Sideritis incana, Stachys ocymastrum, and Thymus fontanesii are medicinal plants belonging to the Lamiaceae family and occurring in semi-arid lands in northern Algeria and in many other countries along the Mediterranean coastline. Despite the ecological and economic interests and also the questionable future these species may meet in their natural habitats, various aspects of their seed biology have not been recognised to this date. This study was intended for in situ conservation of these plants. The problem investigated was the germination response of seeds to different water potential levels attained with using different amounts of polyethylene glycol (PEG6000) (0, –0.03, –0.07, –0.2, –0.5, –1 and –1.6 MPa). In this way, the appropriate conditions and the threshold tolerance of seed germination against water stress were assessed. Seeds of the three species lacked primary/innate dormancy and they germinated abundant and fast in distilled water (S. incana – 65%; S. ocymastrum – 60% and T. fontanseii – 90%). Small seeds of T. fontanesii tolerated more water stress and germinated under up to –1 MPa (–10 bars). Large seeds of S. incana and S. ocymastrum, however, were more sensitive to the drought stress and germinated only under –0.5 MPa (–5 bars). Moreover, more studied parameters were found developing negative reponse under rising drought stress, such as postponed triggering of seed germination, decreased germination velocity and prolonged germination duration, as well as the average time of germination. Therefore, these three plants appear to postpone their establishment until arrival of conditions promising for germination, including sufficient rainfall.
