Hydration-Dehydration Effects on Germination Tolerance to Water Stress of Eight Cistus Species

Seeds in soil are often exposed to cycles of hydration and dehydration, which can prime them by triggering physiological activation without leading to germination. While this phenomenon has been scarcely studied in wild species, it may play a critical role in enhancing drought resilience and maintaining seed viability under the warmer conditions predicted by climate change. In this study, I investigated the effects of hydration-dehydration cycles on germination response under water stress in eight Cistus species typical of Mediterranean shrublands. First, seeds were exposed to a heat shock to break physical dormancy, simulating fire conditions. Subsequently, they underwent one of two hydration-dehydration treatments (24 or 48 h) and were germinated under a range of water potentials (0, -0.2, -0.4, -0.6, and -0.8 MPa). Six out of eight species showed enhanced germination responses following hydration-dehydration treatments, including higher final germination percentages, earlier germination onset (T₀), or increased tolerance to water stress. These findings highlight the role of water availability as a key factor regulating germination in Cistus species and evidence a hydration memory mechanism that may contribute in different ways to post-fire regeneration in Mediterranean ecosystems.