Effects of root fragmentation on generative reproduction of Sonchus arvensis

To develop better mechanical management strategies, more information on the impact of root partitioning on generative reproduction of Sonchus arvensis L. is needed. Therefore, an outdoor experiment was performed in Sweden in 2008, to evaluate the effect of root fragmentation on generative reproduction of S. arvensis. Two artificial populations of S. arvensis with the same total root length per area but with different initial root lengths and different numbers of root fragments were planted. Cumulative numbers of flower receptacles which had shed mature seeds over the season were assessed. Changes in the number of seeds per flower receptacle and average seed weight were monitored over time during the late season. Plants from long root fragments produced more flower receptacles than plants from short ones. Per area, however, the number of mature flower receptacles did not differ. The number of seeds per flower receptacle and individual seed weight were not affected by initial root length for the first cohort of shoots which sprouted from the initially planted roots. A second cohort, from roots produced during the season, resulted, irrespective of its initial root length, in fewer flower receptacles per plant and per area, with less seeds per receptacle, but with the same average seed weight as the first cohort. The number of seeds per flower receptacle was higher in mid-September than earlier or later. Average seed weight slightly decreased over time. The weight of seeds produced in early September was inversely related to the number of seeds per receptacle, but this trade-off disappeared over time. Root fragmentation alone in pure populations of S. arvensis does not impede generative reproduction, but is likely to decrease input of seeds to the seed bank, when combined with crop competition.