Determination of cardinal temperatures of Ziziphus spina-christi and quantification of its characteristics under moisture and salinity stresses

Seed germination is usually the most critical factor in determining the success or failure of plant establishment. The most important environmental factors affecting germination, establishment, and growth of plants are temperature and humidity, which have significant effects on germination characteristics such as germination percentage and rate. For this purpose, to determine the cardinal temperatures of seed germination in Ziziphus spina-christi, in vitro experiment, the germination characteristics of the seeds of this plant at different temperatures were evaluated in a completely randomized design with four replications. Germination response was assessed at the constant temperatures of 5, 10, 15, 20, 25, 30, 35, 40, and 45°C. The cardinal temperatures for the germination of Ziziphus spina-christi seeds were fitted using germination rate and by three models including the beta, segmented, and dent-like models. To select the superior model and measure the accuracy of the model in describing the germination rate of Ziziphus spina-christi seeds for temperature, the statistics of root mean square (RMSE) and coefficient of determination (R2) were used. Accordingly, a higher R2 and a lower RMSE represent a closer correlation of the model with reality.  Based on the results, these statistics were better for the segmented model than the other models, so that they were 0.0004 and 98% for this model, respectively. Therefore, based on the segmented model, which was known as the superior model, the basal temperature, the optimum and maximum germination rate of this plant were calculated to be 8.6, 34.3 and 49 ° C, respectively. Then, the response of germination and seedling growth of Ziziphus was explored in another experiment to different levels of salinity and drought stresses at the optimum temperature.  In this experiment, the seed germination was studied at four levels of salinity and drought stresses with osmotic potentials of 0, -2, -4, -6, -8, and -10 bars. The results of this experiment showed that salinity and drought stress reduced the germination percentage and rate of Ziziphus spina-christi seeds. Germination percentage for control was 97% while in -10 bar salinity and drought stress was 16% and 0, respectively. In addition, compared to drought, seed germination showed more sensitivity to salinity stress.