Morphological and physiological adaptive traits of Mediterranean narrow endemic plants: The case of Centaurea gymnocarpa (Capraia Island, Italy)

A case study on Centaurea gymnocarpa Moris & De Not., a narrow endemic species, was carried out by analyzing its morphological, anatomical, and physiological traits in response to natural habitat stress factors under Mediterranean climate conditions. The results underline that the species is particularly adapted to the environment where it naturally grows. At the plant level, the above-ground/below-ground dry mass (1.73 ± 0.60) shows its investment predominately in the above-ground structure with a resulting total leaf area per plant of 1399 ± 94 cm2. The senescent attached leaves at the base of the plant contribute to limit leaf transpiration by shading soil around the plant. Moreover, the dense C. gymnocarpa leaf pubescence, leaf rolling, the relatively high leaf mass area (LMA = 12.3 ± 1.3 mg cm−2) and leaf tissue density (LTD = 427 ± 44 mg cm−3) contribute to limit leaf transpiration, also postponing leaf death under dry conditions. At the physiological level, a relatively low respiration/photosynthesis ratio (R/PN) in spring results from high R [2.26 ± 0.59 μmol (CO2) m−2 s−1] and PN [12.3 ± 1.5 μmol (CO2) m−2 s−1]. The high photosynthetic nitrogen use efficiency [PNUE = 15.5 ± 0.4 μmol (CO2) g−1 (N) s−1] shows the large amount of nitrogen (N) invested in the photosynthetic machinery of new leaves, associated to a high chlorophyll content (Chl = 35 ± 5 SPAD units). On the contrary, the highest R/PN ratio (1.75 ± 0.19) in summer is due to a significant PN decrease and increase of R in response to drought. The low PNUE [1.5 ± 0.2 μmol (CO2) g−1 (N) s−1] in this season is indicative of a greater N investment in leaf cell walls which may contribute to limit transpiration. On the contrary, the low R/PN ratio (0.05 ± 0.02) in winter is resulting from the limited enzyme activity of the respiratory apparatus [R = 0.23 ± 0.08 μmol (CO2) m−2 s−1] while the low PNUE [3.5 ± 0.2 μmol (CO2) g−1 (N) s−1] suggests that low temperatures additionally limit plant production. The experiment of the imposed water stress confirms that the C. gymnocarpa growth capability is in conformity with the severe conditions of its natural habitat, likewise as it may be the case with others narrow endemic species that have occupied niches with similar extreme conditions.