Study of Dormancy and Germination Dynamics of Eryngium caeruleum L. in Response to Burial Durations in Soil

Eryngium caeruleum L. is a perennial weed from Apiaceae family that grows in the farm and roadside. There is limited information on the reaction of this plant germination to temperature and light conditions. Germination is one of the most important processes for weed success, because it is the first step in competition beginning in an ecological niche. Recognition of E. caeruleum germination and emergence ecology has an important role in weed management and control. Different environmental factors, such as temperature and light, affect dormancy and germination of the seeds. Light requirement is an indicator showing the level of seed dormancy. Given the significance of seed dormancy in weed dispersal and management, this study aimed at determining the relation between light and temperature as the most effective environmental factors in germination stimulation of E. caeruleum. An experiment was conducted to estimate the effect of temperature and light on germination of Eryngium caeruleum seeds at agriculture and Natural Resources College, university of Tehran in 2016. A factorial experiment was undertaken based on completely randomized block design with three replications. Factors were germination temperatures (5, 10, 15, 20, 25, 30 and 35 °C), burial periods in 5 cm depth (30, 60, 90, 120, 150, and 180 days), and light conditions during germination test (Presence and absence of light). The seeds were buried in the soil under environmental conditions at depth of 5 cm and then removed from the soil at one month intervals and were placed at dark and light (Osram, L 18 W/77, 20 µmolm-2s-1) conditions under seven temperature treatments for 14 days. 25 seeds were placed in each petri dish and the germinated seeds were counted for 14 days in each month. Segmented model was used for studying the germination rate to temperature in light and temperature treatments. Thermal parameters of germination were calculated for light and temperature treatments. These parameters were Gmax for the maximum germination rate, Tb or base temperature for germination, To or optimum temperature for germination, and Tc or ceiling temperature for germination. The base and ceiling temperature parameters were also calculated. The findings of this study shed light on seed germination requirements of E. caeruleum and enhance our understanding of this species distribution. These results will be useful in selecting the best time and method for E. caeruleum control. Results showed that temperature and light conditions during germination test influenced germination rate. According to the analysis of variation, all simple effects and interactions were significant on germination rate. Germination rate increased with increasing temperature up to 20 °C. Germination rate, however, decreased when temperature increased up to 35 °C. Temperature parameters of base temperature, optimum temperature and ceiling temperature were affected by burial periods and light conditions during germination test. In general, base and optimum temperatures decreased and ceiling temperature increased with increasing of burial time. The burial time needed for 50 percent decrease of maximum base temperature in the lowest burial time was 117 days in light condition and 82 days in dark condition. The optimum temperature decreased from 23.5°C to 21.2°C in light condition and from 23°C to 21°C in dark condition. The ceiling temperature increased from 28°C to 42°C in light condition, but it had different process in dark condition. The Tc parameter was 30°C in 60 days of burial and was 28°C up to 120 days of burial and had no changes up to 180 days of burial. Parameters of Gmax and ai, which indicate the maximum germination rate and seed non-dormant index, respectively, rose linearly by increasing time of seed burial, irrespective of light condition during germination test. The same results were found in Circium arvense and Bidens pilosa, that their germination thermal parameters were higher in light condition than in dark. This can be attributed to the heat made by the light. The results of the experiment on Plygonum aviculare also showed the effects of light on cardinal temperatures of germination and the start of germination of this plant. The results showed that the thermal parameters of Eryngium caeruleum germination including base temperature (Tb), optimum temperature (To), and ceiling temperature (Tc) changed during the burial periods. The Gmax and awakeness index parameters increased linearly regardless of the presence or absence of light. It can be also concluded that Eryngium caeruleum L. has morphophysiological dormancy and dormancy breaking. Moreover, the optimum temperature range for germination mostly rises by increasing ceiling temperature.