Effects of seed traits variation on seedling performance of the invasive weed, Ambrosia artemisiifolia L

Seed Germination and Growth Improvement for Early Maturing Pear Breeding

Breeding early maturing cultivars is one of the most important objectives in pear breeding. Very early maturing pears provide an excellent parental material for crossing, but the immature embryo and low seed germination of their hybrid progenies often limit the selection and breeding of new early maturing pear cultivars. In this study, we choose a very early maturing pear cultivar 'Pearl Pear' as the study object and investigate the effects of cold stratification, the culture medium, and the seed coat on the germination and growth of early maturing pear seeds. Our results show that cold stratification (4 °C) treatment could significantly improve the germination rates of early maturing pear seeds. A total of 100 days of cold-temperature treatment in 4 °C and in vitro germination on White medium increased the germination rate to 84.54%. We also observed that seed coat removal improved the germination of early maturing pear seeds, with middle seed coat removal representing the optimal method, with a high germination rate and low contamination. The results of our study led to the establishment of an improved protocol for the germination of early maturing pear, which will greatly facilitate the breeding of new very early maturing pear cultivars.

Assessment of the processing conditions which make the Ambrosia seeds non-viable

The European Commission requested EFSA to provide an assessment of the processing conditions which make Ambrosia seeds non-viable in feed materials and compound feed. This assessment also includes information on a reliable procedure to verify the non-viability of the seeds. Ambrosia seeds are known contaminants in feed with maximum levels set in the Directive 2002/32/EC. The manufacturing processes and processing conditions applied to the feed may affect the viability of the Ambrosia seeds. Therefore, the CONTAM Panel compared these conditions with conditions that have been shown to be sufficient to render Ambrosia seeds non-viable. The Panel concluded with a certainty of 99-100% that solvent extraction and toasting of oilseed meals at temperatures of 120°C with steam injection for 10 min or more will make Ambrosia seeds non-viable. Since milling/grinding feed materials for compound feed of piglets, aquatic species and non-food producing animals would not allow particles of sizes ≥1 mm (the minimum size of viable Ambrosia seeds) passing the grinding process it was considered very likely (with ≥ 90% certainty) that these feeds will not contain viable Ambrosia seeds. In poultry, pig, and possibly cattle feed, particle sizes are ≥ 1 mm and therefore Ambrosia seeds could likely (66-90% certainty) survive the grinding process. Starch and gluten either from corn or wheat wet milling would not contain Ambrosia seeds with 99-100% certainty. Finally, ensiling fresh forages contaminated with A. artemisiifolia seeds for more than 3 months is very likely to render all seeds non-viable. The Panel concluded that a combination of the germination test and a subsequent triphenyl-tetrazolium-chloride (TTC) test will very likely (with ≥ 90% certainty) verify the non-viability of Ambrosia seeds. The Panel recommends that data on the presence of viable Ambrosia seeds before and after the different feed production processes should be generated.

Trade-offs between diaspore dispersal and dormancy within a spike of the invasive annual grass Aegilops tauschii

MAIN CONCLUSION

Differences in dispersal and dormancy of heteromorphic diaspores of Aegilos tauschii may increase its flexibility to invade/occupy weedy unpredictable habitats by spreading risk in space and time. In plant species that produce dimorphic seeds, there often is a negative relationship between dispersal and dormancy, with high dispersal-low dormancy in one morph and low dispersal-high dormancy in the other, which may function as a bet-hedging strategy that spreads the risk of survival and ensures reproductive success. However, the relationship between dispersal and dormancy and its ecological consequences in invasive annual grasses that produce heteromorphic diaspores is not well studied. We compared dispersal and dormancy responses of diaspores from the basal (proximal) to the distal position on compound spikes of Aegilops tauschii, an invasive grass with heteromorphic diaspores. Dispersal ability increased and degree of dormancy decreased as diaspore position on a spike increased from basal to distal. There was a significant positive correlation between length of awns and dispersal ability, and awn removal significantly promoted seed germination. Germination was positively correlated with GA concentration and negatively correlated with ABA concentration, and the ABA: GA ratio was high in seeds with low germination/high dormancy. Thus, there was a continuous inverse-linear relationship between diaspore dispersal ability and degree of dormancy. This negative relationship between diaspore dispersal and degree of dormancy at different positions on a spike of Aegilops tauschii may facilitate seedling survival in space and time.

Seed size, endosperm and germination variation in sexual and apomictic Boechera

Asexual reproduction results in offspring that are genetically identical to the mother. Among apomictic plants (reproducing asexually through seeds) many require paternal genetic contribution for proper endosperm development (pseudogamous endosperm). We examined phenotypic diversity in seed traits using a diverse panel of sexual and apomictic accessions from the genus Boechera. While genetic uniformity resulting from asexual reproduction is expected to reduce phenotypic diversity in seeds produced by apomictic individuals, pseudogamous endosperm, variable endosperm ploidy, and the deviations from 2:1 maternal:paternal genome ratio in endosperm can all contribute to increased phenotypic diversity among apomictic offspring. We characterized seed size variation in 64 diploid sexual and apomictic (diploid and triploid) Boechera lineages. In order to find out whether individual seed size was related to endosperm ploidy we performed individual seed measurements (projected area and mass) using the phenoSeeder robot system and flow cytometric seed screen. In order to test whether individual seed size had an effect on resulting fitness we performed a controlled growth experiment and recorded seedling life history traits (germination success, germination timing, and root growth rate). Seeds with triploid embryos were 33% larger than those with diploid embryos, but no average size difference was found between sexual and apomictic groups. We identified a maternal effect whereby chloroplast lineage 2 had 30% larger seeds than lineage 3, despite having broad and mostly overlapping geographic ranges. Apomictic seeds were not more uniform in size than sexual seeds, despite genetic uniformity of the maternal gametophyte in the former. Among specific embryo/endosperm ploidy combinations, seeds with tetraploid (automomous) endosperm were on average smaller, and the proportion of such seeds was highest in apomicts. Larger seeds germinated more quickly than small seeds, and lead to higher rates of root growth in young seedlings. Seed mass is under balancing selection in Boechera, and it is an important predictor of several traits, including germination probability and timing, root growth rates, and developmental abnormalities in apomictic accessions.

Seed viability of common ragweed (Ambrosia artemisiifolia L.) is affected by seed origin and age, but also by testing method and laboratory

Common ragweed (Ambrosia artemisiifolia L.) is an annual Asteraceae species native to North America which is highly invasive across Europe and has harmful impacts, especially on human health and agriculture. Besides its wide ecological range, particularly its high reproductive power by seeds is promoting its spread to various habitats and regions. To prevent further spread and to control the plant, the European Commission funded projects and COST-Actions involving scientists from all over Europe. A joint trial was set up comprising eight different laboratories from Europe to study seed viability variation in different seed samples. Three different testing methods (viability test with 2,3,5-triphenyltetrazolium chloride (TTC), a germination test combined with a subsequent TTC test and a crush test) were tested within the EU-COST-Action SMARTER network to four different seed origins. The viability test results from different laboratories were compared for variation amongst tests and laboratories. The main aim was to optimise the reliability of testing procedures, but results revealed not only significant effects of seed origin and seed age on seed viability, but also considerable differences between the output of the individual testing methods and furthermore between laboratories.

Due to these significant differences in the results of the testing labs, additionally a second test was set up. Twelve Austrian ragweed populations were used for TTC testing to obtain a precise adjustment of the testing method as well as a tight guideline for interpreting the results, particularly for the TTC state “intermediate” since a proper classification of TTC-intermediate coloured seeds is still a challenge when determining viability rates.

The significance of biomass allocation to population growth of the invasive species Ambrosia artemisiifolia and Ambrosia trifida with different densities

Background

Ambrosia artemisiifolia and Ambrosia trifida are globally distributed harmful and invasive weeds. High density clusters play an important role in their invasion. For these two species, the early settled populations are distributed at low densities, but they can rapidly achieve high population densities in a short period of time. However, their response to intraspecific competition to improve the fitness for rapid growth and maintenance of high population densities remains unclear. Therefore, to determine how these species form and maintain high population densities, individual biomass allocations patterns between different population densities (low and high), and plasticity during seedling, vegetative, breeding and mature stages were compared. In 2019, we harvested seeds at different population densities and compared them, and in 2020, we compared the number of regenerated plants across the two population densities.

Results

Most biomass was invested in the stems of both species. Ambrosia trifida had the highest stem biomass distribution, of up to 78%, and the phenotypic plasticity of the stem was the highest. Path analysis demonstrated that at low-density, total biomass was the biggest contributor to seed production, but stem and leaf biomass was the biggest contributors to high-density populations. The number of seeds produced per plant was high in low-density populations, while the seed number per unit area was huge in high-density populations. In the second year, the number of low-density populations increased significantly. A. artemisiifolia and A. trifida accounted for 75.6% and 68.4% of the mature populations, respectively.

Conclusions

High input to the stem is an important means to regulate the growth of the two species to cope with different densities. These two species can ensure reproductive success and produce appropriate seed numbers. Therefore, they can maintain a stable population over time and quickly form cluster advantages. In the management, early detection of both species and prevention of successful reproduction by chemical and mechanical means are necessary to stop cluster formation and spread.

Latitudinal and Longitudinal Trends of Seed Traits Indicate Adaptive Strategies of an Invasive Plant

Invasive plants may change their seed traits to adapt to the environment and facilitate their performance. Studies on variation in seed traits among populations of an invader along latitudes/longitudes may assist in revealing how invasive plants cope with variable climates. In this study, we collected seeds of 26 populations of the global invasive plant Ambrosia artemisiifolia along ranges spanning 23° latitudes and 20° longitudes that are highly correlated in its invasive range in China. We measured over 20 seed traits, including seed morphology, phytohormone, nutrients, and germination, and investigated how the climate along the latitudes affects those traits. We found that germination time was significantly delayed with increasing latitude and longitude, while the reversed patterns were true for the germination rate. From low to high latitude, seed size, abscisic acid, and fatty acid were increased, likely affecting seed germination. Our analysis further demonstrated that temperature is the dominant driver of the variability in seed traits and germination. Germination rates of larger seeds in cold ranges were lower, while smaller seeds from warm ranges germinated faster, likely indicating adaptive strategies of the invasive plant in seed trait functional ecology. Together, our findings provide new insights into understanding the seed adaptation strategies during the invasion process and the underlying physiological and biochemical mechanisms involved.

The influence of environmental factors on seed germination of Xanthium strumarium L.: Implications for management

Xanthium strumarium L. (Common cocklebur) is a noxious weed prevailing in different ecosystems around the world. It incurs significant yield and economic losses in different cropping systems globally. Successful management of any weed species depends on sound knowledge of seed germination biology. However, detailed knowledge on seed germination biology of the species is missing. Therefore, we investigated the impact of different environmental factors on seed germination and seed burial depths on seedling emergence of two X. strumarium populations. The impact of different sorghum mulch doses (0-10 t ha-1) on seedling emergence of the tested populations was also explored. Seed germination was evaluated under different photoperiods (0, 12 and 24), constant temperatures (0-50°C with 5°C stepwise rise), and different levels of pH (3-12), salinity (0-600 mM) and osmotic potential (0 to -1.6 MPa). Seedling emergence was observed for seeds buried at different depths (0-15 cm). Seeds of both populations proved non-photoblastic; however, higher germination was recorded under 12-hour photoperiod. The seeds germinated under a wide range of constant temperatures (10-45°C), pH (4-10), osmotic potentials (0 to -0.8 MPa) and salinity levels (0-400 mM NaCl). However, the highest germination was observed under 30-31°C temperature and neutral pH (7.51-7.52). Seeds were able to withstand 400 mM salinity and -1.00 MPa osmotic potential. Seedling emergence was initially improved with increasing burial depth and then a sharp decline was noted for the seeds buried >3 cm depth. Most of the seeds of both populations did not emerge from >8 cm depth. Different sorghum mulch doses linearly suppressed seedling emergence of tested populations, and 5.83-5.89 t ha-1 mulch application suppressed 50% of seedling emergence. Seedling emergence was completely retarded with 8 t ha-1 sorghum mulch. The tested populations germinated under diverse environmental circumstances indicating that the species can become troublesome in marginal habitats and cropped lands. Deep burial of seeds and application of sorghum mulches suppressed seedling emergence. Thus, deep burial followed by shallow tillage and application of sorghum mulches could be used as a successful strategy to manage the species in agricultural fields. Nonetheless, management strategies must be developed to control the species in other habitats.

Ambrosia pollen source inventory for Italy: a multi-purpose tool to assess the impact of the ragweed leaf beetle (Ophraella communa LeSage) on populations of its host plant

Here, we produce Ambrosia pollen source inventories for Italy that focuses on the periods before and after the accidental introduction of the Ophraella communa beetle. The inventory uses the top-down approach that combines the annual Ambrosia pollen index from a number of monitoring stations in the source region as well as Ambrosia ecology, local knowledge of Ambrosia infestation and detailed land cover information. The final inventory is gridded to a 5 × 5-km resolution using a stereographic projection. The sites with the highest European Infection levels were recorded in the north of Italy at Busto Arsizio (VA3) (European Infection level 2003-2014 = 52.1) and Magenta (MI7) (European Infection level 2003-2014 = 51.3), whereas the sites with the lowest (i.e. around 0.0) were generally located to the south of the country. Analysis showed that the European Infection level in all of Italy was significantly lower in 2013-2014 compared to 2003-2012, and this decrease was even more pronounced at the sites in the area where Ophraella communa was distributed. Cross-validations show that the sensitivity to the inclusion of stations is typically below 1% (for two thirds of the stations) and that the station Magenta (MI7) had the largest impact compared to all other stations. This is the first time that pollen source inventories from different temporal periods have been compared in this way and has implications for simulating interannual variations in pollen emission as well as evaluating the management of anemophilous plants like Ambrosia artemisiifolia.