Three phylogenetically distant shade-tolerant temperate forest herbs have similar seed germination syndromes

Plants growing in the same environment typically share ecophysiological characteristics enabling them to survive and reproduce when exposed to a common selection pressure. As such, seedlings establishing in temperate forest habitats cope with similar issues of low light availability and climate seasonality. We examined similarities in the germination syndromes in three shade-tolerant temperate forest herbs, Allium ursinum (Amaryllidaceae), Mercurialis perennis (Euphorbiaceae) and Dioscorea communis (Dioscoreaceae) with a distant phylogenetic affiliation. Seeds of M. perennis and A. ursinum are dispersed in spring and those of D. communis in autumn. Experiments on phenology of seed germination and seedling emergence in natural conditions revealed that seeds of all three species germinate in the following autumn. Seedlings do not emerge immediately but rather remain below the soil surface until late winter or early spring. Experiments in controlled laboratory conditions showed that seeds germinated best at intermediate autumn or spring temperatures (between 10 and 15°C) following a period at high summer temperatures (around 20°C). Seed germination of Allium ursinum is strongly photoinhibited, while moderate photoinhibition and no photoinhibition at all was observed in Dioscorea communis and Mercurialis perennis, respectively. Although seeds of all three species are endospermic at dispersal, no embryo growth prior to germination was observed. The cotyledons functioned as a haustorium, recuperating the nutrient reserves in the endosperm post-germination. It can be concluded that although phylogenetically unrelated, the three species studied show a remarkable similarity in germination and seedling emergence strategy that is commonly observed in plants adapted to growing in shady conditions.