Abstract
Due to the expansion of road networks and the use of de-icing salt, numerous salt-tolerant plant species spread along roadsides worldwide. One of the most striking examples is a coastal halophyte, Plantago coronopus, which has been dispersing along road networks in Europe. Our aim was to investigate the role of seed dimorphism, seed traits of the two seed types (morphology, germination rate, wind dispersal ability), and salt tolerance in the successful roadside spread of P. coronopus. We compared the seed traits, salt tolerance, and the distribution along roadsides in Hungary of P. coronopus to four species of the Plantago genus native to Central Europe. We found that the native P. lanceolata was the most widespread along roadsides in Hungary, while P. coronopus was the third. Interestingly, P. coronopus was found almost exclusively along motorways. P. coronopus had almost twice as many large seeds as small seeds. Small seeds of this species had the lowest terminal velocity compared to the other species, which may contribute to the successful dispersal by wind and air currents caused by cars. Seeds of all the species germinated at 0.4% salt concentration, but with a reduced rate (except P. major). Specimens from small seeds had higher shoot weight than those from large seeds and proved to have a higher reproductive potential even in salted soil. Our results show that seed dimorphism, through different dispersal capacity and germination success of the two seed types, as well as through different reproductive potential and salt tolerance of specimens developed from the two seed types, may contribute to the current rapid spread of P. coronopus along motorways in Hungary and Europe, and to its persistence under disturbed, spatially and temporally heterogeneous conditions.