Modelling the germination of Stellaria media using the concept of hydrothermal time

The germination characteristics of Stellaria media (common chickweed) were investigated over a range of constant temperatures and degrees of moisture stress in order to assess the suitability of hydrothermal time as a basis f or modelling germination under field conditions. Maximum percentage germina tion occurred over a much narrower temperature range around the optimum tem perature than previously seen for cultivated crop seed. The entire final pe rcentage germination response to temperature in water was well described by two probit curves, and this model mas extended to describe the data at all water potentials at a temperature dose to the optimum. The implications of the reduction in germination at nonoptimal temperatures are discussed with respect to the interpretation of germination progress curves and condition al dormancy. After adjusting for maximum percentage germination, a hydrothe rmal time model was found to fit the data set well within the conditions no rmally encountered in horticultural seedbeds. This separation of the final percentage germination presents a flexible modelling approach that allows f or the different levels of dormancy, typically expressed within weed popula tions. By contrast with many previously reported specics, S. media had a sy nchronous germination rate within the population at any given temperature/w ater potential combination. This synchronous germination of at least a prop ortion of the population over a wide range of temperature and water potenti als might have ecological significance for the opportunistic germination be haviour of this weed species.