Pollen and achene characters of natural interspecific hybrid Solidago ×niederederi Khek were analyzed and compared with putative parental species S. virgaurea L. and S. canadensis L. to estimate the level of disturbances in generative reproduction resulting from its hybrid nature. Pollen viability (stainability) of Solidago ×niederederi from one newly discovered locality in NE Poland was evidently reduced to ~65% in both viability tests (acetocarmine and Alexander). The diameter of viable pollen (median 21.11 µm) fell between S. canadensis (median 19.52 µm) and S. virgaurea (median 23.48 µm). Both parental species produced normally developed achenes with high frequency (~90%) whereas in the hybrid, the seed set was dramatically low (6%). The results clearly indicated that sexual reproduction of hybridogenous taxon S. ×niederederi is disturbed, and its potential impact as an invasive species depends mainly on vegetative propagation.

Hybridization between alien and native species is biologically very important and could lead to genetic erosion of native taxa. Solidago 9 niederederi was discovered over a century ago in Austria and described by Khek as a natural hybrid between the alien (nowadays regarded also as invasive) S. canadensis and native S. virgaurea. Although interspecific hybridization in the genus Solidago is considered to be relatively common, hybrid nature of S. 9 niederederi has not been independently proven using molecular tools, to date. Because proper identification of the parentage for the hybrid Solidago individuals solely based on morphological features can be misleading, in this paper we report an additive polymorphism pattern expressed in the ITS sequences obtained from individuals representing S. 9 niederederi, and confirm the previous hypothesis that the parental species of this hybrid are S. canadensis and S. virgaurea. Additionally, based on variability at the cpDNA rpl32-trnL locus, we showed that in natural populations hybridization occurs in both directions.

Solidago ×niederederi is a natural hybrid between S. canadensis and S. virgaurea, which occurs sporadically in synanthropic habitats together with the parental species. The hybrid was reported from seven countries in Central and Northern Europe and its presence emphasizes the invasive character of S. canadensis. A new locality of the hybrid was found in Mieruniszki village in the Zachodniosuwalskie Lakeland in NE Poland in 2011 (ATPOL square: FB06). Solidago ×niederederi grew on fallow land, at the edge of planted birchen shrubbery and its population comprised two small clumps. Current data suggest that the hybrid is a casual alien in the Polish flora, however, it has the potential to become established by clonal growth or restricted sexual reproduction.

In this study, the authors aimed to revise the ability of Solidago 9 niederederi, a hybrid between S. canadensis and S. virgaurea, to produce sexual ramets and seedlings as a part of its naturalization strategy. Based on a two-season garden cultivation experiment, we showed that the hybrid produces more generative ramets than vegetative ones and the number of generative ramets increases from one season to another with an increasing number of stem buds located on the caudices. We also revealed a spontaneous seedling recruitment by the hybrid during cultivation in the garden. Based on the seed germination test under laboratory conditions, we evidenced that the hybrid can reach a higher final germination percentage than S. canadensis but a lower one than S. virgaurea. Based on field studies conducted in 35 populations in Poland, the hybrid formed the largest populations in tree plantations and on abandoned fields, reaching 16.5 and 15.7 ramet clusters on average, respectively. The most abundant populations were found on abandoned fields; however, the mean number of ramets per cluster did not differ remarkably among habitats (H = 6.5, p = 0.163). In all populations, the mean number of vegetative ramets per cluster reached 0.85, while the generative ones achieved 6.43 on average. The statistical analysis proved that the aforementioned differences are significant (t = -12.6, p = 0.0002). Our results suggest that S. 9 niederederi is able to generate its own offspring by sexual reproduction and that abandoned fields seem to be the most suitable habitats for its establishment.

Solidago ×niederederi Khek (1905: 22) (Asteraceae) was described by Khek (1905) as a natural hybrid between S. canadensis Linnaeus (1753: 878) and S. virgaurea Linnaeus (1753: 880). The hybrid was discovered by Niedereder in the area of Stoder (Austria) in the beginning of the twentieth century. It is morphologically more or less intermediate between the presumed parents, especially in characters such as capitula size, leaf shape and venation (Khek 1905, Nilsson 1976, Karpavičienė & Radušienė 2015). Moreover, it is characterized by reduced pollen viability, and produces only very few well-developed achenes (Nilsson 1976, Migdałek et al. 2014, Karpavičienė & Radušienė 2015). According to the protologue (Khek 1905) the pappus is absent in the hybrid (“Pappus fehlend”), however, I noticed the pappus in all examined generative specimens of the hybrid from Poland. The parental species of S. ×niederederi belong to different taxonomical sections: S. canadensis is a member of S. sect. Unilaterales Don in Loudon (1830: 348), and S. virgaurea is a member of S. sect. Solidago Linnaeus (1753: 878) (Nesom 1993). The hybrid has been reported from several countries in Europe: Austria, the United Kingdom, Denmark, Sweden, Norway, Germany, Poland, Lithuania, and Russia (Nilsson 1976, Burton 1980, Sunding 1989, Mayorov et al. 2012, Pliszko 2013 and literature cited therein, Karpavičienė & Radušienė 2015). It is usually found in anthropogenic habitats together with both parental species, and its presence emphasizes the invasive character of S. canadensis (Pliszko 2013).

It appears that S. ×niederederi is formed easily wherever the parental species grow in proximity, mainly in anthropogenic habitats such as abandoned fields, roadsides, railway embankments, disused quarries, and

The species Erigeron acris L. s. l. is revised in Poland. Three subspecies are recognized (E. acris subsp. acris, E. acris subsp. droebachiensis (O.F. Müll.) Arcang., and E. acris subsp. serotinus (Weihe) Greuter), a diagnostic key is provided, syntypes of two synonyms are discovered. Cauline leaves, capitula, and achenes are studied under scanning electron microscope. The updated distribution of the subspecies in Poland are presented using the ATPOL cartogram method. The distribution and hybridization are discussed.

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