Abstract: The seedlings of three macadamia cultivars were used as experimental material to study their cold resistance. The changes in leaf morphology, relative electric conductivity ( REC) , malondialdehyde(MDA) content, leaf chlorophyll content, free proline content, soluble protein content, and superoxide dismutase activity ( SOD) were measured to evaluate their resistance to low temperature stress. Three macadamia cultivar seedlings showed different degrees of leaf damage symptom. HAES900 produced more frozen spots than H2 and OC. The frozen spots close to leaf vein connected, leading to death of leaves and even the whole plant. H2 only showed slightest injury symptom with some browning spots on leaf epidermis. In OC and H2,under low temperature stress, MDA contents and REC increased significantly, while chlorophyll content decreased, with chlorophyll a decreased more significantly than chlorophyll b. However, with the increasing of content of MDA, free proline content and SOD activity increased, and MDA accumulation became very slow or downtrend. After low temperature stress was relieved, the physiological indexes tended to resume to the pre-cold level. For HAES900, after being exposed to low temperature treatment for three days, REC increased significantly, content of soluble protein began to decrease, and the activity of SOD decreased. With the elongation of cold stress (6 - 9 d) , irreversible physiological changes occurred. Even after the relieve of low temperature stress, no recovery change appeared in physiological index.

Key words: macadamia, chlorophyll, superoxide dismutase, soluble protein, cold resistance