Hydrothermal time as a tool in comparative germination studies

Hydrothermal time (HTT) analysis is an effective method for quantifying germination response to temperature and water potential. Time course curves under any temperature and water potential conditions can be generated from knowledge of the HTT parameters mean base water potential, standard deviation of base water potentials, base temperature, and hydrothermal time requirement. Here we use these parameters as indices for making comparisons among 24 species including shrubs, grasses and perennial herbs from the deserts and semi-deserts of Asia and North America. Halophytes (salt-tolerant species) are characterized by low base water potential values but high hydrothermal time constants. Psammophytes (species that inhabit high-sand soils) are just the opposite, and bodenvags (generalist species with no special soil requirements) display a wide range in all parameter values. variation in the distribution of base water potentials strongly influences uniformity of germination. The results illustrate that germination rate in water or at reduced water potentials is closely associated with HTT parameters. These findings also have important ecological relevance, in that they help explain differences in germination patterns associated with contrasting habitats.