High temperature and burial inhibit seed germination of two perennial Mediterranean Euphorbia species

Abstract

Narbona E., Arista M. and Ortiz P. L. 2007. High temperature and burial inhibit seed germination of two perennial Mediterranean Euphorbia species. Bot. Helv. 117: 169 – 180. Seeds of the Mediterranean Euphorbia boetica and E. nicaeensis are produced in summer but germinate only in autumn; they are actively dispersed by ants, which may cause them to be buried. Here we tested whether the delay in germination is caused by thermodormancy, and whether seedlings are able to emerge after burial. Seeds of the perennial spurges Euphorbia boetica and E. nicaeensis were collected in four populations of southern Spain and were incubated at differing temperatures or after burial at differing soil depths. Most seeds germinated at 18–21 °C, but germination percentages were reduced at 25 °C and nearly zero at 30 °C. In E. boetica, the lack of germination at 30 °C was due to thermoinhibition, as all seeds that had not germinated at 30 °C did germinate when they were subsequently incubated at 18–21 °C. In contrast, about 15% of the viable seeds of E. nicaeensis did not germinate under these conditions, meaning that they had entered secondary dormancy. All viable seeds of E. boetica buried at 1 and 5 cm emerged, but at 10 cm emergence was significantly lower. In contrast, very few buried seeds of E. nicaeensis emerged at all burial depths, and some buried seeds entered secondary dormancy. The different germination strategies of the two species may reflect their different habitats and dispersal modes: E. boetica grows in the lowlands on sandy soils, and its seeds are always buried after ant dispersal, whereas E. nicaeensis lives in montane regions on stony and compact soils, and its seeds may be frequently dispersed at soil surface, reducing the selective pressure for seeds to germinate after burial.