| چکیده انگلیسی مقاله |
Extended abstract Introduction: Seed germination and seedling establishment are the most sensitive stages in the life cycle of a plant. Among the environmental factors, water potential is an important factor affecting the seed germination of various plants. This research aims to evaluate the effects of water potential on germination indices and quantify the effect of water potential the germination responses of Thymus medicinal plant seeds. Materials and Methods: A factorial experiment was carried out in the form of a completely randomized design with four replications at the laboratories of the Department of Agronomy and Plant Breeding, Ilam University in the winter of 2023. The factors of the experiment included two types of Thymus (Thymus daenensis and T. vulgaris) and water potential stress induced by polyethylene glycol (PEG-6000) at six levels (0, -0.1, -0.3, -0.5, -0.7, and -0.9 MPa). Results: The results showed as the water potential decreased to -0.1, -0.3, -0.5, and -0.7 MPa, seed germination percentage respectively went down by 8.43, 43.26, 61.80, and 88.76% in T. daenensis and 19.74, 44.08, 61.18 and 92.76% in T. vulgaris compared with water potential stress-free conditions. Also, T. vulgaris did not germinate at a water potential of -0.9 MPa, whereas some seeds of the T. daenensis plant germinated under this condition. The highest germination rate in both T. daenensis and T. vulgaris species was observed under stress-free conditions, and there was significant difference between the species. Four statistical distributions including normal, logistic, log-logistic, and Gumbel, were compared to quantify the germination response of Thymus to water potential. In order to evaluate the models, corrected Akaike information criterion (AICc), the coefficient of determination (R2adj), and root mean square error (RMSE) were used. The lowest AICc index values for T. daenensis were associated with the log-logistic and logistic distributions (-2012 and -2006), and the Gumbel distribution (-1665) in T. vulgaris, suggesting the superior distributions for quantifying Thymus's response to water potential. Estimation of parameters related to the hydrotime model showed that T. daenensis species had a lower hydrotime constant value (θH)(23.91 MPa hour-1) compared with T. vulgaris (28.06 MPa hour-1), which indicated a higher germination rate in T. daenensis. The value of ψb(50) in T. daenensis (-0.455 MPa) was lower than that of T. vulgaris (-0.388 MPa). Therefore, based on the results, T. daenensis showed a greater ability to tolerate drought during the germination stage. Conclusions: In general, the results showed that the effects of water potential stress on the germination components of T. vulgaris were greater than those of T. daenensis, and according to the parameters of the hydrotime model, T. daenensis was more tolerant than T. vulgaris. Highlights: - The best distribution in the hydrotime model was determined for predicting Thymus daenensis and Thymus vulgaris seed germination under water potential stress conditions.
- The threshold level of water potential stress causing a significant decrease in the germination components of Thymus daenensis and Thymus vulgaris was determined.
- Based on the hydrotime model, Thymus species was determined to be more tolerant to water potential stress during germination.
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