Influence of heat on seed germination of nine woody Cistaceae species

The influence of high temperatures on germination of four Cistus and five H alimium taxa is analysed. Seeds were heated to a range of temperatures (fro m 50 degreesC to 150 degreesC) and a range of exposure times (from 1 to 60 min), simulating those heat conditions registered on the soil surface durin g wildfires. After the thermal pretreatments, seeds were sown in plastic Pe tri dishes and monitored for germination over 60 days. For all the species, germination was increased significantly over the control set (untreated se eds) by at least one of the thermal pretreatments. Temperatures of 120 and 150 degreesC were the most efficient temperatures promoting seed germinatio n, although germination was interrupted at 150 degreesC if exposure times w ere equal to or longer than 5 minutes for the majority of species. The preh eating at 50 degreesC was effective on;y for C. populifolius, but only at l ong exposure times (60 min). H. atriplicifolium had the highest heat requir ements for stimulating germination, requiring at least 120 degreesC for 5 m in. H. halimifolium subsp. halimifolium seeds showed the highest heat resis tance: the final germination level reached at 150 degreesC for 7.5 minutes ranged between 52.5 and 55.5%. The germination rates after preheating were much lower than in mechanically scarified seeds, and closely resembled thos e of the untreated seeds. In general, species sharing the same habitat show ed different heat requirements in promoting germination. For C. crispus and H. halimifolium subsp. halimifolium, the experiment was carried out on see ds collected from two different localities. For both species the germinatio n patterns were similar between populations, although some high temperature pretreatments showed different germination percentages.