Taxonomic revision of the neophyte nightshades (Solanum, Solanaceae) in European Russia and the North Caucasus

The alien <i>Solanum</i> species in European Russia and the North Caucasus have never been revised. Over the past three decades, several exotic species have been found in various parts of the area, but many of them were identified incorrectly or reported using outdated taxonomy. We detailed the distribution data for these species, verified and corrected published reports, and provided updated diagnostic characters and a new identification key. Here, we report and discuss the records and residence status of nine species: <i>S. elaeagnifolium</i>, <i>S. emulans</i>, <i>S. heterodoxum</i>, <i>S. nitidibaccatum</i>, <i>S. rostratum</i>, <i>S. scabrum</i>, <i>S. sisymbriifolium</i>, <i>S. triflorum</i>, and <i>S. villosum</i>. One additional species, <i>S. nigrum</i>, appears to be an archeophyte widely distributed in both regions, except the Arctic zone. <i>Solanum villosum</i>, previously treated as several segregate taxa (<i>S. alatum</i>, <i>S. luteum</i>, <i>S. transcaucasicum</i>, <i>S. zelenetzkii</i>), was first collected in the early 20^th century in the North Caucasus, whereas its collections from European Russia are mostly from the late 20^th century. <i>Solanum rostratum</i> is naturalized in the plains of the North Caucasus and the Lower Volga Region. It has also been found several times in the forest and forest-steppe zones, with one new record reported here. <i>Solanum triflorum</i> is naturalized and rapidly spreading in the Lower Volga and the eastern North Caucasus; it is recorded here as a new casual alien in Kazakhstan. <i>Solanum nitidibaccatum</i> is reported for the first time from several administrative units and is considered naturalized at least in Central Russia. The putative presence of its close relative, <i>S. sarrachoides</i> from South America, which is a naturalized alien in some parts of Europe, is also discussed. At present, the remaining species are considered casual aliens. <i>Solanum scabrum</i> has escaped from cultivation, whereas the other species have arrived with contaminated grain imported from North America.