Development of a hydrothermal time model that accurately characterises how thermoinhibition regulates seed germination

Thermoinhibition is the decline in germinability within a seed population as soil temperatures increase above the optimum for germination. Hydrothermal time (HTT) models have been developed that describe the thermoinhibition response as a function of increases in the threshold water potential for seed germination [seed base water potential, Psi(b)(G)]. Although these models assume a normal distribution of Psi(b)(G) and a linear upward shift in Psi(b)(G) with increasing temperature, little research has tested these assumptions. Using germination data obtained from four unrelated plant species, we fitted HTT models that use the Weibull and normal distribution to describe Psi(b)(G) and compared the accuracy and bias of these two HTT models. For all four species, Psi(b)(G) and germination were more accurately described by the Weibull than the normal distribution HTT model. At supra-optimal temperatures, Psi(b)(G) of the earliest germinating seeds showed little thermoinhibition effect so that the seeds germinated very rapidly under moist conditions. However, for the rest of the population, Psi(b)(G) increased progressively in response to supra-optimal temperatures so that the slower germinating seeds were thermoinhibited. The fitted HTT models reveal aspects of seed thermoinhibition that appear to have adaptational value under variable conditions of soil temperature and moisture.