The toxic weed Heliotropium europaeum L.: Germination ecology, soil seed bank, and emergence dynamics in diversified cropping systems

Heliotropium europaeum is a toxic annual weed that has been found with increasing frequency in the Mediterranean agro-environment. We investigated, both in laboratory and in field, the ecological aspects of this species in order to verify a possible connection between cropping systems and population increase. Its germination, characterized by a high thermal optimum (about 35 degrees C) is markedly reduced by burial. Strong depth-mediated germination inhibition was observed with over 2 cm of burial. Nongerminated seeds undergo a secondary dormancy highlighting their ability to form a persistent seed bank. Indeed the seed bank analysis carried out in an agroecosystem based on minimum tillage cropping systems showed a seed accumulation in the shallowest soil layers. The hypothesis that poor seed burial plays a crucial role in the population dynamics of this species was confirmed by the drastic reduction in seedling emergence after only 1 year of plowing. A strongly reduced emergence percentage was shown which causes not only a lower seedling density but also a lower inflorescence density and respective seed ripening. In conclusion, there is a clear relationship between an increase in this species and cultivation systems based on minimum tillage.