Plant material

Data for the multivariate analyses were recorded from herbarium specimens deposited in UPA and ATH (acronyms follow Thiers 2017+) belonging to Petrorhagia fasciculata (Margot & Reut.) P. W. Ball & Heywood, P. graminea (Sm.) P. W. Ball & Heywood, P. phthiotica (Boiss. & Heldr.) P. W. Ball & Heywood and P. saxifraga. We collected several specimens of P. laconica in the field. All specimens included in the analyses were selected so as to represent the entire distribution range of the taxa in Greece, as well as the morphological variation in each species. Only well-preserved and intact specimens were considered in the analyses. The total number of herbarium specimens included in the analyses was 97. A list of the examined specimens is provided in Table S1 (see  Supplementary Material online (wi.48.48110_Supplementary_tables_and_figures.pdf)). Each specimen was preliminary identified as one of the five aforementioned species. Species identification and nomenclature are according to Ball & Heywood (1964), Georgiou (1997) and Dimopoulos & al. (2013).

Characters recorded

Twenty-seven quantitative and 9 qualitative (binary) morphological characters were scored for the herbarium specimens included in our analyses (Table 1). The selected characters represent those previously quoted as diagnostic and are the ones most commonly used in the latest comprehensive monographs, regional and local floras (Ball & Heywood 1964; Coode & Cullen 1967; Georgiou 1997). Capsule and seed morphological characters, although with a diagnostic value in the genus Petrorhagia (Ball & Heywood 1964), were excluded from the morphometric analyses as most studied specimens were in flower, lacking capsules and seeds. These characters, however, were used for the documentation of morphological differences among P. laconica and its closest relatives. All the characters in the herbarium specimens were measured under a stereomicroscope using a ruler with the precision of 0.5 mm.

Morphometric analyses

Basic statistical parameters (mean, minimum and maximum value, standard deviation, 5^th and 95^th percentiles) were calculated for each species included in the analyses. We used a combination of ordination methods and discriminant analyses in the morphometric analyses of the specimens (Marhold 2011).

We first tested whether any character deviated from a normal distribution using the Shapiro-Wilk statistic and we log₁₀ - transformed any characters that deviated from normality. Then, we computed the correlation coefficients between each character pair in order to reveal highly correlated characters and to ensure that no high correlations (> 0.95 — Španiel & al. 2017) were present that could potentially distort the discriminant analysis. As a second step, we performed several stepwise canonical and classiflcatory Discriminant Analyses (DAs) in order to: (1) identify the variables having the highest potential as diagnostic characters and (2) test the effectiveness of the discrimination rules by using the Leave-One-Out cross-validation (LOO). The value of the inclusion and the exclusion criterion in the stepwise DAs was set at F = 0.05 and F = 0.1, respectively. Wilk's lambda was used to assess the significance of each variable and Discriminant Function (DF). The Maximum Chance Criterion (MCC) and the Proportional Chance Criterion (PCC) were used to determine whether the prediction equation was better than random chance (Huberty & Olejnik 2006). The DAs were checked for outliers. Finally, we examined the morphological variation in our dataset in relation to the species boundaries by analysing the ranges of the characters using core functions in the R computing environment (R Core Team 2015). All the analyses were carried out using IBM SPSS 24. The correlation coefficients were calculated with the “usdm” (Naimi & al. 2014) package, while the stepwise discriminant analysis plots were visualized via the ggplot2 (Wickham, 2009) package in the R computing environment. The Maximum and Proportional Chance criteria were ran under the Zclass algorithm kindly provided by John D. Morris (Florida Atlantic University).

Table 1.

Characters used in the morphometric analyses.

Characters scored

The basic statistical parameters for all species are given in Table S2. The only character that had a correlation coefficient above the threshold was a10 (eglandular hairs present at the upper ⅓ of the stem) and was thus excluded from further analyses.

Multivariate analyses

The DA clearly discriminated the five Petrorhagia species. Sixteen of the characters emerged as statistically significant (Table 2), with LOO showing a 100 % correct classification, being significantly better than random chance for both the PCC and MCC (P < 0.001, Table S3). These characters showed no to little overlap across the five species (Fig. 1). The first (DF1), the second (DF2), the third (DF3) and the fourth (DF4) discriminant function explained 86.6%, 11.5%, 1.3% and 0.6% of the total variation, respectively (Table 2, Fig. S1). Among the morphological characters used, the presence and the type of the indumentums are the most informative characters for species distinction in P. subsect. Saxifragae. The highest discriminative function (DF1) is primarily influenced by the presence of eglandular hairs at the middle part of the stem (a9). The presence of glandular hairs on calyx (a34) and eglandular hairs at the lower part of the stem (a8) have also high discriminant validity in DF1 (Table 2). The presence of eglandular hairs on calyx (a35) and glandular hairs at the middle/upper part of the stem (a6/a7) are the characters loading most heavily on DF2 and DF3, respectively (Table 2). The five Petrorhagia species included in the analyses are clearly distinct from one another (Table S3, Fig. 1, S1, S2).

Fig. 1.

Stepwise discriminant analysis plot for the five Petrorhagia species. The squares indicate the function group centroid of each species.

Taxonomy

Petrorhagia laconica Trigas, Kalpoutz. & Kougioum., sp. nov. — Fig. 2.

Holotype: Greece, Peloponnisos, sand dunes c. 3.5 km NW of Neapoli village, 36°31.721′N, 23°01.273′E, 2 m, 22 May 2017, Trigas 6311 (ACA; isotypes: B, UPA).

Diagnosis — Related to Petrorhagia saxifraga, from which it differs in the following morphological characters: underground stock rooting at nodes; stems procumbent to ascending or erect, glaucous-green, simple or rarely with few (1-3[-6]) lateral branches at upper ⅓, minutely papillose-scabridulous except glabrous at 1–4 upper internodes; leaves closely appressed to stem, rigid; flowers solitary or rarely paired, 1(or 2) per flowering stem, rarely up to 6; calyx minutely glandular pubescent; ribs purplish brown, obscurely 1-veined; capsule 2.5–3 mm long, sparsely tuberculate at proximal ½; seeds with flat, non-thickened margin.

Description — Plants perennial, with branched, slender, woody underground stock rooting at nodes, producing few to several flowering stems. Flowering stems procumbent to ascending or erect, glaucous-green to purplish brown, 7–40 cm long, simple or rarely with few (1-3[-6]) lateral branches at upper ⅓, minutely papillose-scabridulous except glabrous at 1–4 upper internodes; intemodes markedly unequal, larger at middle of stem, diminishing in size toward base and apex. Leaves linear-subulate, ⅓–1¼ as long as internodes, rigid, 1-veined, united at base into a sheath 1–1.5 mm long, with a purplish brown ring at base, apex acuminate to caudate; lower leaves 7-19 × 0.5-1 mm, minutely roughly papillose at margin, otherwise glabrous; cauline leaves closely appressed to stem, 3-16 × 0.5-1 mm, glabrous. Flowers solitary or rarely paired at apices of stems and branchlets, 1(or 2) per flowering stem, rarely up to 6, subtended by 4 straw-coloured, lanceolate, hyaline, glabrous epicalyx bracts, sometimes purplish-tinged at midrib. Calyx cylindric, 3–6 mm long, minutely glandular pubescent; ribs purplish brown, obscurely 1-veined; teeth oblong-triangular, 0.5–1 mm long, apex obtuse. Petals white, 4-8 × 1.5-3 mm, abaxial surface purple-veined, adaxial surface with three longitudinal purplish stripes at base, with glabrous claw not distinctly delimited from emarginate lamina. Anthers white. Ovary oblong-ovoid, c. 1 mm long; anthophore green, c. 0.5 mm long, glabrous. Capsule pale brown, ovoid, 2.5–3 mm long, smooth or sparsely tuberculate at proximal ½. Seeds blackish brown, ovate-oblong, 1-1.2 × 0.7-0.8 mm, both surfaces reticulate, margin flat, not thickened, apex long mucronate.

Fig. 2.

Petrorhagia laconica — A: sand-dune habitat NW of Neapoli village in SE Peloponnisos with evergreen shrubs (locus classicus), 22 May 2017; B: individual with simple, procumbent-ascending stems at the locus classicus, 16 May 2014; C: middle part of stem with leaves; D: upper part of stem with flower; E: flower, lateral view; F: flower, apical view; G: immature capsule; H: seed, dorsal face; I: seed, ventral face. — Scale bars: C–F: 3 mm; G: 1 mm; H, I: 500 µm. — All photographs by Panayiotis Trigas except G by Eleftherios Kalpoutzakis.

Distribution — Petrorhagia laconica is distributed in the Malea Peninsula in SE Peloponnisos. Two populations have been discovered so far; the first one is located at the Gulf of Neapoli, right across Elafonisos Island, while the second one was found close to Sykea village, c. 25 km N-NW of the first population. It is the southernmost distributed species of P. subsect. Saxifragae in Europe.

Ecology — Petrorhagia laconica is known to grow in two different habitats. In Neapoli, all individuals were observed to grow in the inner sand dune zone, usually surrounded by sclerophyllous shrubs, such as Anthyllis hermanniae L., Ceratonia siliqua L., Pistacia lentiscus L., Pyrus spinosa Forssk. and Smilax aspera L. Petrorhagia laconica grows in stabilized sand dunes among the shrubs, together with other sand dune adapted taxa, such as Anthemis tomentosa L., Asphodelus ramosus L., Centaurea sonchifolia L., Elytrigia juncea (L.) Nevski, Linaria tenuis (Viv.) Spreng, and Scirpoides holoschoenus (L.) Soják.

Close to Sykea village, Petrorhagia laconica grows in small remnant patches of macchie and phrygana formations that form a mosaic among the abundant olive groves. The population occupies a small area at 120–140 m a.s.l., on flysch. Shrub species composition of macchie is similar to that of the locus classicus, with Lavandula stoechas L., Olea europaea L., Phlomis fruticosa L., Pistacia lentiscus and Pyrus spinosa the predominant species. Petrorhagia laconica usually grows in small patches among the shrubs, together with Ballota acetabulosa (L.) Benth., Cistus salviifolius L., Globularia alypum L., Hypericum triquetrifolium Turra, Sarcopoterium spinosum (L.) Spach, Silene nocturna L., etc.

Conservation status — In Neapoli Lakonias, Petrorhagia laconica grows on a large sandy beach c. 3.5 km long, between Neapoli and Viglafla village. The width of sand dunes along this beach ranges from 30 to 150 m, and the species is exclusively confined to the inner sand dune zone, where it is fairly rare. It usually forms small groups of 5–30 individuals, but single remote plants were also observed. The total population of Neapoli is estimated to include 300–400 individuals, mainly concentrated at the E part of the beach. The Sykea population counts c. 500 individuals distributed in an area of c. 0.1 km².

Habitat loss and touristic development are the main threats that Petrorhagia laconica faces at its locus classicus. Sand dunes have been partly turned into cultivated fields at the E half of the beach, almost certainly leading to a significant population decline. The habitat also hosts thousands of swimmers during summer, being an unpredictable risk factor for its long term survival. The whole area, however, belongs to the Natura 2000 network of protected areas (Periochi Neapolis kai Nisos Elafonisos, GR2540002), and numerous human activities that could create additional risks for the population of P. laconica in Neapoli area are fortunately forbidden.

Habitat loss is also the main threat that Petrorhagia laconica faces in Sykea area, as more than 50% of the area previously covered with natural vegetation have been turned into cultivated fields. As a result, it is irrefutable that the species has suffered a severe population decline in the near past, which can be estimated as probably approaching the percentage of habitat loss.

The flora of SE Peloponnisos has been intensively explored during the last decades (e.g. Kalpoutzakis & Constantinidis 2005, 2006; Greuter 2012; Kalpoutzakis & al. 2012). The recent discovery of Petrorhagia laconica indicates that the species should not be common in this area. The habitat types in which it grows (especially the mosaic with macchie and phrygana formations), however, are abundant in SE Peloponnisos. Consequently, the existence of additional populations of this tiny and easily overlooked species cannot be ruled out. The extent of occurrence (EOO) does not exceed 25 km² and since P. laconica is a very local species, its area of occupancy (AOO) is much smaller, apparently less than 1 km². Ergo, due to: (1) the restricted EOO and AOO, (2) the current existence of max. 900 mature individuals distributed in two populations and (3) the high possibility that the extant localities and populations could be eradicated as a result of human interference, P. laconica is assigned to the Endangered (EN) IUCN (2001) category, following criteria Blab(i,ii,iii,iv)+2ab(i,ii,iii,iv). Petrorhagia laconica should be carefully monitored, cultivated ex situ in botanical gardens and special conservation measures be taken to safeguard its populations.

Table 3.

Main morphological differences among Petrorhagia laconica, P. saxifraga and P. graminea.

Etymology — The specific epithet refers to Laconia (Lakonia, also known as Lacedaemonia), a region that covers almost the entire SE part of the Peloponnisos, where the new species was collected. This name goes back to the historical times, when Laconia was the principal region of the Spartan state.

Taxonomic relationships — Petrorhagia laconica undoubtedly belongs to P. sect. Petrorhagia, as it is indicated by its 1-veined leaves, the presence of epicalyx bracts, the not abruptly clawed petals, the reticulate, blackishbrown seeds and the perennial life-cycle. The thickened seed margin is not a common feature of all species in P. sect. Petrorhagia, as it is indicated by Ball & Heywood (1964). Petrorhagia dianthoides (Sm.) P. W. Ball & Heywood, P. fasciculata, P. grandiflora Iatrou and P. thessala (Boiss.) P. W. Ball & Heywood have a flat, thin seed margin, similar to that of P. laconica.

Within Petrorhagia sect. Petrorhagia, P. laconica shows close affinities to the members of P. subsect. Saxifragae. The solitary or paired flowers and the lanceolate, 1-veined epicalyx bracts advocate for the inclusion of P. laconica within P. subsect. Saxifragae. Flower and seed morphological characters further support this taxonomic view. The simple or sparingly branched stems, however, resemble the members of P. subsect. Thessalae, which otherwise have significant morphological differences from P. laconica (e.g. capitate inflorescence, broadly ovate or sub-orbicular bracts, larger and different petals, etc.).

Petrorhagia laconica appears to be one of the most distinct species in P. sect. Petrorhagia. The results of multivariate morphometric analyses (Tables 2, S3, Fig. 1, S1, S2) support its distinct taxonomic position within P. subsect. Saxifragae. The closest related species seem to be P. saxifraga and P. graminea, which, however, show significant morphological differences from P. laconica (see also Table 3). The possibility of vegetative reproduction (underground stolons connecting different individuals have been observed in the field) and the tuberculate capsule have not been recorded in other Greek Petrorhagia species, and they probably represent unique features within the whole genus. The discovery of P. laconica is in line with Trigas & al. (2007, 2012), which state that SE Peloponnese is one of the important regions in Greece in terms of endemic plant species richness and conservation, as well as a significant diversity centre for Petrorhagia, because more than 30 % of the species diversity of the whole genus is hosted in this small region.

Additional specimens examined — Greece: Peloponnisos, c. 3.5 km S-SE of Sykea village, 36°43.970′N, 22°57.079′E, 130 m, macchie and phrygana among olive groves, on flysch, 6 May 2013, Kalpoutzakis 4294 (ACA); ibid., 29 Jul 2017, Kalpoutzakis 4959 (ACA); c. 2.5 km S-SE of Sykea village, 36°44.552′N, 22°57.209′E, 120 m, macchie and phrygana among olive groves, on flysch, 22 May 2017, Trigas 6312 (ACA).