Modeling the effects of salt stress and temperature on seed germination of cucumber using halothermal time concept

The Halothermal time model (HaloTT) as a robust framework can accurately be used to simultaneously quantify the effect of both salt stress and temperature (T) on the dynamics of seed germination (SG) in different crops. The parameters of this model can be easily physiologically interpreted and explain how environmental factors interfere in regulating the SG response into seed population. A total of 75 seeds were used for germination tests. Five constant Ts, including 15, 20, 25, 30 and 35 °C, which at each of the following four levels of salinity (75, 150, 225 and 300 mM) were used. The effects of T and salinity on SG characteristics were studied using a HaloTT model. Also, cardinal temperatures were estimated and accumulation of Na+ and Cl− into the seeds were investigated. Results showed that the HaloTT model could quantify well the effect of salinity on SG of cucumber over sub- and supra-optimal Ts (R2 > 0.80). Based on the model, the cardinal Ts in water were 10.4, 28.7 and 38.5 °C. At To, the NaClb(50) was 337 mM equal to − 1.49 MPa when it was converted to the ψb(50). The thresholds decreased linearly at T ≥ To (slope of the linear model (kT) = 33.9 mM °C−1), then reached 0 mM at Tc while the ψb(50) increased linearly becoming less negative at T ≥ To (kT = 0.150 MPa °C−1), then reached 0 MPa at Tc. Seed Na+ and Cl− uptake increased significantly with T, and seed ψ was lower at each T under salt stress (depending on NaCl concentration), when compared to the control. Our results indicate that seed salt uptake was sufficient for germination at lower ψs under low and moderate salinity levels, and that SG is primarily controlled by the osmotic effect and, then, by the ion toxicity.