Modelling the effects water stress and temperature on seed germination of Soybean (Glycine max L.) and Velvetleaf (Abutilion thephrasti med.)

Water and temperature are determinant factors for seed germination. Both factors can, separately or jointly, affect the germination percentage and germination rate. The effect of temperature on germination and water potential on germination can often be described by the thermal time and hydrotime models, respectively. Additionally, the thermal time model and the hydrotime model have been combined into a hydrothermal time model to describe the combined effect of water and temperature on germination. The objective of this study was investigation interaction effect of five levels of temperature (7, 10, 15, 20 and 25 °C) and five drought stress levels (0, -0.2, -0.4, -0.6 and -0.8 MPa, by PEG-6000) on the germination of the Velvetleaf plant (Abutilion thephrasti med.) and two soybean cultivars (DPX and Williams) seeds. This experiment was done in during 2010 in the seed research laboratory of Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran. Results indicated that the base temperature and the thermal time to 50% of germination (°Cday-1) in each three plants increase significantly with decrease in water potential. Also, in low water potentials thermal time in the Velvetleaf was lower than two soybean cultivars. The base water potential (from -0.9 to -0.6 MPa) and hydrotime to 50% of germination (from 1.4 to 10 MPaday-1) in each three plants increase significantly with decrease in temperature from 25 to 7 °C. Generally, in low temperatures of amount hydrotime in the Velvetleaf was lower than two soybean cultivars. After fitted the hydrothermal time model the base temperature, the hydrothermal time constant and the base water potential for germination of the Velvetleaf, DPX and Williams were 3.9, 4.3 and 5.2 °C, 26.7, 26.5 and 27.3 MPa°Cday-1 and -0.9 MPa for each three plants, respectively. Results this study indicated that Velvetleaf plant in conditions of drought stress and temperatures similar had higher competition power than soybean cultivars studied.