A STUDY ON ACHENE MACRO-AND MICROMORPHOLOGICAL CHARACTERS OF POLISH SPECIES OF THE Senecio jacobaea GROUP

A study on achene macroand micromorphological characters of the following five Polish species of the Senecio jacobaea group was carried out by LM and SEM: S. aquaticus (var. aquaticus and var. barbareifolius), S. erucifolius (subsp. erucifolius and subsp. tenuifolius), S. jacobaea, S. paludosus, and S. subalpinus. The achenes differed in several characters, but the most distinctive differences concerned their indument, especially the length and morphology of hairs. The study clearly shows that two morphologically similar species, S. jacobaea and S. erucifolius, which are often confused can be easily distinguished based on the morphological characters of achenes. Additionally, we showed differences between the achenes of S. erucifolius subsp. erucifolius and subsp. tenuifolius, the two taxa which were not distinguished in floristic studies and herbarium collections from Poland. The key to determine Polish species of the Senecio jacobaea group, based on the morphological characters of achenes, is also given.


INTRODUCTION
Senecio L. (Asteraceae; Senecioneae) is a large and taxonomically difficult genus comprising between 1000 and 3000 species (J e f f r e y et al. 1977; N o r d e n s t a m , 1978; B r e m e r , 1994; V i nc e n t , 1996).The genus is divided into approximately 150 sections which are not clearly distinguished from one another.According to different authors, the section Jacobaea comprises from 3 to 15 species (P e ls e r et al. 2002).Such different views of the species composition of the section resulted from a very general section description by different authors (e.g.C h a t e r and W a l t e r s , 1976; J e f f r e y , 1992; S h i s h k i n , 1995) which could also be applicable to many species from other sections of Senecio.Three species: S. aquaticus, S. erucifolius and S. jacobaea, are considered to form the core of sect.Jacobaea with S. jacobaea as a type species of the section.The delimitation of sect.Jacobaea, its phylogenetic position as well as the relationships within and among Senecio sections were recently examined by P e l s e r et al. (2002, 2003, 2004, 2007).
The descriptions of Senecio achenes based on macromorphological characters are available in many floras, although they are not always consistent.From the Senecio jacobea group, only S. jacobaea achenes were investigated using SEM (M c E v o y , 1984).The aim of the present study was to describe the macro and micro-morphological characters of achenes of Polish species of the Senecio jacobaea group.We were interested in finding additional taxonomic characters which could be useful in identification and distinguishing taxa within sect.Jacobaea.

MATERIALS AND METHODS
Achenes of the following five species of the Senecio jacobea group were examined: S. aquaticus (var.aquaticus and var.barbareifolius), S. erucifolius (subsp. erucifolius and subsp. tenuifolius), S. jacobaea, S. paludosus, and S. subalpinus.Plant material, 1-5 specimens per taxon, was obtained from various sources, both from natural localities and herbarium collections (Table 1).All examinations were carried out on fully developed dry fruits.Achene size (length and width), shape, outline, and colour as well as pappus length were determined from 30 fruits of each taxon using a PZO type 131 stereoscope microscope.Some macrographs were obtained with a Zeiss Stereo Lumar V12 stereoscope microscope.Five achenes of each taxon were mounted on aluminium stubs with "Leit-Tabs" and coated with gold in an Agar sputter coater.Electron micrographs were obtained with a Zeiss EVO 40 scanning electron microscope at an accelerating voltage of 12 kV.Micromorphological characters were determined on the basis of SEM images.The terminology describing fruit shape and surface followed B o j ň a n s k ý and F a r g a š o v á (2007) as well as B a r t h l o t t (1981,1984).

RESULTS
The main macro-and micromorphological characters of the examined achenes are summarized in Table 2, while the selected macrophotographs and SEM microphotographs of the fruit showing its general view, the achene surface and pappus details are presented in Plates I-III.In case of two species -S.aquaticus (var.aquaticus and var.barbareifolius) and S. erucifolius (subsp.erucifolius and subsp.tenuifolius), infraspecific taxa were investigated.In S. aquaticus, no significant differences were found at the species level, so further we present the results only for nominal taxa.In S. erucifolius, the differences between the achenes of subsp.erucifolius and subsp.tenuifolius were well visible, so achenes of both taxa are presented and compared.Among the studied species, only Senecio jacobaea had dimorphic achenes (Plate I 2A*, 2A), whereas the remaining ones had homomorphic fruits.

Macromorphological characters
The achenes of the investigated species were oblong-cylindrical, in S. aquaticus sometimes narrowly obovoid, or linear in S. paludosus (Table 2, Plates I-III).They were longitudinally ribbed, with the ribs more or less distinct and the furrows sometimes darker than the rest of achenes.The colour of achenes differed from whitish-yellow to pale-brown.The apex of achenes had usually the form of a distinct white (in S. erucifolius yellowish) crest.The basal scar of achenes was more or less distinct, white or yellowish.The achenes were equipped with non-deciduous (except external S. jacobaea achenes) white pappus, usually about twice as long as the achene (Table 2).

Micromorphological characters
The achene surface of all species showed the reticulate type of microsculpture with a net-like cell arrangement.The epidermal cells of the exocarp were elongated, prosenchymatic, running parallel to the achene axis.Usually they were shorter on the ribs, where the hairs occurred, and longer between the ribs.The most elongated cells occurred in S. erucifolius and the shortest ones in S. paludosus (Plate II 2E, Plate III 1E).The anticlinal walls of exocarp cells were channelled in S. jacobaea and raised in the remaining species.
The outer periclinal walls were usually concave, with a smooth surface or with delicate cuticular striations running usually lengthways.In S. paludosus, the stripes were the most distinctive (Plate III 1E).Considering different micromorphological characters, the most distinctive differences related to hairs occurring on the surface of achenes (Plates I-III, D and E).Only S. subalpinus and external S. jacobea achenes were bare.The remaining ones were covered with simple hairs.In S. aquaticus and S. jacobaea, hairs were scattered, short (up to about 70 μm), finger-like and tapering, occurring only on the ribs (Plate I 1D, 2D) In S. erucifolius subsp.Erucifolius, achenes were medium hirsute with hairs, occurring mainly on the ribs, of medium length (up to about 150 μm), slightly spirally twisted and flattened, with a sharp bifurcate or single tip.In S. erucifolius subsp.tenuifolius, achenes were dense hirsute, with long (up to about 250 μm) hairs, distinctly spirally twisted and flattened, most often with a sharp single tip (Plate II 2D, 2E).In S. paludosus, achenes were medium hirsute with hairs, occurring on the entire surface, of medium length (up to about 100 μm) and with a characteristic bifurcate tip (Plate III 1D, 1E).SEM analysis of pappus bristles did not show any significant differences between the examined species.They were all simple, usually with two (sometimes one) terminal cells, more or less divergent.
A key to the Polish species of the Senecio jacobaea group, based on the morphological characters of achenes:

DISCUSSION
Both the macro-and micromorphological characters of achenes have rarely been taken into consideration in taxonomic studies at the infrageneric and infraspecific level in the genus Senecio (P e l s e r and H o u c h i n , 2004; P e l s e r et al. 2004; H o d á l ov á et al. 2007, 2010; X u e -i n and G u o -h a i l , 2007).Most often, the presence and length of achene hairs were compared, and scarcely also their internal structure and the tips of pappus bristles (M a c l os k i e , 1883; D r u r y and W a t s o n , 1965).Both cited authors described achene hairs of the species Senecio as superficially duplex, each having two tubes with a partition between.They also noticed that closer examination proved that there is always an additional small cell associated with the base of the hair.M a c l o s k i e (1883) described interesting duplex achenial hairs of S. vulgaris with a bifurcate tip and elaters protruding with excess of moisture.We observed a similar morphological type of hairs (but with no elaters) in S. paludosus and partially in S. erucifolius subsp.erucifolius.D r u r y and W a t s o n (1965) distinguished two main morphological types of hairs: short, bent and blunt to more or less clavate, and more elongated, straight and tapering.The first type was not observed in the species observed by us, while the other one appeared on the achene surface of S. aquaticus and S. jacobaea.D r u r y and W a t s o n (1965) also noticed that in a given species the hairs were always of the same type.The type of achenial hairs that we observed in S. erucifolius subsp.tenuifolius -very long, spirally twisted and flattened -had not been described before.The information about the length of achenial hairs is inconsistent.For example, D r u r y and W a t s o n (1965) described these hairs as long in S. jacobaea, S. erucifolius and S. paludosus, whereas Bojňanský and F a r g a š o v á (2007) as short ones.In turn, in our study achenial hairs in these three species were distinctly different according to their length.D r u r y and W a t s o n (1965) also examined pappus bristles in several European Senecio species.They noticed that in some species each pappus bristle had three terminal cells, while in others there were never more than two.In the latter case, the cells of the terminal pair were either divergent or appressed to their tips.According to their observations, in the Senecio jacobaea group they were divergent.Our study confirmed their observations, although we also noticed bristles with a single terminal cell.
In the introduction, we underlined that we were interested in finding additional taxonomic characters which could be useful in identification and distinguishing taxa within sect.Jacobaea.It is especially crucial for two morphologically similar species, S. jacobaea and S. erucifolius, which are often confused.
Our study clearly shows that by comparing the morphological characters of achenes these two species can be easily distinguished.Additionally, we showed differences between the achenes of S. erucifolius subsp.erucifolius and subsp.tenuifolius, the two taxa which were not distinguished in floristic studies and herbarium collections from Poland.