Alyssum rossetii Španiel, Bovio & K.Kaplan, 2018

Španiel, Stanislav, Kaplan, Klaus, Bovio, Maurizio, Mártonfiová, Lenka & Cetlová, Veronika, 2018, Alyssum rossetii (Brassicaceae), a new species from the Aosta Valley in Italy based on morphological and genome size data, Phytotaxa 360 (3), pp. 269-281 : 275-278

Alyssum rossetii Španiel, Bovio & K.Kaplan , sp. nov.

Type: — ITALY, Aosta , Valpelline valley , Bionaz, NE of Chez Les Chenaux, between La Tsa and Pas des Feuilles, above and along footpath no. 3, open soil with sparse vegetation cover, 45°53′26.6″N, 7°26′43.6″E, 2,293 m.a.s.l., 10 Jun 2017, Španiel 560FEU (holotype SAV 0006950 !, barcode SAV0006950 , http://ibot.sav.sk/herbarium/object/ SAV0006950 ; isotype SAV 0006951 !, barcode SAV0006951 , http://ibot.sav.sk/herbarium/object/ SAV0006951 )

Diagnosis: Similar to Alyssum montanum L. but differing in having shorter rays of stellate trichomes on the lower surface of middle cauline leaves (0.11–0.15 mm vs 0.18–0.30 mm) and a higher trichome density on the leaf underside (28–42 vs 14–27 stellate trichomes per 0.5 mm 2) and upper surface (20–32 vs 4–15 stellate trichomes per 0.5 mm 2) of middle cauline leaves. It also differs to some extent from the latter species by shorter stems (2.6–8.5 cm vs 7.5–24.5 cm), narrower petals (0.93–2.13 mm vs 1.62–3.59 mm) and shorter four inner filaments of stamens (2.02–2.97 mm vs 2.56–4.38 mm). In addition, it differs from A. montanum and many other perennial Alyssum taxa by the absence of pronounced wings and teeth on the four inner filaments (vs pronounced wings and teeth on the four inner filaments) and purplish colour of sepals or at least the presence of purplish spot at the apex of sepals (usually missing in other perennial Alyssum taxa), see Fig. 6 and 7 and Table 3.

Etymology: The specific epithet is derived from the name of the botanist Piero Rosset, who was the first to point out to the peculiarity of the population between La Tsa and Pas des Feuilles in the Valpelline valley.

Chromosome number: 2 n = 2 x = 16 ( Fig. 3A )

Distribution and Ecology: Alyssum rossetii is a stenotopic endemic so far known to occupy a single highmountain site between La Tsa and Pas des Feuilles. It shares its habitat with the widespread annual species A. alyssoides ( Linnaeus 1753: 652) Linnaeus (1759a: 487 , 1759b: 1130), from which it can be easily recognized by its perennial life form, different ploidy level (tetraploid in A. alyssoides, Španiel et al. 2010 ) and numerous morphological differences. It grows on more or less steep south-facing slopes in a subalpine-alpine grassland with a stair-step structure ( Fig. 1 ). The locality is situated somewhat below the tree line, as lone trees nearby demonstrate. As is typical for this kind of grassland, dense vegetation alternates with open soil. Grazing has been practised for hundreds of years at this location and may have reinforced the stair-step structure of the grassland. Along the footpath there are other small patches of open ground, possibly created by small landslides. These areas of open soil are the typical microhabitats of A. rossetii . However, it can also be found in clumps with other alpine plants or in juniper bushes. The plant communities of the surrounding steep south-facing slopes probably mostly belong to the Festucion variae-alliance; however, we have not analysed them in detail. In this part of the Valpelline valley there are some of the rare areas of calcareous substrate. Actually, the dominant paragneiss (kinzigits) of the Valpelline valley alternates at the site of A. rossetii with numerous little outcrops of calcareous rocks (ancient silicate-rich marbles). The consequence is that calcifuge plants and a certain number of calcicoles grow side by side or close to each other; for instance, Festuca scabriculmis s.l. and Juniperus nana Willdenow (1796: 159) grow next to the very rare Viola pinnata Linnaeus (1753: 934) (cf. Bovio et al. 2017), Helictotrichon parlatorei ( Woods 1850: 405) Pilger (1938: 7) , Senecio doronicum ( Linnaeus 1753: 880) Linnaeus (1759b: 1215) and Satureja alpina Scheele (1843: 577) .

Conservation: The conservation status of the new species should be classified as Endangered (EN), based on current information and following the guidelines of the IUCN (2012). Only one population of A. rossetii composed of several dozen individuals has been found along and above a short section (100 m) of mountain trail no. 3. All the individuals have been found along the margins of a hiking path on open soil with sparse vegetation cover and within surrounding plant communities. No individuals have been spotted outside of this small area; however, A. rossetii is easy to overlook, especially when not in anthesis, because of its small size (2–10 cm) and low population density.

Figures

FIGURE 1. Alyssum rossetii: unripe fruits (A), in bloom (B) and the habitat of the holotype between La Tsa and Pas des Feuilles (C, D).

FIGURE 3. A. Metaphase plate of a root tip of Alyssum rossetii with the chromosome number 2n = 16. Photo by L. Mártonfiová. B. A simultaneous flow cytometric analysis illustrating difference in relative genome size between A. rossetii and diploid A. montanum. For population codes, see Table 1.

FIGURE 6. Morphology of Alyssum rossetii (A-H) and of inner and outer filaments of A. montanum (I), A. rhodanense (J), A.flexicaule (K) and A. orophilum (L). A—habitus, B—fruiting raceme, C—silicule, D—flower, E—morphology of a silicule trichome, H–L—filaments. Drawings by Zlata Komárová.

FIGURE 7. Variation of the indumentum on the lower surface of middle cauline leaves among Alyssum rossetii (A, B; pop. 560FEU), A. montanum (C, D; pop. 293CHA), A. flexicaule (E, F, 274VEX), A. orophilum (G, H; pop. 316MGR) and A. rhodanense (I, J, 254TAI). Scale bars: 100 μm (B, D, F, H, J) and 200 μm (A, C, E, G, I). SEM microphotographs by V. Cetlová and S. Španiel.