Physiological And Biochemical Process Of Freezing Tolerant Improvement In Seedlings Of Ammopiptanthus Mongolicus Under Cold-hardening

Ammopiptanthus mongolicus is the only evergreen broadleaf shrub endemic to the Alashan desert, northwest sand area of China, and can survive–30oC or even lower temperature in winter. In order to investigate cold acclimation process of A. mongolicus, the seedlings was acclimated at 2℃using 14 hours photoperiod (150μmol m-2s-1 photosynthetically active radiation) for 14 days. Seedling freezing tolerance increased after 15 d of cold-hardening.Cytochemical localization of phenolics in cotyledon cells was observed by electron microscopy using caffeine, which forms electron-dense deposits. Results showed that phenolics were mainly localized in the vacuole and in close proximity to tonoplast. Deposits also could be observed in the cytoplasm. No phenolic deposits were found in cell walls. After cold hardening, the amount of phenolics increased obviously in vacuole and in close proximity to tonoplast in cotyledon cells. Spectrophotometric measurement revealed that the content of phenolics in cotyledons of Ammopiptanthus mongolicus grown at 25℃was 5.3 mg/gFw, whereas in the cold-hardening seedlings it increased to 8.4mg/gFw. As hydrogen- or electron-donating agents, phenolics protect plant cells against reactive oxygen species formed during chilling or freezing stress. So these results suggested that the amount of phenolics may be closely associated with the cold hardiness of Ammopiptanthus mongolicus seedlings.Patterns of isozymes peroxidase, catalase ,PPO and superoxide dimutase, as well as the contents of protein, proline and soluble carbohydrates were investigated.The activities of SOD, peroxidase ,and catalase in cold acclimated plants were increased.New isoenzyme bands appeared after cold acclimation. Protein, proline,and solute carbohydrate contents increased in the seedlings after cold acclimation.