Comparative germination ecology of the endemic Centranthus amazonum (Valerianaceae) and its widespread congener Centranthus ruber

Centranthus amazonum Fridl. & A. Raynal (Valerianaceae) is a critically endangered narrow endemic to Central–Eastern Sardinia (Italy), where it occurs in rocky habitats. In the present study, the germination ecology of this species was investigated and compared with the relatively widespread species Centranthus ruber (L.) DC. subsp. ruber. In particular, the effects of a range of constant temperatures (5–25°C) and two alternating temperature regimes (25/10 and 30/15°C) on seed germination were analyzed. For the two species, cardinal temperatures were determined as the reciprocal of time to 50% germination (t50). In addition, seed persistence in the soil was assessed by experimental seed burials. Seeds of both species were non-dormant; however, C. amazonum produced more empty seeds and germination was slower and reached lower final percentages than C. ruber. Moreover, seed and seedling mass of C. ruber were higher than for C. amazonum and the lack of a persistent soil seed bank detected for C. amazonum increases vulnerability to extinction for this species. The present study provides new data on the germination ecology of the previously unstudied C. amazonum that may explain the rarity of this species when compared with the commonness of C. ruber.