A new halothermal time model describes seed germination responses to salinity across both sub- and supra-optimal temperatures

We quantified the seed germination responses of chicory ( Cichorium intybus L.; Asteraceae) to heat, water and salt stresses using hydrothermal time (HTT) and halothermal time (HaloTT) models. We extended the HaloTT model based on NaCl concentration to include supra-optimal temperatures. The HTT and HaloTT models enabled quantitative comparison of the effects of reduced water potentials and increased salinity on germination. Below 30 °C, the median threshold water potential permitting germination estimated by the HTT model (− 0.83 MPa) was higher than that estimated by the HaloTT model (− 1.30 MPa, after converting the median threshold NaCl concentration to osmotic potential). Ion uptake into seeds imbibed in salt solutions accounted for an average osmotic adjustment of 36% in the presence of salt stress compared to seeds imbibed in polyethylene glycol solutions. These thresholds became more positive above 30 °C with a common median ceiling temperature of 43 °C. The HaloTT models developed here accurately described germination responses to NaCl concentrations across all temperatures and incorporated both water potential and osmotic adjustment effects. With respect to seed germination, chicory could be considered as a moderately water stress tolerant species and highly tolerant to salt stress.