Influence of heat on seed germination of nine woody Cistaceae species

Abstract

The influence of high temperatures on germination of four Cistus and five Halimium taxa is analysed. Seeds were heated to a range of temperatures (from 50ºC to 150ºC) and a range of exposure times (from 1 to 60 min), simulating those heat conditions registered on the soil surface during wildfires. After the thermal pretreatments, seeds were sown in plastic Petri dishes and monitored for germination over 60 days. For all the species, germination was increased significantly over the control set (untreated seeds) by at least one of the thermal pretreatments. Temperatures of 120 and 150ºC were the most efficient temperatures promoting seed germination, although germination was interrupted at 150ºC if exposure times were equal to or longer than 5 minutes for the majority of species. The preheating at 50ºC was effective on;y for C. populifolius, but only at long exposure times (60 min). H. atriplicifolium had the highest heat requirements for stimulating germination, requiring at least 120ºC for 5 min. H. halimifolium subsp. halimifolium seeds showed the highest heat resistance: the final germination level reached at 150ºC 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 those of the untreated seeds. In general, species sharing the same habitat showed different heat requirements in promoting germination. For C. crispus and H. halimifolium subsp. halimifolium, the experiment was carried out on seeds collected from two different localities. For both species the germination patterns were similar between populations, although some high temperature pretreatments showed different germination percentages.