Halothermal and hydrothermal time models describe germination responses of canola seeds to ageing

Hydrothermal time (HTT) and halothermal time (HaloTT) models were used to quantitatively characterize the combined effects of temperature (T), water potential (ψ) and NaCl concentration on seed germination of canola after different accelerated ageing periods (AAP) at 42 °C and 100% humidity. Seed germination time courses were observed and electrical conductivities of seed leachates were measured in all experiments. The cardinal temperatures estimated by both models were 4.6, 28 and 35 °C for the base (T_b ), optimum (T_o ) and ceiling (T_c ) temperatures in water (0 MPa), respectively, when the seeds were not aged. The T_b increased with AAP, while the T_c decreased and the T_o remained constant at all AAP. Below T_o , the median base water potentials (ψ_b (50)) were -1.07, -0.73, -0.48 and -0.39 MPa for the AAP levels of 0, 24, 36 and 48 h, respectively. These values were more negative when germination occurred in salt solutions (the base NaCl concentrations converted to ψ were -1.25, -0.81, -0.51 and -0.41 MPa, respectively), due to uptake of salt ions. Thresholds became more positive above T_o and reached zero at T_c in all AAP. Seed osmotic adjustment capacity declined linearly with increasing AAP while the conductivity of seed leachates increased in association with the loss of seed vigour. Significant correlations between conductivity results and the parameters of HTT and HaloTT models suggest that they could be considered as effective descriptors of canola seed vigour.