Overcoming physical dormancy: Optimizing seed germination in Helianthemum and Cistus species for desert truffle mycorrhizal seedling production

Seed germination is a critical stage in the plant reproductive cycle, significantly influencing species fitness. Variations in germination rates are often considered adaptations to specific ecological conditions. Understanding this process is essential for developing effective conservation strategies, particularly for producing mycorrhizal seedlings in greenhouse settings. A key challenge in desert truffle cultivation is the low germination rate of seeds from their host plants, which impedes the establishment of mycorrhizal plants under greenhouse conditions. This study investigates the germination behaviour of four Helianthemum species (H. lippii, H. salicifolium, H. ledifolium, H. almeriense) and two Cistus species (C. ladanifer, C. laurifolius) under controlled conditions. Three pre-sowing treatments, manual scarification, soaking intact seeds in distilled water for 24 hr at ambient temperature and sulphuric acid treatment were applied. Among all Helianthemum and Cistus species tested, manual scarification yielded the highest germination percentages. While other pre-sowing treatments promoted germination in some species, none matched the efficacy of manual scarification. The consistently high germination rates following mechanical disruption of the seed coat suggest that dormancy is primarily governed by seed coat impermeability. Physical dormancy, resulting from an impermeable seed coat, appears to be the primary cause of low germination in untreated seeds of the studied Helianthemum and Cistus species.