Abstract:The membrane fluidity of seedling mitochondria of chilling-sensitive rice and that of chilling-tolerant rice were compared by using spin labeled stearic acid: 5, 12 16-NS and fluorescent probe DPH. From the ESR spectra using 5-NS as a spin labeled probe it clearly showed that the calculated order parameter (S) of seedling mitochondria of chilling-sensitive rice Qiu Guang was obviously higher than that of chilling-resistant rice Ji Geng 44. Similar results were obtained when seedling‘mitochondria of another species of chilling sensitive Zao Jin were compared with those of chilling tolerant rice Ji Geng 60. Moreover, the difference of order parameters between Ji Geng 44 and Ji Geng 60 was quite small, but both of them are obviously lower than those of chilling-sensitive rice Qiu Guang or Zao Jin. The results using spin labeled probe 12-NS, 16-NS clearly showed that the relative correlation times (τc) of seedling mitochondria of chilling-sensitive rice Qiu Guang or Zao Jin was markedly higher than that of the chilling tolerant rice Ji Geng 44 or Ji Geng 60. A comparison of membrane fluidity of seedling mitochondria of chilling-sensitive and chilling-resistant rice using fluorescent probe DPH was also carried out. Similar results were obtained and showed that the fluidity of mitochondrial membrane of chilling resistant rice seedling was obviously higher than that of the chilling-sensitive ones. Thus, it seemed that the fluidity of mitochondrial membrane might be used as a biophysical test for screening chilling tolerance of rice at seedling stage.