An after-ripening thermal-time model for Lithospermum arvense seeds based on changes in population hydrotime parameters

Lithospermum arvense seeds show primary physiological dormancy. Changes in population hydrotime parameters during after-ripening were used to model primary dormancy loss. Lithospermum arvense seeds were dry-stored at constant temperatures of 5, 15, 24 and 30 degrees C for 180 days. After different storage periods, seeds were incubated at 10 degrees C at a range of water potentials (0 to) -1.2 MPa). Experimentally obtained cumulative-germination curves were analysed by repeated probit regressions to obtain seed population hydrotime parameters. The population mean base water potential (Psi(b)(50)) showed a progressive decrease as after-ripening time progressed and the dormancy release rate was positively related to storage temperature. The hydrotime constant (theta(H)) and the SD of the base water potential (sigma(Psi b)) were unaffected by after-ripening time or storage temperature. To account for the effect of after-ripening time and temperature on dormancy release, an after-ripening thermal-time model was developed. The model consisted of the description of Psi(b)(50) changes as a function of an after-ripening thermal-time index (theta(AT)). An exponential decay function accurately described (R(2) = 0.92) the decrease pattern of Psi(b)(50) as function of theta(AT). Model evaluation under fluctuating soil water regimes showed a good correlation between observed and predicted data (r = 0.94 and 0.96). This indicated that the after-ripening process could be adequately described as a thermal-time response, further suggesting the potential applicability of the model to predict L. arvense emergence in the field.