Seed germination, salt-stress tolerance, and the effect of nitrate on three Tyrrhenian coastal species of the Silene mollissima aggregate (Caryophyllaceae)

The Silene mollissima (L.) Pers. aggregate is part of section Siphonomorpha Otth. and currently comprises 11 narrow endemic species of the Western Mediterranean Basin. Three of these taxa (S. velutina Pourr. ex Loisel, S. ichnusae Brullo, De Marco & De Marco f., and S. badaroi Breistr.) have a distribution range centred in the northern Tyrrhenian area and occurring in coastal habitats. Inter- and intra-specific variability in the responses to light, constant (5–25 °C) and alternating temperatures (25/10 °C), salt (NaCl, 0–600 mmol/L), and levels of nitrate (KNO3, 20 mmol/L) under salt stress, as well as recovery of seed germination were evaluated for these species to more effectively support their in-situ and ex-situ conservation. Our results highlighted that the seeds of these three taxa were nondormant, and that light significantly improved their rate of germination, which was higher (>80%) at low temperatures (5–15 °C) and under the alternating temperature regime (25/10 °C), but decreased significantly at the highest temperature tested (25 °C). Seeds from Silene velutina and S. ichnusae germinated in up to 300 mmol/L NaCl, and S. badaroi germinated in up to 100 mmol/L. For all of the species except S. badaroi, salt did not affect seed viability, and recovery germination did not decrease with increasing salinity and temperature. Interpopulation variability, both in salt tolerance and recovery germination, was detected for S. velutina. The addition of KNO3 did not affect germination or recovery germination under salt conditions. The lack of effect from KNO3 suggests that nutrient availability is not a requirement for seed germination in these species. Our results show that all species experience an optimum period of germination during autumn–winter, which is when water availability is highest and soil salinity levels are minimal because of the Mediterranean rainfalls, but seeds from S. velutina and S. ichnusae will germinate up until spring.