Introduction

The Balkan Peninsula boasts a rich and diverse flora, recognized as one of the most biodiverse regions in Europe. This remarkable biodiversity is underscored by a substantial number of endemic plant species (Griffiths et al. 2004; Petrova and Vladimirov 2010; Tomović et al. 2014; Vuksanović et al. 2016). With approximately 8000 vascular plant species and subspecies (Stevanović 2005) it harbours a remarkable 2600 to 2700 endemic species and subspecies (Stevanović et al. 2007). Furthermore, the Balkan Peninsula has long been recognized as a glacial refugium for numerous vascular plant species (Thompson 2005, 2020; Hewitt 2011; Nieto Feliner 2014). Therefore, a part of the Balkans within the Mediterranean region has been recognized as a World Biodiversity Hotspot (Mittermeier et al. 1999, 2005; Myers et al. 2000).

However, despite extensive botanical exploration, the study of the plant diversity in this unique region remains far from complete. This is evidenced by the continuous discovery and description of new taxa, both previously unknown to science and novel to the Balkan Peninsula (Stevanović et al. 2005, 2009; Tashev 2013; Brullo et al. 2019; Cecchi et al. 2020; Raca et al. 2022; Aneva et al. 2022; Hashani et al. 2023). Additionally, the ever-accumulating data from molecular, cytological, and morphological investigations necessitate the revision of many taxa within this flora.

One such taxonomically challenging genus is Portulaca L. (Portulacaceae), primarily due to the recent descriptions of numerous taxa and their subsequent nomenclatural combinations within the P. oleracea L. aggregate over the past 45 years (Danin et al. 1978, 2008, 2012; Ricceri and Arrigoni 2000; Danin and Reyes-Betancort 2006; Banfi et al. 2011; Tison et al. 2021). Consequently, the information regarding the Portulaca genus in floristic publications of Balkan countries has become highly heterogeneous.

To illustrate it, according to the most recent floras, checklists, and identification guides, Portulaca is simply represented by two species (P. grandiflora Hook. and P. oleracea) in Serbia, including Kosovo (Josifović 1971). However, it is worth to note that though the new revision of the Serbian flora has already started (Niketić and Tomović 2018), the genus Portulaca has not been processed yet. The same species with two subspecies of P. oleracea (subsp. oleracea and subsp. sativa (Haw.) Čelak.) were noted in Albania, but the presence of these subspecies is unconfirmed (Vangjeli 2015; Barina et al. 2018). In North Macedonia, only P. oleracea is recorded (Micevski 1995), while in Slovenia, P. oleracea is present alongside its two subspecies (subsp. oleracea and subsp. sativa) (Martinčič et al. 2007). Bosnia and Herzegovina's Portulaca data is incomplete, with P. oleracea documented in its Republic of Srpska region (Stupar et al. 2021). P. nitida (Danin & H.G. Baker) Ricceri & Arrigoni and P. oleracea have been noted for the flora of Montenegro (Stešević and Caković 2013).

In Croatia, only P. oleracea was initially known (Domac 2002), but later assessments considered Portulaca as a genus comprising two species (P. grandiflora and P. oleracea) and four subspecies of P. oleracea: subsp. sativa (Haw.) Schübl. et G. Martens, subsp. nitida Danin et H.G. Baker, subsp. granulato-stellulata (Poelln.) Danin et H.G. Baker, subsp. oleracea (Nikolić 2019). Moreover, Rottensteiner et al. (Rottensteiner et al. 2020) reported instances of P. umbraticola Kunth escaping cultivation on the Island of Krk.

There is no consensus about the number of Portulaca species in the Bulgarian flora: according to some data, there is only P. oleracea (Assyov et al. 2012), according to others – P. grandiflora, P. oleracea, P. papillato-stellulata (Danin & H.G. Baker) Danin, P. trituberculata Danin, Domina & Raimondo and P. umbraticola (Stoyanov et al. 2021).

Special situations are for floras of Romania and the continental part of Greece. P. oleracea agg. including P. cypria Danin, P. granulato-stellulata (Poelln.) Ricceri & Arrigoni, P. nitida, P. oleracea s.str., P. papillato-stellulata, P. rausii Danin, P. trituberculata, and P. zaffranii Danin was noted for the continental part of Greece (Dimopoulos et al. 2016). Also, P. grandiflora has been mentioned for this area (Arianoutsou et al. 2010), however the last species is not included in the final floristic list, because of its unconfirmed escaping from cultivation (Dimopoulos et al. 2016).

Portulaca has been considered as a genus simply represented by two species (P. grandiflora and P. oleracea) in Romania for a long time (Sârbu et al. 2013). Though there were also two subspecies of P. olearacea (subsp. oleracea and subsp. sativa) reported for the Romanian flora early (Sîrbu and Oprea 2011). Almost at the same time, P. granulato-stellulata and P. trituberculata were discovered in the country (Danin 2011; Greuter and von Raab-Straube 2011), as well as P. pilosa L. was found two year later (Nagodă et al. 2013). But, the newest study of seeds of the P. oleracea agg. has recently allowed documenting new nine microspecies (P. cypria, P. daninii Galasso, Banfi & Soldano, P. macrantha (Maire) Ricceri & Arrigoni, P. nitida, P. oleracea s.str., P. papillato-stellulata, P. rausii, P. sardoa Danin, Bagella & Marrosu, and P. socotrana Domina & Raimondo) for Romania (von Raab-Straube and Raus 2024).

Also, there is tangled information about the Portulaca genus from Turkey where eight species noted for the whole country, but only six ones for its European part: P. nitida, P. oleracea, P. papillato-stellulata, P. rausii, P. trituberculata, and P. zaffranii (Güner 2012). But, P. granulato-stellulata and P. stellata (Danin & H.G. Baker) Ricceri &: Arrigoni are considered as synonyms of P. oleracea, and P. grandiflora, as a naturalized species, is specified only for Asian part of Turkey in this publication.

As evident, the species composition and distribution of the Portulaca genus within the Balkans are far from clear. Therefore, this study aims to address the following questions: 1) Which Portulaca taxa are present in the Balkan Peninsula? 2) What is currently known about their distribution in the region?

Materials and methods

Study period

The study was conducted from 2022 to 2024 and consisted of three distinct research stages: a comprehensive literature review of data pertaining to the distribution of taxa within the genus Portulaca across the Balkan Peninsula, the collection of seeds from these species within the region, followed by their subsequent identification, and finally, the analysis and synthesis of the obtained results.

Taxonomic approach

In our study, we adopted Danin's conception of microspecies within the P. oleracea aggregate (Danin et al. 1978, 2008, 2012, 2014, 2016; Danin and Reyes-Betancort 2006; Danin and Raus 2012). Though it is important to acknowledge that this viewpoint is subject to debate (Matthews et al. 1993; Ocampo and Columbus 2012; El-Bakatoushi et al. 2013; Walter et al. 2015). Nevertheless, this approach is actively used still (Amini Rad et al. 2017; Kurtto et al. 2019; Stoyanov et al. 2021), but often with the substitution of concepts when microspecies is named as morphotypes (Bulakh et al. 2019, 2022; von Raab-Straube and Raus 2023, 2024). More detailed exploration why we chose this conception for the present research is given in Discussion.

Identification of microspecies from Portulaca oleracea aggregate

The identification of these microspecies was conducted using relevant identification keys that consider various seed characteristics, including seed diameter, the shape of testa cells (digitated, elongated, isodiametric), surface of testa cells (smooth or with protuberances), type of protuberances (tubercles, papillae, granula), and the number and position of protuberances in the testa cells (Danin et al. 2008, 2012, 2016; Danin and Raus 2012).

Study area

Given the lack of a universally accepted definition of the geographical boundaries of the Balkan Peninsula, which is often influenced by political context (Schaefer 2008; Allcock et al. 2023), we provide precise details regarding the countries and regions considered in our research. Our study encompassed the territories of Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Kosovo, Montenegro, North Macedonia, Romania, Serbia, and Slovenia in their entirety. Additionally, we included the continental part of Greece (excluding its islands in the surrounding seas) and the European part of Turkey. This approach is rooted in a natural-geographic context and involves delineating the northern border of the Balkan Peninsula by the Kupa or Soča–Sava–Danube river line. However, for the sake of practicality, we retained the inclusion of the entire territories of Romania, Serbia, Croatia, and Slovenia, as excluding these regions would have presented considerable challenges in handling literature sources, particularly floras and checklists.

Literature sources

We refrain from citing the literature sources reviewed during our investigation in this section since comprehensive information about all presently known locations of Portulaca species within the Balkan Peninsula is available in other parts of this paper and in the Supplementary materials. In general, the literature contains accurate information about 145 locations of the Portulaca taxa within the Balkans.

Seed collection

A total of 26 samples of Portulaca seeds were collected in field conditions. Detailed information regarding the locations, collection dates, and collectors is provided below:

  • Croatia: 1) Krk Island, a garden, 45.164890ºN, 14.545202ºE, 24.09.2022, S. Dragićević, A. Katnić.

  • Montenegro: 2) Bar, old town Bar, 42.093298ºN, 19.136221ºE, 29.10.2022, S. Dragićević, B. Knežević, N. Dragićević, I. Dragićević; 3) Bar, old town Bar, a parking lot, 42.092685ºN, 19.133120ºE, 29.10.2022, S. Dragićević, B. Knežević, N. Dragićević, I. Dragićević; 4) Bar, by the park Šumice, 42.107578°N, 19.089267°E, 29.10.2022, S. Dragićević, B. Knežević, N. Dragićević, I. Dragićević; 5) Bar, port Bar, a passenger terminal, 42.097019ºN, 19.090580ºE, 29.10.2022, S. Dragićević, B. Knežević, N. Dragićević, I. Dragićević; 6) Bar, Šušanj, Žukotrlica, 42.109307ºN, 19.090127ºE, 29.10.2022, S. Dragićević, B. Knežević, N. Dragićević, I. Dragićević; 7) Bar, Tomba settlement, 42.081581°N, 19.129131°E, 29.10.2022, S. Dragićević, B. Knežević, N. Dragićević, I. Dragićević; 8) between Bar and Ulcinj, next to the highway, 41.964092°N, 19.191164°E, 29.10.2022, S. Dragićević; 9) Danilograd, Spuž, garden of house by Dragane Saveljić, 42.535028ºN, 19.200408ºE, 5.11.2022, S. Dragićević, D. Saveljić, M. Jovićević; 10) Danilograd, Spuž, garden of house Dragane Saveljić, 42.535028ºN, 19.200408ºE, 5.11.2022, S. Dragićević, D. Saveljić, M. Jovićević; 11) between Golubovci and Vranjina, near the train station Zeta, 42.296310ºN, 19.159904ºE, 29.10.2022, S. Dragićević; 12) Herceg Novi, along the road towards Kameno, 42.468611ºN, 18.542680ºE, 26.10.2022, S. Dragićević, I. Ćetković, S. Malidžan; 13) Herceg Novi, Savina, in front of the building at the roundabout, 42.454883ºN, 18.558764ºE, 26.10.2022, S. Dragićević, I. Ćetković, S. Malidžan; 14) Kotor, Perast, garden of the restaurant “Đardin”, 42.486554ºN, 18.6978080ºE, 22.10.2022, S. Dragićević; 15) Podgorica, park forest Kruševac, near a pedestrian trail, 42.438506°N, 19.250975°E, 8.11.2022, S. Dragićević; 16) Podgorica, Zlatica, a garden around house by Vuko Milikić, 42.467281°N, 19.290186°E, 3.11.2022, S. Dragićević; 17) Podgorica, Vektra square, 42.442469°N, 19.246753°E, 25.08.2022, S. Dragićević; 18) Podgorica, near to the monument of St. Petar Cetinjski, 42.442283°N, 19.244228°E, 25.08.2022, S. Dragićević; 19) Podgorica, Stari Aerodrom, 42.425169°N, 19.270244°E, 20.9.2022, S. Dragićević, I. Ljuljđurović; 20) Podgorica, Stari Aerodrom, 42.425169°N, 19.270244°E, 20.09.2022, S. Dragićević, I. Ljuljđurović; 21) Podgorica, near Sahat kula, 42.435772°N, 19.260106°E, 24.08.2022, S. Dragićević; 22) Podgorica, near Sahat kula, 42.435772°N, 19.260106°E, 24.08.2022, S. Dragićević; 23) Sutomore, next to the highway, 42.143913ºN, 19.044148ºE, 29.10.2022, S. Dragićević; 24) Virpazar, center, near a fountain, park, 42.246305ºN, 19.090339ºE, 29.10.2022, S. Dragićević; 25) Virpazar, near hotel Vir, 42.246595ºN, 19.092522ºE, 29.10.2022, S. Dragićević.

  • Slovenia: 26) Ljubljana, Ig, parking of the Research Centre of the Slovenian Academy of Sciences and Arts, 45.963140ºN, 14.517828ºE, 12.10.2022, S. Dragićević.

Scanning electron microscopy

Scanning electron microscopy was performed at the Centre for Material Analysis Research Infrastructure of the University of Oulu. For this purpose, dry seeds were directly mounted on stubs using double-sided adhesive tape and coated with a 20 nm platinum layer in a sputter coater to provide conductivity and enhance signal quality. A field emission scanning electron microscope (FESEM), Zeiss ULTRA plus FESEM (Zeiss, Oberkochen, Germany), was employed to capture micrographs and measure the size of seeds.

Mapping

The maps of currently known locations of the taxa of the genus Portulaca was produced by available tools at the website "Simplemappr” (https://www.simplemappr.net). Used data for mapping were collected from literature sources and during our field studies. Also, citizen science data about the distribution of P. grandiflora were taken from iNaturalist (https://www.inaturalist.org). All currently known and confirmed location of Portulaca taxa within the Balkans are listed in Supplementary materials.

Identification key

The identification key for the genus Portulaca within the Balkan flora was compiled by synthesizing previously published data (Josifović 1971; Danin et al. 1978, 2008, 2012; Danin and Raus 2012; Nagodă et al. 2013; Nikolić 2019; Stoyanov et al. 2021).

Results

Within the flora of the Balkan Peninsula, the genus Portulaca is comprised of four distinct and easily distinguishable species: P. oleracea agg., P. grandiflora, P. pilosa and P. umbraticola. Notably, the latter three are cultivated plants as ornamental ones in the region, and there are documented cases of their escape from cultivation. In addition to these four species, there are twelve microspecies found within the complex of P. oleracea. These microspecies, namely P. cypria, P. daninii, P. granulato-stellulata, P. macrantha, P. nitida, P. oleracea s. str., P. papillato-stellulata, P. rausii, P. sardoa, P. socotra, P. trituberculata, and P. zaffranii, can currently only be distinguished based on their seed micromorphology. Therefore, if we accept Danin's conception of microspecies, there are a total of fifteen presently known and recognized species of Portulaca within the Balkan Peninsula.

It is noteworthy that five species, namely P. daninii, P. granulato-stellulata, P. rausii, P. sardoa, and P. trituberculata, have been newly identified in Montenegro, and one species, P. nitida, has been reported for the first time in the Slovenian flora.

To aid in the identification of these Portulaca species within the Balkans, we have given an identification key, brief taxonomic citations, and up-to-date information regarding their distribution in the region.

The identification key for Portulaca species within the Balkan Peninsula

  • 1. Leaves are cylindrical …………………………………………………………………………………...…...…. 2

  • * Leaves are flat ……………………………………………...…………………………………………………… 3

  • 2. Flowers are large, 3-4 cm in diameter …………………………………………………………. 1. P. grandiflora

  • * Flowers are small, 0.5-1.2 cm in diameter ……………………………………………………………. 2. P. pilosa

  • 3. Leaves' tips are acute. Flowers are large, 3-4 cm in diameter. The corolla is red, pink, orange, yellow or white ……………………………………………………………………………………...……………..3. P. umbraticola

  • * Leaves' tips are obtuse. Flowers are small, about 0.6 cm in diameter. The corolla is yellow ……4 (P. oleracea agg.)

  • 4. Testa cells flat devoid of emergences, star-shaped …………………………………………..……...…………. 5

  • * Lateral testa cells with at least one kind of tubercles and/or papillae …………………………………………... 6

  • 5. Major seed diameter > 0.85 mm .......................................................................................................4. P. oleracea

  • * Major seed diameter < 0.85 mm ................................................................................................ 5. P. nitida (Fig. 1)

    Fig. 1
    figure 1

    Seeds of Portulaca nitida collected in Croatia (sample #1): A) general view of the seed; B) a fragment of the seed surface

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  • 6. Testa cells only papillate ………………………………………………………………………………..…….... 7

  • * Testa cells with, at least, one large tubercle at the center ………………………………………………..…….. 13

  • 7. Seed surface covered with small papillae of almost equal size ……………………………………………...…. 8

  • * Testa cells covered unevenly by papillae, mostly or only on the rays …………………………………..……… 9

  • 8. Major seed diameter > 0.85 mm ...............................................................................................6. P. rausii (Fig. 2)

    Fig. 2
    figure 2

    Seeds of Portulaca rausii collected in Montenegro (sample #7): A) general view of the seed; B) a fragment of the seed surface

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  • * Major seed diameter < 0.85 mm ........................................................................................................ 7. P. zaffranii

  • 9. Major seed diameter > 0.85 mm ......................................................................................................................... 10

  • * Major seed diameter < 0.85 mm .......................................................................................................................... 12

  • 10. Cell interior with 2-7 papillae, rays short, many of them with terminal papillae …………….... 8. P. macrantha

  • * Cell interior smooth (without papillae), rays long, with or without terminal papillae ……………………........ 11

  • 11. Rays long, many with terminal papillae, but not all papillate ........................................ 9. P. papillato-stellulata

  • * Most rays papillate, forming circles or ellipses of papillae among neighbouring testa cells .. 10. P. sardoa (Fig. 3)

    Fig. 3
    figure 3

    Seeds of Portulaca sardoa collected in Montenegro (sample #6): A) general view of the seed; B) a fragment of the seed surface

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  • 12. Testa cells star-shaped, the papillae emerging from the tips of the rays; rays 1.5-2 as long as wide ……………………………………………………………………………........ 11. P. granulato-stellulata (Fig. 4)

    Fig. 4
    figure 4

    Seeds of Portulaca granulato-stellulata collected in Montenegro (sample #9): A) general view of the seed; B) a fragment of the seed surface

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  • * Testa cells star-shaped, the papillae emerging from the base of each ray; rays as long as wide; cells with papillae only at the periphery of the lateral face ………………………………………………………...…. 12. P. socotrana

  • 13. Testa cells tuberculate with single or paired tubercles at the center or 2-3-4 tubercles in the line ………….. 14

  • * Testa cells with tubercles in their central part and papillae on some of the rays …………… 13. P. daninii (Fig. 5)

    Fig. 5
    figure 5

    Seeds of Portulaca daninii collected in Montenegro (sample #12): A) general view of the seed; B) a fragment of the seed surface

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  • 14. Testa cells tuberculate with single or paired tubercles at the center ……………………………….. 14. P. cypria

  • * Testa cells tuberculate with 2-3-4 tubercles in the line ……………………………... 15. P. trituberculata (Fig. 6)

    Fig. 6
    figure 6

    Seeds of Portulaca trituberculata collected in Montenegro (sample #19): A) general view of the seed; B) a fragment of the seed surface

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A synopsis of the Portulaca taxa within the Balkans

Portulaca L. in Sp. Pl.: 445 (1753)

  1. 1.

    Portulaca grandiflora Hook. in Bot. Mag. 56: t. 2885 (1829)

    Distribution within the Balkans: presently known from Bulgaria, Croatia, Romania, Turkey (Fig. 7), also it is questionable for Greece, however most likely P. grandiflora might be found in all countries of the peninsula.

    Fig. 7
    figure 7

    Currently known locations of Portulaca grandiflora within the Balkans (“□”)

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    Note: an alien species, known as naturalized one or from the cases of escaping from its cultivation.

  2. 2.

    Portulaca pilosa L. in Sp. Pl.: 445 (1753)

    Distribution within the Balkans: presently known only from the single area in Bucharest (Romania); most likely P. pilosa is more widely spread in the region.

    Note: an alien species, known only from the case of escaping from its cultivation.

  3. 3.

    Portulaca umbraticola Kunth in F.W.H. von Humboldt, A.J.A. Bonpland & C.S. Kunth, Nov. Gen. Sp. 6: 72 (1823)

    Distribution within the Balkans: presently known from the Krk island (Croatia) and unspecified location/s in Bulgaria; most likely P. umbraticola is more widely spread in the region.

    Note: an alien species, known only from the cases of escaping from its cultivation.

  4. 4.

    Portulaca oleracea L. in Sp. Pl.: 445 (1753) ≡ P. stellata (Danin & H.G. Baker) Ricceri & Arrigoni in Parlatorea 4: 93 (2000) ≡ P. oleracea subsp. stellata Danin & H.G. Baker in Israel J. Bot. 27: 198 (1978).

    Distribution within the Balkans: presently known from Greece, Romania, Serbia and Turkey (Fig. 8); most likely might be found within all countries of the peninsula.

    Fig. 8
    figure 8

    Currently known locations of Portulaca oleracea within the Balkans (“★”)

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    Note: There are no morphological descriptions of seeds of the species from P. oleracea aggregate here and next, because detailed ones were given in several previous publications (Danin et al. 1978, 2008, 2012; Domina et al. 2010; Danin and Raus 2012; Bulakh et al. 2019, 2022). So, we do not see any sense to repeat..

  5. 5.

    Portulaca nitida (Danin & H.G.Baker) Ricceri & Arrigoni in Parlatorea 4: 93 (2000) ≡ P. oleracea subsp. nitida Danin & H.G. Baker in Israel J. Bot. 27: 194 (1978)

    Distribution within the Balkans: presently known from Croatia, Greece, Montenegro, Romania, Slovenia and Turkey (Fig. 9); most likely might be found within other Balkan countries too.

    Fig. 9
    figure 9

    Currently known locations of Portulaca nitida within the Balkans (“▲”)

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  6. 6.

    Portulaca rausii Danin in Fl. Medit. 18: 92 (2008) ≡ P. oleracea subsp. rausii (Danin) J. Walter in Fl. Slovenska 6(3): 657 (2012)

    Distribution within the Balkans: presently known from Greece, Montenegro, Romania and Turkey (Fig. 10); most likely might be found within all countries of the region.

    Fig. 10
    figure 10

    Currently known locations of Portulaca rausii within the Balkans (“▼”)

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  7. 7.

    Portulaca zaffranii Danin in Quad. Bot. Amb. Appl. 21: 189 (2010) ≡ P. oleracea subsp. zaffranii (Danin) J.Walter in Fl. Slovenska 6(3): 657 (2012)

    Distribution within the Balkans: presently known from Greece, Serbia and Turkey (Fig. 11); most likely might be found within all countries of the region.

    Fig. 11
    figure 11

    Currently known locations of Portulaca zaffranii within the Balkans (“✳”)

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  8. 8.

    Portulaca macrantha (Maire) Ricceri & Arrigoni in Parlatorea 4:93 (2000) ≡ P. oleracea var. macrantha Maire (1929) Bull. Soc. Hist. Nat. Afrique Nord, 20:18 ≡ P. oleracea ssp. macrantha (Maire) Maire in Jahandiez et Maire (1932) Cat. Pl. Maroc:195

    Distribution within the Balkans: known only from Romania for now (Fig. 12), but most likely might be found within all Balkan countries

    Fig. 12
    figure 12

    Currently known locations of Portulaca macrantha within the Balkans (“☆”)

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  9. 9.

    Portulaca papillato-stellulata (Danin & H.G.Baker) Danin in Lagascalia 26: 76 (2006) ≡ P. oleracea subsp. papillato-stellulata Danin & H.G.Baker in Israel J. Bot. 27: 200 (1978)

    Distribution within the Balkans: presently known from Croatia, Greece, Romania and Turkey (Fig. 13); most likely might be found within other Balkan countries too.

    Fig. 13
    figure 13

    Currently known locations of Portulaca papillato-stellulata within the Balkans (“⬢”)

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  10. 10.

    Portulaca sardoa Danin, Bagella & Marrosu in Pl. Biosystems 146 (Suppl.): 139 (2012)

    Distribution within the Balkans: known only from Montenegro and Romania for now (Fig. 14), but most likely might be found within all Balkan countries

    Fig. 14
    figure 14

    Currently known locations of Portulaca sardoa within the Balkans (“○”)

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  11. 11.

    Portulaca granulato-stellulata (Poelln.) Ricceri & Arrigoni in Parlatorea 4:93 (2000) ≡ P. oleracea var. granulato-stellulata Poelln. in Occas. Pap. Bernice Pauahi Bishop Mus. 9:5 (1936) ≡ P. oleracea subsp. granulato-stellulata (Poelln.) Danin & H.G.Baker in Israel J. Bot. 27: 189 (1978)

    Distribution within the Balkans: presently known from Croatia, Greece, Montenegro, Romania, Serbia, Slovenia and Turkey (Fig. 15); most likely might be found within all Balkan countries.

    Fig. 15
    figure 15

    Currently known locations of Portulaca granulato-stellulata within the Balkans (“■”)

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  12. 12.

    Portulaca socotrana Domina & Raimondo in Webbia 64:10 (2009)

    Distribution within the Balkans: presently known only from the single location in Bucharest (Romania).

  13. 13.

    Portulaca daninii Galasso, Banfi & Soldano in Atti Soc. Ital. Sci. Nat. Mus. Civico Storia Nat. Milano 152: 96 (2011) ≡ P. oleracea subsp. tuberculata Danin & H.G. Baker in Israel J. Bot. 27: 194 (1978) ≡ P. tuberculata (Danin & H.G.Baker) Danin in Lagascalia 26: 76 (2006), nom. illeg. ≡ P. avinoamii García-Beltrán in Brittonia 73: 300 (2021), nom. superfl.

    Distribution within the Balkans: known only from Montenegro and Romania for now (Fig. 16), but most likely might be found within all Balkan countries.

    Fig. 16
    figure 16

    Currently known locations of Portulaca daninii within the Balkans (“⬤”)

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  14. 14.

    Portulaca cypria Danin in Fl. Medit. 18:92

    Distribution within the Balkans: presently known from the single location in Buzău (Romania)

  15. 15.

    Portulaca trituberculata Danin, Domina & Raimondo in Fl. Medit. 18: 91 (2008) ≡ P. oleracea subsp. trituberculata (Danin, Domina & Raimondo) J. Walter in Fl. Slovenska 6(3): 657 (2012)

    Distribution within the Balkans: presently known from Bulgaria, Greece, Montenegro, Romania, Serbia and Turkey (Fig. 17); most likely might be found within the whole region.

    Fig. 17
    figure 17

    Currently known locations of Portulaca trituberculata within the Balkans (“⬡”)

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Discussion

Distribution of P. grandiflora, P. pilosa and P. umbraticola

The situation regarding the spread of P. grandiflora, P. pilosa and P. umbraticola within the Balkan Peninsula, while conceptually straightforward, is currently marred by fragmented and insufficient data. It appears that researchers have not given due attention to the phenomenon of these species escaping cultivation and undergoing subsequent naturalization processes. We suppose that wherever these species are cultivated, instances of their escape are either already documented or can potentially be reported. This is due to the fact that these three species exhibit well-defined naturalization strategies that have been observed in numerous European countries (Tutin et al. 1993; Sîrbu and Oprea 2011; Feráková et al. 2012; Nagodă et al. 2013; Galasso et al. 2018). A good example is Romania, where many naturalized populations of P. grandiflora are documented (Sîrbu and Oprea 2011). It is evident that similar investigations will need to be conducted in other Balkan countries to comprehensively understand the contemporary distribution of these species within the region, as well as threats that may result from the spread of these alien species. Additionally, a potential challenge lies in distinguishing between P. grandiflora, P. pilosa and P. umbraticola, as their initially similar appearance can lead to misidentification. However, the proposed identification key is poised to address this issue effectively.

Distribution of the P. oleracea complex: the current knowledge and a prediction

In light of our research findings and generalizations, it is evident that we can anticipate the discovery of several new microspecies within the P. oleracea aggregate in the Balkans and throughout the various countries of the region in the near future. This expectation arises from the fact that the P. oleracea complex exhibits a notably rich diversity in the immediate vicinity, as well as throughout the continental expanse of Europe as a whole. We have compiled recent data on the distribution of the P. oleracea aggregate from countries and regions from Southern to Northern Europe located relevant close to the Balkan Peninsula (Danin et al. 2008, 2014, 2016; Greuter and von Raab-Straube 2011; Feráková et al. 2012; von Raab-Straube and Raus 2015, 2023, 2024; Bulakh et al. 2019, 2022; Kurtto et al. 2019), as shown in Table 1.

Table 1 Tracking species' ranges from the P. oleracea aggregate from Southern to Northern Europe: " + "—confirmed microspecies; "?"—microspecies likely likely present, but not yet identified
Full size table

The presented data not only underscores the current fragmented state of knowledge concerning the P. oleracea complex across different countries and regions but also serves as a valuable tool for predicting the presence of as-yet-unidentified species in these areas based on available information regarding their distribution. Thus, there is no compelling reason to assume the absence of almost all identified species within the Balkans in any of the Balkan countries. We can reasonably predict the detection of eleven microspecies within P. oleracea aggregate (P. cypria, P. daninii, P. granulato-stellulata, P. macrantha, P. nitida, P. oleracea s. str., P. papillato-stellulata, P. rausii, P. sardoa, P. trituberculata, and P. zaffranii), for instance, in Albania or Bosnia and Herzegovina in the near future.

Even taken into account the representation of all microspecies from the P. oleracea complex mostly within ruderal and segetal communities, the only exception may be P. socotrana, because the case of this species presents a more complex scenario. Initially, P. socotrana was considered an endemic species confined to the island of Soqotra (Domina and Raimondo 2009). However, subsequent observations have found it in Tehran province (Iran) too (Amini Rad et al. 2017). Later, this taxon was noted from the southern coast of the Crimea Peninsula (Ukraine) where may have been inadvertently introduced through imported building materials (von Raab-Straube and Raus 2023). But it also has been recently discovered in Bucharest (Romania), though the authors require additional research for the final confirmation (von Raab-Straube and Raus 2024). Therefore, now we can only suggest new findings of this microspecies in the Black Sea basin, for instance, in Bulgaria.

Besides, P. sardoa needs special attention, because this microspecies has been known only from regions with a Mediterranean climate or its characteristics: Sardinia, Corsica (Danin et al. 2012, 2016), the southern coast of Crimea (von Raab-Straube and Raus 2023) and Montenegro (our data, Fig. 14). Though six locations of P. sardoa have been recently discovered in Romania in regions with temperate climate (von Raab-Straube and Raus 2024), there is a chance that this microspecies is absent in the territory of Serbia, including Kosovo, and North Macedonia.

Simultaneously, we should anticipate findings of P. sativa in each Balkan country, even though it has not yet been identified here. As well, this concerns P. cypria and P. macrantha which are known only from Romania within the Balkans at present. This anticipation is due to the fact that P. cypria and P. sativa are already known from Italy and some Mediterranean islands (Danin et al. 2008, 2016), as well as some of eastern and northern countries: P. cypria – from Ukraine (Bulakh et al. 2019; von Raab-Straube and Raus 2023) and Belarus' (von Raab-Straube and Raus 2015); P. sativa – from Slovakia (Feráková et al. 2012), Belarus (von Raab-Straube and Raus 2015) and even Finland (Kurtto et al. 2019). The situation is distinct for P. macrantha, which had long been known only from North Africa (Domina et al. 2010). Though, this species was noted for Italy too (Ricceri and Arrigoni 2000), later this point of view was criticized (Domina et al. 2010), and then again confirmed (Banfi et al. 2011). However, recent findings in Ukraine (von Raab-Straube and Raus 2023), Romania (von Raab-Straube and Raus 2024) and Poland (Bulakh et al. 2022) suggest a significantly wider distribution of P. macrantha, including the Balkan Peninsula and the entire Mediterranean region.

The data we have acquired also lends itself to making predictions about the potential presence of certain species from the P. oleracea complex beyond the scope of our study region. For instance, there is a likelihood of encountering P. zaffranii in Ukraine, particularly along the southern coast of Crimea. It is also plausible that the species' range occupies Austria, Hungary, and Poland, given its documented occurrence in Slovakia (Feráková et al. 2012), even though it primarily inhabits Mediterranean regions (Danin et al. 2008, 2016; Dimopoulos et al. 2016).

In a broader context, we anticipate an increase in the number of these species recorded in many Central and Eastern European countries. For instance, the confirmation of P. papillato-stellulata in Belarus is expected due to its presence in Finland, Poland, Ukraine, and Slovakia. Similarly, Poland is likely to report findings of P. cypria, P. rausii, and P. sativa, while Ukraine may discover P. sativa. Slovakia may record the presence of P. cypria, P. daninii, and P. macrantha. Hungary also may document a wider array of species, including P. cypria, P. daninii, P. macrantha, P. nitida, P. rausii, P. sativa, and P. zaffranii. Lastly, Austria may see the confirmation of P. cypria, P. daninii, P. granulato-stellulata, P. macrantha, P. oleracea, P. rausii, P. sativa, P. trituberculata, and P. zaffranii within its borders.

Special attention is warranted for P. sicula Danin, Domina & Raimondo which is listed in Table 1 based on our literature review. P. sicula is currently documented in only a handful of locations in Sicily (Danin et al. 2008). However, considering the common distribution patterns observed within the P. oleracea complex, it is unlikely to be an endemic species. Therefore, there is a reasonable expectation that P. sicula may also be found on other Mediterranean islands, including those in Greece.

Taxonomical difficulties in the P. oleracea complex

From our analysis, it is evident that many microspecies within the P. oleracea aggregate share overlapping ranges, adding another layer to the ongoing discussion surrounding their taxonomic status. Remarkably, we find ourselves in a unique situation where the accepted taxa exhibit minimal documented differences in morphology (except for variations in seeds), ecological preferences, and distribution ranges. It was previously believed that they have different chromosome numbers, but recent studies cast doubt on this fact (Walter et al. 2015). Even the present molecular data largely fail to substantiate their taxonomic classification (Ocampo and Columbus 2012; El-Bakatoushi et al. 2013). Notably, within this context, the rich diversity in seed shapes and sizes could serve as a compelling basis for identifying new species in other taxonomic groups, however this approach in the P. oleracea complex does not work clearly.

According to Legrand (Legrand 1962), seed surface characters in P. oleracea complex are not dependable because the variation is unpredictable; each population often presents a different pattern, therefore seed surface cannot be used to separate even varieties because infinite transitions are produced between nontuberculate and tuberculate and between rugose and stellate surface patterns. This transitions were partly confirmed by Danin et al. (Danin et al. 1978, 2014, 2016), as well as in our research. Namely, Danin et al. (Danin et al. 1978, 2014) noted that mixed populations of P. nitida and P. granulato-stellulata occur in several locations of California, Israel and in Italy. Later, studying the P. oleracea aggregate in the Italian peninsula and adjacent islands, multiple occurrences, the presence of different microspecies in some of the sites, were documented too. For example, five microspecies growing together were noted in two small public flowerbeds in Sorrento (Naples). However, the main pattern consists in that present known mixed populations are presented by only tetraploid, hexaploid or tetraploid-hexaploid microspecies. We also confirmed this fact. Namely, two mixed populations (locations #9, 10 and 21, 22) of P. nitida and P. granulato-stellulata were found in Danilovgrad and Podgorica (Montenegro). Furthermore, we documented P. granulato-stellulata and P. trituberculata growing together in Podgorica (locations #19, 20), as an example of the mentioned tetraploid-hexaploid mixed population.

Matthews et al. (Matthews et al. 1993) claim that "the provision of a name attracts attention to a taxon", that is why so much attention is paid to the taxonomic puzzle in the P. oleracea aggregate, but they do not think that these microspecies should be treated as even subspecies, because a subspecies should have several conspicuous morphological differences, should be cohesive geographically and largely allopatric, with multigenic control of the differences; additionally, there is the possibility of hybridization along contact zones but with markedly reduced fertility of hybrids. We concur with this perspective and suggest that, unless additional evidence emerges to support the independent species or subspecies status of taxa within the P. oleracea complex, most should be considered at the variety level, that already is offered for the "Flora Gallica" by Tison et al. (Tison et al. 2021). However, one important caveat preventing us from insisting on such taxonomic changes at this stage is the recommendation by Ocampo and Columbus (Ocampo and Columbus 2012) to refrain from describing new taxa or making nomenclatural transfers until we gain further insights into the evolution and morphology of these taxa. This approach is logical and prudent. Therefore, we guess that all diversity of known microspecies from the P. oleracea aggregate from the Balkans will be easy in any moment to reduce to three variations (P. oleracea var. oleracea, P. oleracea var. granulatostellulata Poelln., P. oleracea var. trituberculata (Danin, Domina & Raimondo) J.-M. Tison), as it did by Tison et al. (Tison et al. 2021), if such irrefutable facts appear denying their species status. Moreover, some latest publications show that we are not alone who still share the same views accepting or partially accepting the Danin's conception of microspecies in the P. oleracea complex (Amini Rad et al. 2017; Bulakh et al. 2019; Kurtto et al. 2019; Stoyanov et al. 2021, Bulakh et al. 2022; von Raab-Straube and Raus 2023; von Raab-Straube and Raus 2024), as well as it concerns some leading taxonomic databases as POWO (https://powo.science.kew.org/) or Euro + Med PlantBase (https://europlusmed.org/).

Recommendations for future studies of the P. oleracea complex

We outline our plan for future actions aimed at uncovering the definitive answers to the open questions in the P. oleracea complex below. Firstly, it is imperative to continue the global exploration of P. oleracea microspecies distribution to gain a comprehensive understanding of their geographic peculiarities. Secondly, it is crucial to figure out the questions, at least within Europe: How often can mixed populations be met? What is their composition? Thirdly, although controlled cultivation experiments involving P. cypria, P. sicula, P. nitida, P. zaffrani, P. oleracea s.str., and P. rausii were conducted for three generations in Jerusalem and Palermo (Danin et al. 2008), it is essential to extend these experiments to encompass all recognized microspecies of P. oleracea. Lastly, there is a pressing need for new molecular investigations employing advanced methodologies, covering all documented microspecies and including multiple samples of each microspecies, as previously recommended (Ocampo and Columbus 2012).

Conclusion

Thus, our research allowed to establish that the genus Portulaca is represented by four distinct species in the flora of the Balkans: P. oleracea agg., P. grandiflora, P. pilosa and P. umbraticola. Additionally, within the P. oleracea complex, twelve microspecies have been identified, primarily differentiated based on seed micromorphology. Besides, based on our field collected samples, five microspecies (P. daninii, P. granulato-stellulata, P. rausii, P. sardoa, and P. trituberculata) have been documented in Montenegro, and one microspecies (P. nitida) – in Slovenia for the first time. Also, the present knowledge about the distribution of Portulaca species (Portulacaceae) within the Balkan Peninsula are collected which contribute a lot to ongoing discussion about the status of taxa in the P. oleracea aggregate as well as to add the current data about the cases of escaping of alien plants from their cultivation. Though this exploration does not give final answers to all considered questions, the offered identification key for Portulaca species within the Balkan Peninsula and their synopsis will be useful for future investigations and publications of floras, checklists, and identification guides in the region. Finally, this study provides a valuable contribution to the botanical knowledge of the Balkan Peninsula, offering the recommendations for future studies of the P. oleracea complex for the solution of the taxonomic puzzle in this aggregate.