Abstract: Thermal time model is used to examine the germination response of seeds at temperatures below and above the optimal one, while hydrotime model is used to fit the relationships between germination rate and water potential. We examined the germination responses of six alfalfa cultivars from the northern China (viz. Aohan, Dongmu 1, Gannong 3, Gongnong 1, Longmu 803 and Caoyuan 3) to temperature and simulated drought. Polyethylene glycol (PEG 6000) was used to simulate drought conditions. Germination tests were carried out at eight temperatures (5, 10, 15, 20, 25, 30, 35, and 40 ℃) and six water potentials (0, -0.2, -0.4, -0.6, -0.8, and -1.0 MPa). Results showed that different alfalfa cultivars responded differently to temperature and drought. The base germination temperatures (T_b) for the six cultivars were between -2.3 and 6.2 ℃, with Caoyuan 3 the lowest and Longmu 803 the highest. Germination rate of all the cultivars was the highest at 20 or 35 °C. 40 ℃ high temperature significantly inhibited germination of all alfalfa cultivars. The base water potentials (Y_b) of germination for the six cultivars were between -0.91 and -0.68 MPa, with Aohan the lowest and Gannong 3 the highest. Low water potentials -0.8 and -1.0 MPa greatly inhibited germination of all alfalfa cultivars. From the perspective of seed germination response, Caoyuan 3 and Gongnong 1 are more cold tolerant, while Aohan and Longmu 803 are more drought tolerant than other cultivars. These results provide important information for alfalfa cultivation and breeding.

Key words: germination rate, polyethylene glycol (PEG), thermal time model, water hydrotime model, Medicago sativa, cold tolerance, drought tolerance