Inter‐ and intra‐variability of seed germination traits of Carpobrotus edulis N.E.Br. and its hybrid C. affine acinaciformis

Abstract

Invasions by alien Carpobrotus spp. have been recognised as one of the most severe threats to Mediterranean climate coastal ecosystems, and Carpobrotus is considered one of the most widespread invasive alien genera in the Mediterranean Basin. The aims of this study were to characterise seed germination of both C. edulis and its hybrid C. aff. acinaciformis, in terms of photoperiod, temperature and salinity. Inter- and intra-specific variability in the responses to photoperiod (12/12 h light and total darkness), constant temperatures (5, 10, 15, 20, 25 °C) and an alternating temperature regime (25/10 °C), salt stress (0, 125, 250, 500 mm NaCl) and the recovery of seed germination were evaluated for two seed lots of C. edulis and two of its hybrid C. aff. acinaciformis. All the tested seed lots achieved higher germination percentages in the light, with respect to total darkness. In relation to temperature, the two C. edulis seed lots did not show a preference, while the two C. aff. acinaciformis seed lots differed in their germination response, one germinating more at the lowest temperatures (5 and 10 °C) and one at the highest (20 and 25 °C). For all seed lots, highest germination occurred without NaCl (0 mm) and germination decreased with increasing salinity. Different germination requirements in a saline substrate were not detected for C. edulis, while they were observed for C. aff. acinaciformis. Marked differences were detected in recovery responses between the two taxa. C. edulis has the ability to germinate over a wide time window throughout the year. This study identified significant differences in seed production, seed mass, germination requirements (temperature) and salinity tolerance for both C. edulis and C. aff. acinaciformis. Our results indicate the extreme versatility of the hybrid forms to germinate in a wide range of natural conditions and habitats.

Invasive Alien Flora on Coastal Mediterranean Habitats

“…(Fig. 13) showed their ability to germinate also at high NaCl concentrations (500 mM) (Bacchetta et al 2010;Podda et al 2015Podda et al , 2017Podda et al , 2018. Other invasive species show less tolerance to high salt concentrations, such as Acacia saligna (Labill.)…”

“…The species able of surviving in seawater or even higher concentrations of salt have raised the interest of many researchers who have worked to analyze the mechanisms that allow them to survive in such extreme conditions. Some authors have addressed tolerance to salt of alien plants as one of the common characteristics of coastal habitats, highlighting the presence of halophytes or even euhalophytes (Bacchetta et al 2010;Podda et al 2018).…”

“…14), which however shows a germination capacity of up to 2% NaCl (about 250 mM) (Meloni et al 2015). Moreover, the limit of tolerance to salt is species specific (Podda et al 2018). For some species salinity tolerance during germination depends on temperature, being higher at low temperatures (El-Keblawy and Al-Shamsi 2008) with respect to higher temperatures.…”

“…The seeds even if treated with high salinity can recover their ability to germinate after washing with distilled water (see an example in Fig. 17), although they can differ in recovery, depending on temperature (Podda et al 2018).…”

“…As shown above, the ability of Carpobrotus seeds to germinate with high NaCl concentrations (up to 500 mM NaCl) reveals potential growth of seedlings of these species in close coastline areas and in soils with high salt concentrations (e.g., areas surrounding salt marshes). In addition, the ability to tolerate high saline concentrations in the soil, enabling later germination, when salinity values decrease due to rainfall, increases the invasive power of C. edulis and its hybrids (Podda et al 2018). The habitats colonized by Carpobrotus are the Mediterranean fixed dunes of the psammophilous vegetation series (psammophilous camaephytic vegetation codified as 2210 "Crucianellion maritimae fixed beach dunes" and psammophilous terophytic vegetation codified as 2230 "Malcolmietalia dune grasslands") not subjected to continuous high salt concentrations but to possible sea storms.…”