Elevation difference of Ca²⁺ levels in young leaf cells of bromegrass and induced cold-tolerant enhancement under different controlled chilling temperatures

The changes of Ca²⁺ levels in young leaf cells of bromegrass under different controlled chilling temperatures were investigated by an antimonite precipitation cytochemical method. The main results were as follows:under 25/20℃ (day/night) temperature and 14 h photoperiod, electron-dense Ca²⁺ antimonite precipitates, indicators of Ca²⁺ localization, were mainly localized in the vacuoles, cell walls and intercellular spaces; few Ca²⁺ deposits were observed in the cytosol and nuclei. After a 3℃ chilling treatment for 3 h, many Ca²⁺ precipitates appeared in the cytosol and nuclei, indicating that Ca²⁺ influx had occurred in the cytosol and nuclei. When the 3℃ treatment was prolonged to 8 h, more Ca²⁺ deposits appeared in the nuclei and cytosol, but the amount of Ca²⁺ deposits in both the cytosol and nuclei decreased markedly after a 24 h treatment and most Ca²⁺ deposits were returned to the vacuoles and intercellular spaces after an 8 d treatment. When bromegrass was exposed to 7℃ for 3 h, the Ca²⁺ distribution in the cells had no visible changes, compared with that of the 25/20℃ grown control plants. However, when the chilling treatment of 7℃ was prolonged to 8 h, a Ca²⁺ influx occurred, where many Ca²⁺ deposits were observed in the nuclei and cytosol. More Ca²⁺ deposits appeared in the nuclei and cytosol after a 24 h treatment, but the amount of Ca²⁺ deposits in the cytosol and nuclei was reduced markedly after an 8 d treatment. After a 14 d treatment, the remaining low level of Ca²⁺ was recovered in both the cytosol and nuclei and the Ca²⁺ deposits were again located in the vacuoles and the intercellular spaces. The dynamics of subcellular Ca²⁺ localization in young leaf cells of bromegrass during a 12℃ chilling treatment were similar to those of the 7℃ treatment. Besides, the results showed that the frost tolerance of bromegrass exposed to 3℃ for 8 d increased by 6℃, for 7℃ and 8 d by 4℃ and for 12℃ and 14 d by 3℃, compared with the controls. Finally, the relationship between different Ca²⁺ dynamics and induced frost tolerance was also explored.