Belowground responses of bacterial communities to foliar SA application over four plant generations

Background and aims Jacobaea vulgaris plants grow better in sterilized than in live soil. Foliar application of SA mitigates this negative effect of live soil on plant growth. To examine what causes the positive effect of SA application on plant growth in live soils, we analyzed the effects of SA application on the composition of active rhizosphere bacteria in the soil. Methods We studied the composition of the microbial community over four consecutive plant cycles (generations), using mRNA sequencing of the microbial communities in the rhizosphere of J. vulgaris. We initiated the experiment with an inoculum of live soil collected from the field, and at the start of each subsequent plant cycle, we inoculated a small part of the soil from the previous plant cycle into sterile bulk soil. Results Application of SA did not significantly increase or decrease the Shannon diversity at genus level within each generation, but several specific genera were enriched or depleted after foliar SA application. The composition of bacterial communities in the rhizosphere significantly differed between plant cycles (generations), but application of SA did not alter this pattern. In the first generation no genera were significantly affected by the SA treatment, but in the second, third and fourth generations, specific genera were significantly affected. 89 species out of the total 270 (32.4%) were present as the “core” microbiome in all treatments over four plant cycles. Conclusions Overall, our study shows that the composition of bacterial genera in the rhizosphere significantly differed between plant cycles, but that it was not strongly affected by foliar application of SA on J. vulgaris leaves. Further studies should examine how activation of the SA signaling pathway in the plant changes the functional genes of the rhizosphere bacterial community.

Regulation of Jacobaea vulgaris by varied cutting and restoration measures

The growth of the noxious grassland weed Jacobaea vulgaris Gaertn. in pastures is a threat to grazing animals. This is especially true when it dominates vegetation cover, which often occurs on non-intensively used pastures that are managed with the nature-conservation goal to maintain and promote biodiversity. Thus, we wanted to find management techniques to reduce J. vulgaris without harming the floral biodiversity on the pastures.We tested six different mechanical and cultural methods to reduce the presence and spread of J. vulgaris. Seven study sites in Northern Germany (Schleswig-Holstein) were treated with various measurements, including: milling and seeding, hay transfer, mowing regimes at different times and frequencies, and combinations of mowing and seeding.Our results show that cutting within the bloom of the plant at the end of June and again four weeks later, when the plant is in its second bloom was the most effective treatment in reducing J. vulgaris abundance. This was the only treatment leading to a significant reduction in population growth rate and density without reducing surrounding plant species richness.The study reveals that management of J. vulgaris in non-intensively used pastures is possible, while preserving species-rich grasslands.

Conspecific and heterospecific grass litter effects on seedling emergence and growth in ragwort (Jacobaea vulgaris)

Jacobaea vulgaris Gaertn. or common ragwort is a widespread noxious grassland weed that is subject to different regulation measures worldwide. Seedling emergence and growth are the most crucial stages for most plants during their life cycle. Therefore, heterospecific grass or conspecific ragwort litter as well as soil-mediated effects may be of relevance for ragwort control. Our study examines the effects of conspecific and heterospecific litter as well as ragwort conditioned soil on seedling emergence and growth. We conducted pot experiments to estimate the influence of soil conditioning (with, without ragwort), litter type (grass, ragwort, grass-ragwort-mix) and amount (200 g/m², 400 g/m²) on J. vulgaris recruitment. As response parameters, we assessed seedling number, biomass, height and number of seedling leaves. We found that 200 g/m² grass litter led to higher seedling numbers, while litter composed of J. vulgaris reduced seedling emergence. Litter amounts of 400 g/m² had negative effects on the number of seedlings regardless of the litter type. Results for biomass, plant height and leaf number showed opposing patterns to seedling numbers. Seedlings in pots treated with high litter amounts and seedlings in ragwort litter became heavier, grew higher and had more leaves. Significant effects of the soil conditioned by ragwort on seedling emergence and growth were negligible. The study confirms that the amount and composition of litter strongly affect seedling emergence and growth of J. vulgaris. Moreover, while conspecific litter and high litter amounts negatively affected early seedling development in ragwort, those seedlings that survived accumulated more biomass and got taller than seedlings grown in heterospecific or less dense litter. Therefore, ragwort litter has negative effects in ragwort germination, but positive effects in ragwort growth. Thus, leaving ragwort litter on pastures will not reduce ragwort establishment and growth and cannot be used as management tool.

Spread of Jacobaea vulgaris and Occurrence of Pyrrolizidine Alkaloids in Regionally Produced Honeys from Northern Germany: Inter- and Intra-Site Variations and Risk Assessment for Special Consumer Groups

Pyrrolizidine alkaloids (PA) and PA N-oxides (PANO) are secondary plant metabolites exhibiting genotoxic and carcinogenic properties. Apart from the roots and leaves, PA/PANO are particularly present in pollen and nectar. Therefore, the spread of Jacobaea vulgaris in certain regions of northern Germany has an impact on the safety of honey produced in that region. In this study, raw honey samples (n = 437) were collected from usually three individual beehives per site (n = 73) in the district of Ostholstein and analyzed for 25 PA/PANO. The results reveal mean levels of 8.4, 1.5, and 72.6 µg/kg and maximum levels of 111, 59.4, and 3313 µg/kg, depending on the season (summer 2015 and spring/summer 2016, respectively). As far as individual data are concerned, sites near areas with J. vulgaris growth did not necessarily result in high PA/PANO values. Furthermore, intra-site investigations revealed remarkable differences in PA/PANO levels of raw honey collected by different bee colonies at the same site. Consumption of these regionally produced honeys entails an increased exposure to PA/PANO, especially in children and high consumers. Margin of exposure values of <10,000 and an exceedance of the health-based guidance value highlight that regionally produced and marketed honey must be considered with care for a proper risk assessment and risk management.

Efficient Extraction of Pyrrolizidine Alkaloids from Plants by Pressurised Liquid Extraction – A Preliminary Study

Pyrrolizidine alkaloids and their corresponding pyrrolizidine alkaloid-N-oxides are secondary plant constituents that became the subject of public concern due to their hepatotoxic, pneumotoxic, genotoxic, and cytotoxic effects. In contrast to the well-established analytical separation and detection methods, only a few studies have investigated the extraction of pyrrolizidine alkaloids/pyrrolizidine alkaloid-N-oxides from plant material. In this study, we have applied pressurized liquid extraction with the aim of evaluating the effect of various parameters on the recovery of pyrrolizidine alkaloids. The nature of the modifier (various acids, NH3) added to the aqueous extraction solvent, its concentration (1 or 5%), and the temperature (50 – 125 °C) were systematically varied. To analyse a wide range of structurally different pyrrolizidine alkaloids, Jacobaea vulgaris (syn. Senecio jacobaea), Tussilago farfara, and Symphytum officinale were included. Pyrrolizidine alkaloids were quantified by HPLC-MS/MS and the results obtained by pressurised liquid extraction were compared with the amount of pyrrolizidine alkaloids determined by an official reference method. Using this approach, increased rates of recovery were obtained for J. vulgaris (up to 174.4%), T. farfara (up to 156.5%), and S. officinale (up to 288.7%). Hence, pressurised liquid extraction was found to be a promising strategy for the complete and automated extraction of pyrrolizidine alkaloids, which could advantageously replace other time- and solvent-consuming extraction methods.