choice tests, and/or between tests conducted in the lab versus field conditions. We investigated how the seed-feeding weevil, Mogulones borraginis, distinguishes its host plant, Cynoglossum officinale, from three native confamilial non-target species in North America. In behavioral bioassays, M. borraginis responded to olfactory and visual cues individually and, to an even greater extent, to both plant cue modalities when offered simultaneously. In tests with the combined cues, M. borraginis was attracted to C. officinale but responded with indifference or was repelled by non-target plants. In electrophysiological experiments, we identified that M. borraginis responded to ten volatile compounds and four wavelengths of lights from inflorescences of C. officinale. We propose that studies of responses to multimodal plant cues can advance our understanding of how biocontrol candidate species discriminate among host plants and closely related non-target species, thereby increasing the accuracy of environmental safety assessments pre-release.     

Fructan contents and dry matter deposition in different tissues of the wheat grain during development

The role of fructan metabolism in the assimilate relations of the grain of wheat (Triticum aestivum L.) was investigated by determination of the dry matter and fructan content of grain components at short intervals during grain filling. During the initial phase of rapid expansion, most of the assimilates entering the grain were partitioned to the outer pericarp. A large fraction of these assimilates were used for the synthesis of fructan. Dry matter deposition and fructan synthesis in the outer pericarp ceased at about 5d after anthesis. At the same time, the endosperm and the inner pericarp and testa started to accumulate dry matter at a fast rate. This was also associated with significant fructan synthesis in the latter tissues. The outer pericarp lost about 45% of its former maximum dry weight between 9 and 19 d after anthesis. This loss was due almost entirely to the near complete disappearance of water-soluble carbohydrates, most of which was fructan. The inner pericarp and testa accumulated dry matter until about mid-grain filling. The fructan contents of the inner pericarp and testa and the endosperm decreased slowly towards the end of grain filling. Most of the fructans in the inner pericarp and testa and the endosperm had a low molecular weight, whereas higher molecular weight fructans predominated in the outer pericarp. The embryo did not contain fructan. The presence of low molecular weight fructans in the endosperm cavity at mid-grain filling was confirmed. It is suggested that fructan synthesis is closely linked to growth-related water deposition in the different tissues of the wheat grain and serves to sequester the surplus of imported sucrose.     

Improved Coated Vesicle Isolation Allows Better Characterization of Clathrin Polypeptides

Postmicrosomal pellets from plant sources are grossly contaminatedwith nbosomes. Previous purifications of clathrin coated vesicles(CCV) from such subcellular fractions have therefore often involvedan RNase treatment. Performed at 30°C, this step inherentlycarries with it the dangers of proteolysis. We document herea method for CCV isolation which avoids this. Through the inclusionof suitable antiproteases in the homogenizing and subsequentisolation media, we have also been able to improve the qualityof CCV recovered from plant tissues. As a result we have tentativelybeen able to identify clathrin light chains from zucchini hypocotyland pea cotyledon CCV. Similar to light chains from bovine brainthese polypeptides are heat stable, can be solubilized fromneutralized TCA precipitates, bind calcium and clathrin heavychains. However, in contrast to brain CCV the two light chainsof plant CCV are some 10 kDa heavier. Key words: Antiproteases, Ca²⁺-binding, clathrin coated vesicles, clathrin heavy chains, clathrin light chain(s), heat stability, pea cotyledons, RNase, zucchini hypocotyls     

Enemy release does not increase performance of <Emphasis Type="Italic">Cirsium</Emphasis> <Emphasis Type="Italic">arvense</Emphasis> in New Zealand

Cirsium arvense (L.) Scop. (Californian, Canada, or creeping thistle) is an exotic perennial herb indigenous to Eurasia that successfully established in New Zealand (NZ) approximately 130 years ago. Presently, C. arvense is considered one of the worst invasive weeds in NZ arable and pastoral productions systems. A mechanism commonly invoked to explain the apparent increased vigour of introduced weeds is release from natural enemies. The enemy-release hypothesis (ERH) predicts that plants in an introduced range should experience reduced herbivory, particularly from specialists, and that release from this natural enemy pressure facilitates increased plant performance in the introduced range. In 2007, surveys were carried out in 13 populations in NZ (7 in the North Island and 6 in the South Island) and in 12 populations in central Europe to quantify and compare growth characteristics of C. arvense in its native versus introduced range. Altitude and mean annual precipitation for each population were used as covariates in an attempt to explain differences or similarities in plant traits among ranges. All plant traits varied significantly among populations within a range. Shoot dry weight was greater in the South Island compared to Europe, which is in line with the prediction of increased plant performance in the introduced range; however, this was explained by environmental conditions. Contrary to expectations, the North Island was not different from Europe for all plant traits measured, and after adjustment for covariates showed decreased shoot density and dry weight compared to the native range. Therefore, environmental factors appear to be more favourable for growth of C. arvense in both the North and South Islands. In accordance with the ERH, there was significantly greater endophagous herbivory in the capitula and stems of shoots in Europe compared to both NZ ranges. In NZ, capitulum attack from Rhinocyllus conicus was found only in the North Island, and no stem-mining attack was found anywhere in NZ. Thus, although C. arvense experiences significantly reduced natural enemy pressure in both the North and South Islands of NZ there is no evidence that it benefits from this enemy release.     

Nitrous oxide emissions from a cropped soil in a semi-arid climate

Understanding nitrous oxide (N₂O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N₂O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha^?1 yr^?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography. Daily N₂O emissions were low (?1.8 to 7.3 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 0.11 kg N₂O‐N ha^?1 from N‐fertilized soil and 0.09 kg N₂O‐N ha^?1 from nonfertilized soil. Over half (55%) the annual N₂O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N₂O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N₂O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N₂O emission, demonstrating that a proportion of N₂O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N₂O emissions.     

Effects of pasture competition and specialist herbivory on the performance of Cirsium arvense

Combining specialist herbivory with interspecific plant competition can be an effective means of controlling pasture weeds. Cirsium arvense (Canada thistle, Californian thistle, creeping thistle) is one of the worst weeds of pastoral production systems in New Zealand (NZ). The oligophagous leaf-feeding beetle, Cassida rubiginosa, was recently released in NZ for control of C. arvense. To assess the impact of this biocontrol agent we conducted an outdoor potted-plant experiment with low and high densities of Cassida larvae combined with different levels of interspecific competition from typical NZ pasture species. Secondly, we carried out a field-release experiment to quantify the impact of high densities of Cassida under more natural conditions. Interspecific competition reduced all measured plant parameters of C. arvense except mean shoot height and base diameter. Herbivory by Cassida only reduced root biomass, and showed a weak additive response when combined with competition. All other measured parameters of C. arvense showed a substitutive response, with competition being the only factor having a significant impact on the weed. There were no significant synergistic interactions with competition and herbivory on C. arvense. Interestingly, the number of root buds per plant was significantly greater in the presence of herbivory by Cassida, suggesting that C. arvense may compensate for defoliation. Similar to the potted-plant experiment, Cassida had no significant effect on shoot growth and development in the field-release experiment. The results of this study indicate that competition from typical NZ pasture species is a more important factor than herbivory by Cassida, and unless Cassida reaches outbreak densities, it will likely have an insignificant impact on this weed.