Quantification of Retama raetam seed germination response to temperature and water potential using hydrothermal time concept

Retama raetam (Forssk Webb; Fabaceae) is a xerophytic shrub that is distributed from northern to southern Tunisia and which is of interest for medical use and for revegetation. To quantify seed germination responses of R. raetam to temperature (T) and water potential (psi), and to determine cardinal Ts, a hydrothermal time model (HTT) was used. The experiment was conducted at six constant Ts (5, 10, 15, 20, 25, and 30 degrees C) at each of the following psi s (0, -0.3, -0.7 and -1 MPa; provided by PEG 6000). Results showed that germination rate (GR) and germination percentage (GP) were significantly influenced by psi, T and their interactions (P < 0.01). Cardinal Ts for R. raetam was 4.2 degrees C for T-b (the base T), 17 degrees C for T-o (the optimum T) and 30 degrees C for T-c (the ceiling T) in the control condition (0 MPa). The T-d (the T at which psi(b(50)) starts to change) was estimated to be 10 degrees C. By increasing T to T-o, hydrotime constant (theta(H)) decreased nonlinearly and then stabilized to 30 MPa degrees C h at Ts ranging from 15 to 30 degrees C. The study showed that when the HTT model is applied, it can accurately describe the time course of germination across all Ts and psi s for this plant (R-2 = 0.90). Moreover, estimated parameters using HTT model were 388.6 MPa degrees C h for the hydrothermal time constant (theta(HTT)), -0.913 MPa for the base water potential (psi(b(50)))and 0.047 MPa degrees C h for the k(T) (the slope of the relationship between psi(b(50)) and T >= T-d). The parameters presented in this study can easily be used in R. raetam SG simulation models for characterizing the physiological status of R. raetam seed populations and also for predicting germination time courses of this plant under different environmental conditions.