Phenotypic variation of wild radishes Raphanus pugioniformis and R. raphanistrum associated with local conditions in the southeast Mediterranean

The geographical distribution of Raphanus pugioniformis (Brassicaceae), endemic to the southeast Mediterranean, covers distinct and heterogeneous habitats over a slight rainfall and elevation gradients. In contrast, the closely related R. raphanistrum is widely distributed only in well-drained sandy soils along the Mediterranean coastal plain. Here we hypothesize that the allopatric distribution of the two species, assisted by short- and long-seed dispersal of R. pugioniformis and R. raphanistrum, respectively, is also associated with inter- and intra-species phenotypic variation. In a common garden experiment, we assessed various phenological, morphological and fitness-related traits in five populations of each species. The results showed significant differences between the two species, generally in phenological and fitness-related traits. The results point to the role of early flowering and large reproductive output in the wide distribution of R. raphanistrum. In contrast, phenotypic variation in most of the measured traits among populations of R. pugioniformis suggests that short dispersal distance contributed to increased phenotypic differentiation among populations. Moreover, significant correlations between several of the measured traits with rainfall and elevation (e.g., bolting date and duration of flowering) suggest adaptive phenotypic differentiation. In R. raphanistrum, differences in most phenotypic traits were not found between populations, supporting the hypothesis that long distance dispersal of single seeded units contributes to relatively low genetic variation. Finally, we suggest that it is also possible that R. raphanistrum exhibits strong plastic responses, explaining its worldwide distribution, although this remained to be investigated.