Effects of Temperature, Salinity, and Planting Depth on Seed Germination and Emergence of Tall Morningglory (Ipomoea spp.)

Introduction: Tall morning-glory is an annual broadleaf weed and a problem in many crops globally. Tall morning-glory belongs to the family Convolvulaceae. It is a summer climbing vine with distinctive, heart shaped, alternating leaves and large, showy, white to pink to dark purple flowers. It is a very competitive weed in annual crops. This plant was introduced as an invasive plant in soybean fields of Golestan province. Because of the lack of these weed management recommendations, every year large quantities of crops have been lost. Germination is one of the most critical stages in weed establishment. A better understanding of the germination ecology of tall morning-glory would facilitate the development of more effective management strategies for its control. Materials and Methods: To study the effect of some environmental factors on seed germination of tall morning-glory (Ipomoea spp.), two experiments were conducted in Plant Protection Laboratory of Agriculture Jihad Organization of Golestan in 2013. In the first experiment, effect of six planting depths (1, 3, 5, 7, 10 and 13 cm) and two seed sizes (small and large) on emergence of morning glory at 30˚c as factorial block design in four replications were performed. In the second experiment, effect of salinity and temperature on the germination of seeds of morning glory were studied a completely randomized design with four replication .Temperature and Salinity stress treatments were in 7 levels (0,-0.2, -0.4, -0.6 , -0.8 and 1.2 MPa) and 7 levels (15, 20, 25,30, 35, 38 and 40˚c) respectively. Germination was monitored daily until germination ceased and the number of the germinated seeds was recorded. Seeds were observed twice daily and considered germinated when the radical was approximately >2mm long. To quantify the response of germination rate to temperature and to determine cardinal temperatures for germination, beta, and beta changed, segmented and dent models were used (Soltani et al., 2006). Water potential data showed a sigmoid trend and a three-parameter logistic model was fitted to data (Equation 3). where G is the total germination (%) at concentration x, Gmax is the maximum germination (%), x50 is the osmotic potential required for 50% inhibition of the maximum germination and Grate indicates the slope of the curve in x50. Statistical Analysis System (SAS) was used for analyzing data. Results and Discussion: Depth and interaction between seed size and depth showed significant effect on seeds emergence rate and percentage. Result indicated that higher planting depth than 5cm, reduced the percentage of seedling emergence. The highest and lowest emergence observed for 1 and 13 cm depth with 62% and 34% respectively. The interaction between seed size and planting depth, indicated that for the depth of 1 cm, the germination of large seeds were more than small seeds, but just at the depth of 13 cm the significant difference between two seed size was observed. A linear trend in reduced germination rate was observed with increasing planting depth. The lowest and the highest emergence rate with 0.013 and 0.007 seedling per hour observed for 1 and 3 cm depth. Results showed that the temperature, salinity and interaction between them had significant effects on germination percentage and rate, and the time required to reach 5% (D05), 10% (D10), 50% (D50), 90%(D90) and 95% (D95) germination. Germination of tall morning-glory occurred over a wide range of salt concentrations. According to three-parameter logistic model, the salt concentration required for 50% reduction in the tall morning-glory germination rate and percentage were -0.59 MPa and -0.73 MPa, respectively. These results indicated the germination rate is more sensitive than germination percentage to salinity stress. To describe the rate of germination to temperature and water potential, the original beta, beta changed, segmented and teeth models were evaluated. The results showed that the segmented model compared to other models better described speed germination response to temperature and water potential. Thus segmented model used to determine the cardinal temperature of tall morning glory. Based on this model, base, optimum and ceiling temperatures of the plant in control treatment were 9, 30, 42 0C respectively. The result indicated that by increasing the salt concentration, the base temperature and biological hour for germination increased and optimum and ceiling temperature decreased. For a unit reduction of water potential, base temperature and the numbers of biological hours increased 6 0C and 31 hours respectively. However the reduction per unit of water potential reduced the optimum and ceiling temperature respectively 8 and 40C. Conclusion: These results suggest that the tall morning-glory can tolerate some level of salt stress and a proportion of tall morning-glory seed may still germinate even at salinity levels up to 1 Mps. This could be an important parameter for successful adaptation in the saline areas. Based on the segmented model, base, optimum and ceiling temperatures in control treatment were 9, 30, 420C, respectively. The results of this research showed that environmental factors affected the germination of tall morning-glory, and this information could help to predict the spread of tall morning-glory in new areas