Potentilla × angarensis Popov (Rosaceae Juss.) – a new taxon for the Oka-Don Lowland

A new locality of the taxon from the genus Potentilla, endemic to the flora of Russia and Kazakhstan, was discovered in Eastern Europe. Previously, P. × angarensis was recorded for the European part of Russia based on collections from Moscow Region and the Republic of Udmurtia. The paper reports the first record of this hybrid for the Oka-Don Lowland, which was found in the northeast of the Voronezh Region. The main morphological characters of the hybridogenic taxon, which distinguish it from closely related species, are listed. The general distribution of P. × angarensis is specified. An image of the herbarium specimen of the new record is presented.


Introduction
The Oka-Don Lowland (also referred to as the Oka-Don Plain) is located in Eastern Europe in the territory of Russia and occupies a part of Ryazan, Tambov, Lipetsk, Voronezh, and Volgograd Regions, as well as the Republic of Mordovia. The plain is a vast depression located in the basins of the Oka and Don Rivers. It borders with Central Russia in the west, with the Volga River in the east, with the Smolensk-Moscow Uplands in the north, and in the south, it gradually narrows towards the mouth of the Khoper and Medveditsa Rivers. The northern part of the Oka-Don Lowland bounded by the Oka River was named the Meshchera Lowland. Its central and southern parts are named the Tambov Plain. The relief of the plain features vast plateaus with absolute heights of 160-180 m, which alternate with wide terraced valleys of the Don, Voronezh, Bityug, Khoper, Medveditsa, Tsna, Pra, and others rivers that stretch almost along the meridional direction. In addition to river valleys, the Oka-Don Lowland includes areas with varying degrees of relief dissection by ravines and gullies (Zanin 1967).
The paper reports the first record of the taxon Potentilla × angarensis Popov from the Oka-Don Lowland, which is endemic to the flora of Russia and Kazakhstan. According to the botanical geographical zoning proposed in Flora of the European Part of the USSR (Bobrov et al. 1974), the Oka-Don Lowland is located in the Volga-Don and Nizhne-Don floristic subregions of Eastern Europe. For these subregions, 27 and 21 species of the genus Potentilla are reported, respectively, with no P. × angarensis Popov recorded (Kamelin 2001). In individual regions located on the Oka-Don Lowland, the following number of species of the genus Potentilla is reported. The number of species in the flora of Volgograd Region (Reshetnikova 2018) and the Republic of Mordovia (Silaeva et al. 2010) amounts to 18, and 16 species each are found in Ryazan (Kazakova 2004), Lipetsk (Kazakova et al. 1996), and Tambov Regions (Sukhorukov et al. 2010). The number of species in the flora of Voronezh Region is 25 species (Kamyshev 1978;Tsvelev 1988;Agafonov 2006). No P. × angarensis Popov has been found in all the above regions. In the flora of Eastern Europe, this hybrid is recorded only for the Moscow Region (Vinogradova et al. 2017;Shcherbakov, Lyubeznova 2018), and the Republic of Udmurtia (Baranova, Puzyrev 2012).

Material and methods
Two interesting herbarium specimens were found during the revision of the materials of the genus Potentilla L. in the fund collections of the Herbarium named after Professor B. M. Kozo-Polyansky, Voronezh State University (VOR). Each specimen contained one plant in the bud-formation period collected in the same area. We identified these herbarium specimens using identifiers for the genus Potentilla, including the territories of Eastern Europe and Asia (Kamelin 2001;Soják 2004Soják , 2012a. The plant was identified as P. × angarensis, a hybrid species described from Eastern Siberia. The specimens used for comparison were the type specimens stored in Herbaria A (Index of Botanical Specimens 2020); E (Rob Cubey 2018); K (Royal Botanic Gardens, Kew 2020); NSK (Kovtonyuk et al. 2020); MW (Seregin 2020); PE (China, Beijing); US (The United States National Herbarium 2020), as well as Herbarium ALTB specimens (Russia, Barnaul). The specimens collected in Voronezh Region were morphologically identical to P. × angarensis and materials collected in Siberia and Kazakhstan. Herbarium acronyms are listed according to Index Herbariorum (Thiers 2020).

Results and discussion
For the first time, P. × angarensis is reported for the flora of the Oka-Don Lowland. The hybrid was collected in Voronezh Region. This region is located in the central zone of the European part of Russia. The administrative-territorial division includes 31 municipal districts and three urban districts: Voronezh, Borisoglebsk, and Novovoronezh. According to the herbarium labels, plants were found near the railway station in the Borisoglebsk town on the northeast of the region. Below we present the original label (it is similar for both samples) that indicates the corresponding barcodes and provides an image of one of the herbarium specimens of P. × angarensis.
According to the author, P. × angarensis is a hybrid taxon resulting from crosses between P. multifida L. and P. impolita Wahlenb. (= P. argentea L. s. l.) (Popov 1957). The hybrid was described based on collections from South Siberia. Below we cite the original protologue data and provide information on the type material of P.  J. Soják, a well-known researcher of the genus Potentilla, in one of his works suggested that P. × angarensis could have resulted from crosses between P. argentea and P. tergemina Soják (Soják 1987). In this work, J. Soják (1987) described P. omissa Soják, which, in his opinion, was derived from the hybridization between P. argentea and P. multifida. Authentic P. omissa materials are numerous and originate from various locations in North Kazakhstan, the southern part of East Siberia, and the Russian Far East (Soják 1987). It is extremely difficult to distinguish P. × angarensis from P. omissa. According to J. Soják, the hair length of P. omissa is 0.3-1.3 mm, and the hair length of P. × angarensis is 1.2-2.5 mm (Soják 1987(Soják , 2004(Soják , 2009(Soják , 2012a(Soják , 2012b. Therefore, the length of petiole hairs is the only character to distinguish these closely related taxa. The length of petiole hairs of P. × angarensis collected in Voronezh Region exceeds 1.5 mm, which corresponds to the main character distinguishing it from the closely related P. omissa.
Since the difference between P. × angarensis and P. omissa is insignificant and not always obvious, J. Soják attempted to prove their hybrid nature experimentally (Soják 1987(Soják , 2012a. The author conducted the following hybridization experiments. First, he artificially crossed the putative parental species: exact copies of the natural species P. × angarensis were obtained from hybridization between P. argentea and P. tergemina; hybrids P. argentea and P. multifida were found to be identical to specimens of P. omissa from natural populations studied in nature in South Siberia (Soják 1987(Soják , 2012b. P. × angarensis was recently reported as a new taxon for the flora of Kazakhstan, with subsequent records for East Kazakhstan Region (Kechaykin et al., 2020a(Kechaykin et al., , 2020b. Based on the literary data and our own data, we specify the species distribution.
General distribution. Eastern Europe: Moscow (Vinogradova et al. 2017;Shcherbakov, Lyubeznova 2018) and Voronezh Regions, the Republic of Udmurtia (Baranova, Puzyrev 2012); southern parts of West and East Siberia and the Russian Far East: from the Urals to Vladivostok (Popov 1957;Soják 1987Soják , 2004Soják , 2009Soják , 2012; Kazakhstan: East Kazakhstan and Almaty Regions (Kechaykin et al., 2020a(Kechaykin et al., , 2020b. The range of P. omissa is narrower and extends from North Kazakhstan to the southern regions of the Russian Far East (Soják 2004). According to some reports, P. omissa also grows in northwestern China (Soják 2012a). P. × angarensis and P. omissa can often be found in the areas of intersection of the populations of parental species. However, they began to spread along the roads regardless of their parents. The outbreak of these hybrid species probably co-occurred with the colonization of Siberia and the construction of roads and large oil and gas processing enterprises. Their typical habitat is grassland along the banks of water bodies and various semicultural communities along the roads. Both taxa tend to spread in areas of considerable human activity: in clear-cuts, near mines and pits, on dumps, and around construction sites.

Conclusion
In Eastern Europe P. × angarensis grows in three regions: Moscow and Voronezh Regions and the Republic of Udmurtia. It has been experimentally proven that the species resulted (and results) from hybridization between P. argentea and P. tergemina. The taxon prefers habitats subject to considerable human activity and actively spreads along roads and railways regardless of the populations of parental species. Therefore, it can be assumed with reasonable assurance that P. × angarensis can be found in many other regions from Moscow (possibly to the west) to Vladivostok, along the Trans-Siberian Railway.