According to the latest taxonomic revision, genus Leiotulus Ehrenb., belonging to the Apiaceae tribe Tordylieae Koch., contains 10 species and subspecies, previously mainly assigned to the genera: Malabaila and Pastinaca (POWO 2022). Species Leiotulus aureus is known under the following homotypic synonims: Heracleum aureum Sm. (basionym), Lophotaenia aurea (Sm.) Griseb., Malabaila aurea (Sm.) Boiss., and Pastinaca aurea (Sm.) Calest., while the accepted name is proposed as a new name combination by Pimenov and Ostroumova (1994), based on carpological investigations. The genus included species characterized as intermediate between Pastinaca L. (incl. Malabaila s.str.) and Zosima Hoffm. (e.g. Malabaila involucrata Boiss. & Spruner, M. pastinacifolia Boiss. & Balansa etc.).

L. aureus is a herbaceous biennial, a plant of the native range from SE Europe to NW Turkey (POWO 2022), prefering warm and sunny places along with open rocky habitats. It is caracterised by a semirosette growth form, erect, hollow, striate and somewhat viscid stem up to 50 cm tall, pinnately divided leaves with ovate leaflets near the ground and linear-lanceolate in upper stem ones. Bright yellow flowers are grouped in terminal and subterminal, regularly compound umbels (Tutin et al. 1981). The mature fruit is obcordate - suborbicular, flattened, surrounded by a wide somewhat thickened margin, cordate at the apex, with persistent style (Fig. 1). Although the plant is slightly aromatic, a literature search did not reveal any reference to its local use in traditional medicine or cooking.

Fig. 1

Habitus of Leiotulus aureus (A), inflorescence close up (B), detail of immature fruits (C) (photo: Danijela Stešević).

Pollen grains within the Apiaceaeare usually stenopalynous, radially symmetrical, isopolar and prolate to perprolate in shape (Erdtman 1971). Grains are commonly tricolporate with slit-like ectocolpi and very distinctive and broadband-like costae (Punt 1984, Perveen and Qaiser 2006). The tectum is commonly striate-rugulate or simple striate (Punt 1984), psilate to granulate (Perveen and Qaiser 2006). There are a few different criteria accepted for classification of the pollen grains of the Apiaceae. Various characters have been used, such as polar/equatorial ratio (P/E), exine pattern, etc. For example, Cerceau-Larrival (1962) proposed 5 pollen types occurring within the family based on shape index: sub-rhomboidal (type 1, P/E:1-1.5), sub-circular (type 2, P/E:1-1.5), oval (type 3, P/E:1.5-2), sub-rectangular (type 4, P/E:2), and equatorially constricted (type 5, P/E:over 2). According to this classification, pollen grains of L. aureus in the current study should fit into type 5. In addition, according to Van Zeist and Bottema (1977) the pollen grains of the Apiaceae could be divided into 9 pollen types: Anisosciadium, Bunium, Bupleurum, Eryngium, Ferula, Malabaila, Pimpinella, Sium erectum and Turgenia types. The pollen features characterizing the Malabaila type are as follows: subrectangular to slightly oval shape with rounded poles, P/E is 2, fairly long colpi, transversal furrow oval-rectangular, distinct columellae 0.5 to 0.8 μm in diameter, slight variations in the wall thickness (exine up to 2 μm thick), grain size ranging from 35 to 40 μm. This type comprises Malabaila secacul, M. kotschyi, Orlaya grandiflora, Heracleum maximum, H. lasiopetalum, H. persicum, Ormosciadium aucheri, Scandix iberica, Stenotaenia nudicaulis andTurgeniopsis foeniculacea. Recently, Orlaya has already been described as having its own pollen type (Beug 2015), while Malabaila secacul and M. kotschyi were transferred to genus Leiotulus (as L. secacul (Mill.) Pimenov & Ostr and L. kotschyi (Boiss.) Pimenov & Ostr.)) (Pimenov and Ostroumova 1994, POWO 2022). The results of palynomorphplogical research of L. aureus (which previously also belonged to Malabaila genus) are not completely in accordance with characters described for the Malabaila pollen type, such as P/E ratio and grain size, provided by Van Zeist and Bottema (1977). The pollen grains, described in the current paper, are slightly larger. It is known that the size may vary depending on the liquid medium used to mount samples on microslides for light microscopy (Faegri and Iversen 1989, Pospiech et al. 2021). However, the measurements of pollen size in the present paper are based on micrographs obtained by SEM, without using any mounting media which may cause enlargement of pollen grains. Extensive palynological research provided by Punt (1984), who recognized 50 types of pollen, as well as by Perveen and Qaiser (2006) who distinguished three pollen types based on tectum features within 27 Apiaceae genera, did not cover Malabaila/Leiotulus species.

The Apiaceae species have a uniform umbel and flower structure, however within the family diverse pollination systems occur varying from completely self-pollination to obligate cross-pollination (Koul et al. 1993). Flowers of umbellifers attract numerous insect taxa from several taxonomic orders supplying them with pollen and nectar (Niemirski and Zych 2011). Hence, umbellifers are often connected with generalized pollination system, indicating that their floral nectar is easily accessible to flower-visiting insects of different pollination efficiency due to the various degree of mouthpart modification for feeding on nectar, pollen or petals (Proctor et al. 1996, Olesen et al. 2007). These include mostly flies, but also hymenopterans and coleopterans (Proctor et al. 1996, Lamborn and Ollerton 2000, Zych 2007, Davila and Wardle 2008, Carvalheiro et al. 2008, Mačukanović-Jocić et al. 2016). In the current study, flowers of L. aureus, representing an open dish-shaped blossom type, were visited by twelve insect species from four taxonomic orders. With regard to the observed flower visitors, the following pollination types occurred: melittophily, myophily, sapromyophily and cantharophily. Dipterans were the most frequent group of visitors. Most of the recorded dipterans were of the “hoverfly type” (i.e. members of the family Syrphidae) and the “muscoid type” (i.e. members of the families Sarcophagidae (flesh flies) and Calliphoridae (blow flies)). Although not highly specialized in pollination, flower-pollinating species have been reported in the Bibionidae in previous research (Goldblatt et al. 2005). Adults of march flies (Bibionidae) are known as pollen-collecting and nectar feeding flower visitors pollinating fruit trees and some other crops (Freeman and Lane 1985, Woodcock et al. 2014). In the present study carrion flies were found to be less important from the pollination point of view, since Leiotulus is an aromatic plant more attractive to Apidae and Syrphidae. Despite a few Coleopteran species observed in the current and similar studies on umbellifers (Zych 2006), they are of minor importance as they are not considered to be efficient pollinators (Mačukanović-Jocić 2010). Hemipterans observed in this study could not be considered as pollinators, since their pollination activity is negligible, except in rare cases (Ishida et al. 2009). Unlike previous research on some other Apiaceae species (Zych 2006, Mačukanović-Jocić et al.2016), in the present study neither moths nor butterflies were observed visiting L. aureus umbels. Low visitation rate of lepidopterans can be explained by the length of their proboscis, which is more adapted to tubular corollas, or by weaker attractiveness of flowers which can be attributed to the scent or floral nectar components. L. aureus could be considered an ecologically generalized species in terms of the need for specialized pollinators. There are many reasons for this. Although L. aureus like many other umbellifers lack any visible or invisible floral signs such as honey guides leading to nectar, the flowers are slightly aromatic and could emit some chemical signals that attract specific groups of pollinators, as previously reported for this family (Tollsten et al.1994, Tollsten and Øvstedal 1994, Niemirski and Zych 2011). Regardless of their floral uniformity, some umbellifers are suggested to exhibit cryptic flower specialization enabling oligolectic relationships with bee pollinators (Lindsey 1984, Lindsey and Bell 1985, Niemirski and Zych 2011). Unlike Zych (2006) who did not observe any honey bee on Heracleum sphondylium, L. aureus flowers were very frequently visited, which is in line with the findings of other authors who pointed out the importance of honey bees in pollinating umbellifers (Langenberger and Davis 2002b, Davila and Wardle 2002). Following the diurnal dynamics of honey bee visits, plants were the most frequently visited about midday. Considering the frequency of honey bee visits L. aureus could be regarded as a melliferous plant contributing to honey bee pasture. However, regarding its rather “unspecialized” floral morphology and its being visited by numerous species of flower visitors, this plant species is without ecological specialization to particular insect species.