Breaking Seed Dormancy and Determining Cardinal Temperatures for Syrian Bean-Caper (Zygophyllum fabago L.) Using Regression Models

 Syrian bean-caper is one of the important weeds in orchards, especially pistachio orchards in Kerman. Seed germination is a critical event in determining the success of a weed species in an ecosystem and is regulated by several factors such as temperature, light, soil salinity, moisture and pH. Seed dormancy is the most important factor that prevents seed germination, especially in weeds. Breaking seed dormancy by sulfuric acid scarification is one of the most common methods of stimulating seeds for germination. Temperature is one of the factors controlling the germination and distribution of plants in natural and agricultural ecosystems. All biological events respond to temperature, and all responses can be summarized in three main temperatures. The effects of temperature on plant development are the basis of the models used to predict germination time. The seeds of any species could germinate in a certain temperature range, which is described as the cardinal temperatures (minimum, optimum and maximum) Knowledge of seed germination characteristics, seed dormancy mechanism and response of these phenomena to environmental conditions in weeds, including Syrian bean-caper, can be useful in predicting its spread potential to other areas and its management, so the aim of this study was to identify the methods of dormancy breaking (knowledge of seed dormancy mechanism), the effect of light and alternating temperature on seed germination and also to determine the cardinal germination temperatures of this species.

 To evaluate the effect of some methods on breaking seed dormancy, and to evaluate the effect of alternative temperatures and light on seed germination and to determine the cardinal temperatures of Syrian bean-caper seed germination, experiments were performed in the weed research laboratory of Ferdowsi University of Mashhad in 2016. The experiments included: Experiment 1: Evaluation of different treatments for breaking Syrian bean-caper seed dormancy: This experiment was performed as a factorial in a completely randomized design with three replications. The first factor is constant germination temperatures at five levels (15, 20, 25, 30, 35 °C) and the second factor is different methods of breaking seed dormancy at 5 levels (without treatment (control), concentrated sulfuric acid (98%) at times 10, 15, 20, 25, 30 and 35 minutes, seed stratification at 5 °C for 7 and 22 days, water soaking the seeds at 25 °C for 7 days and seed scarification using sandpaper for two minutes. Experiment 2: The effect of fluctuating temperatures, light regimes and breaking seed dormancy treatment on Syrian bean-caper seed germination: This experiment was performed as a factorial of three factors in a completely randomized design with three replications. The first factor is alternating temperatures at four levels (10/20, 25/15, 10/30 and 20/30 °C (Day/Night)), the second factor is the light regime at two levels (light and dark treatment (12 hours of light and 12 hours of darkness) and continuous dark and the third factor was the breaking seed dormancy treatment with concentrated sulfuric acid, (the best treatment of the first experiment) at two levels (application and non-application). Experiment 3: Determining the cardinal temperatures of Syrian bean-caper seed germination, this experiment was performed completely randomize design in three replicates. Experimental factors included constant temperatures of 0, 5, 10, 15, 20, 25, 30, 35 and 40 ° C with a light / dark period of 12/12 hours. SAS 9.1 software was used to analysis of variance and compare different treatments. To calculate the germination cardinal temperatures were used, 1. Segmented 2. Dent-like and 3. 4-parameter beta models.

The effect of breaking seed dormancy treatments, temperature and interaction of temperature and breaking seed dormancy treatments on Syrian bean-caper seed germination were significant at 1% level. Maximum germination (96%) was obtained in sulfuric acid treatment for 15 minutes. The results of this study showed that all of treatments have a significant effect (at 1% level) on seed germination rate and total seed germination of Syrian bean-caper. Seed germination of Syrian bean-caper was in the range of 10 to 35 ° C. Seed germination increased with increasing temperature from 10 to 40 °C. Because this plant is native to dryland, it also had significant seed germination at high temperatures. The best temperature for seed germination of this plant was between 20.5 and 25.5 °C. Seed germination stopped at temperatures below 5 °C and at 40 °C. Besides, seed germination was strongly affected by alternating temperatures. Light had not impact on the seed germination of Syrian bean-caper. The highest total seed germination was observed in breaking seed dormancy treatments related to sulfuric acid treatment for 15 minutes at 30 °C and in alternating temperature of 25/15 °C (Day/Night). The four-parameter beta model described the relationship between seed germination rate of Syrian bean-caper and temperature, better than other models. Based on the estimation of this model, the base, optimum and ceiling temperatures for Syrian bean-caper seed germination were 9.83, 16.33 and 39.29 ° C, respectively. Knowledge of these ecological parameters can be useful in quantifying the behavior of weeds in response to various climatic variables, especially temperature. 

The wide range of Syrian bean-caper seed germination from 5 to 35 °C indicates the ability of this weed species to grow in different seasons and in various climatic conditions. Considering the mechanisms affecting the breaking seed dormancy of Syrian bean-caper seeds (remove of seed coat (hardness) and alternating temperatures) as well as the temperature range of seed germination of this plant, it seems that the spread of this plant would be possible in temperate and semi-arid regions. Also, due to the high competitiveness of this species and the ability to produce abundant seeds, this weed might be included in the list of problematic weeds in these areas.