Dormancy and germination of sixRhinanthus species in relation to climate

The genusRhinanthus (Orobanchaceae) consists of annual hemiparasites that occur in a wide range of climates. Patterns of dormancy and germination were studied for six species sampled in areas ranging from the Pyrenees to Northern Scandinavia, and from sea level up to about 2500 m altitude in the Alpine region. Dormancy was broken by a 2 to 6 months period of cold stratification. Optimal temperature and length of the stratification period appeared to vary between and within species. Two patterns of dormancy and germination were distinguished. Seeds of the first group, including the widely distributedR. minor andR. angustifolius, further referred to as the LW group, require a relatively long period of cold stratification. Moreover, their germination is accelerated if they are subjected to a widening range of higher temperatures in the last weeks of the stratification period. In the other species (R. alectorolophus, R. glacialis, R. mediterraneus and perhapsR. antiquus) the release of dormancy is completed in a rather short period. Higher temperatures in the last weeks of the stratification period hardly affect the germination process of this SN-group, with short dormancy and no accelerated germination at higher temperatures. In both groups, temperatures above the low values during stratification reduce germination percentages and induce secondary dormancy in non-germinated seeds. The effects strongly vary with timing and temperature. The patterns have a genetic basis and seem to be species-specific. They do not vary with climate conditions, since samplesR. minor, whether collected in sub-arctic or sub-alpine areas or at sea level, generally react according to the LW pattern, and samples ofR. alectorolophus from areas at sea level up to montane regions according to the SN one. The ecological significance of the various stratification requirements and effects of higher temperatures on germination is discussed in relation to the local climate of the species and the evolutionary history of the genus.