It takes all sorts

The young researcher therefore wanted to find out whether the characteristics of the seeds have an influence on the subsequent development of the plants. Ragweed seeds are indeed highly variable, as regards their size, mass, colour, density, etc. It therefore seemed like a good idea to check whether these characteristics had an impact on the development of the seedlings and on the plant’s viability in the long term. This approach is original because rather than just limiting himself to measuring seed mass, as is the case in the majority of the research conducted in this domain, William Ortmans also focused on their surface. "I worked on 900 seeds from nine different ragweed populations. I selected 10 mother plants in each population and I collected 10 seeds per plant. After weighing them, I photographed them in standardised conditions. A software program allowed me to precisely measure the colour of every seed and estimate the surface of each one based on the measurement of the largest ellipse present on the seed’s surface. Besides the originality of the photographic technique, I wanted, above all, to go beyond simply measuring mass, and thereby avoid overlooking other possibly decisive characteristics. If I had limited myself to mass, I wouldn’t have been able to estimate the true quantity of food reserves that exist for the future seedling, for instance". 

The seeds were then sown in two seed chambers that were identical except for the ambient temperature. One – the warm chamber – was supposed to provide the ideal growing conditions (simulating southern Europe), while the other one – the cold chamber – provided suboptimum conditions such as those in Belgium. At the age of two weeks, the seedlings were photographed and their foliage cover was estimated. After two months, the 900 plants were cut in order to estimate their aerial biomass. "Biomass is an excellent indicator of the plant’s performance, in particular the amount of pollen and seeds produced. For instance, I wasn’t surprised to see that fewer of the seeds sown in the cold chamber germinated and that germination was slower. But what interested me above all was understanding whether the characteristics of the seed, i.e. the colour, size or mass, influenced the seedling’s growth, which is the stage when the plant is most vulnerable”.

Ready for invasion

And the result? "We may well have expected all the seeds to more or less identical, regarding the traits taken into account, within each population (geographical area) considered. But this wasn’t the case. Variability exists at all levels: geographical areas, local populations and the plants themselves. The seed traits didn’t all have a very significant impact, but the big and heavy seeds produced seedlings that grew more quickly and had a greater biomass. The extensive variability of the seeds therefore allows the species to grow in a wide range of environments, including colder ones". This fundamental observation is worrying. It means that ragweed is perfectly capable of growing in temperatures found in Belgium; albeit more slowly than elsewhere, but with an excellent germination rate and a complete cycle, i.e. the fructification and dispersal of pollen in the environment.

"For the time being, the ragweed hotspots in Europe are still far from Belgium, separated by a few hundred kilometres”, says Arnaud Monty, senior research fellow at the BIOSE Department’s Biodiversity and Landscape Unit, and promoter for William Ortmans’ thesis. “But if human activities were to bring considerable seed stocks into our regions, it is clear that we would have to react very quickly. An early detection system is essential. Even if the authorities only wait two or three years before reacting, it will already be very difficult to be effective. If they wait 10 years before they adopt limitation or eradication measures, it will really be too late". Among the range of monitoring measures suggested by the expert from Gembloux, is a focus on the "fauna strips" developed within the framework of greening the Common Agricultural Policy (CAP). Indeed, some sowing provided for within the framework of the agri-environment measures includes the use of sunflowers, potentially contaminated by ragweed seeds.