The research was made under the investigation of hybrid citrus cultivar\’s cultivation and production in Sichuan province.The hybrid citrus cultivar\’s frozen fruits were taken as the experiment material.The separation purification and identification of ice nucleation active bacteria,was used to search for the species of INA bacteria and to detect its activity in hybrid citrus cultivar of Sichuan,and to study the factors influencing ice nucleation activity of INA bacteria.The back inoculation of INA bacteria to hybrid citrus cultivar \’Skiranui tangerine\’ normal fruits,and treating the fruits under tress of low temperatures through simulated frost experiment in laboratory,were used to observe and measure the change low of REC,MDA,SOD,POD,CAT,thus explore the relation between INA bacteria and frost injury of fruits,so as to reveal the mechanism which INA bacteria induced frost injury of fruits.This research paper draws the following main conclusions.1.Twenty samples of fruit frost from hybrid citrus cultivars were collected in Sichuan province.Thirty-one strains of bacteria were isolated from these twenty samples in this test, from which sixteen ice nucleation active bacterial strains were fond.Those showed that INA bacteria were existent in four kinds of hybrid citrus cultivars,and it was first discovery of INA bacteria in hybrid citrus cultivar.Freezing percentage to isolated ice nucleation active bacterial strains were above 85%except for No.3 strains,P.syringae pvs.\’s freezing percentage were higher than E.ananas\’s.2.By bacteriological determination of sixteen ice nucleation active bacterial strains, those were fond that six stains were identified as E.ananas.Other,six stains were identified as P.syringae pvs,while the others were P.viridiflava.Assay of ice nucleation activity showed that the activity of the Pseudomonas strains were higher than the Erwinia ones.This was first report about INA bacteria studied in hybrid citrus cultivars.3.The back inoculation of INA bacteria to hybrid citrus cultivar \’Skiranui tangerine\’ normal fruits,and treating the fruits under stress of low temperatures through simulated frost experiment in laboratory,the change law of Relative electric conductivity was: P.syringae pvs.＞P.viridiflava＞E.ananas＞CK,and the changes increased with decreasing temperature.By significance analysis on all data,we knew:the REC ofpericarp inoculated with INA higher than those of CK(the non-inoculated),this showed INA bacteria could make the frost injury of fruits worse under the stress of same low temperatures,and make the frost temperatures improved 1～2℃.We also fond the effect of different INA was different to aggravate frost injury of fruits,it related to the activity of INA bacteria.There was obviously positive correlation between the degree of frost injury to fruits and the sizes of activity of INA bacteria.So,our results demonstrated that INA bacteria are one major factor to incite frost damage to fruit.4.The change law of MDA was:P.syringae pvs.＞P.viridiflava＞E.ananas＞CK.The MDA of pericarp inoculated with INA was higher than those of CK(the non-inoculated), and the change tendency of MDA was different under treated pericarp by different INA. Those showed that,under stress of low temperatures,INA bacteria could promote obviously Membrane-Lipid Peroxidation in pericarp,and decreased the cold hardiness of \’Skiranui tangerine\’fruits.By significance analysis,the results showed,the difference of MDA content was insignificant between the pericarp inoculated with INA and the noninoculated under treating on 0℃,but,with decreasing temperature,the Difference gradually became significant,on -3℃the difference reached very significant level.All of these showed that INA bacteria really aggravated the frost injury of fruits,and the three kinds INA bacteria had different ice nucleation activity.5.The determination results of Protective Enzyme Activity of friuts,whether pericarp or flesh,the change tendency of SOD activity was similar in all treatments.And SOD activity of pericarp was higher than these of flesh,and whole stress of low temperatures process,SOD activity of CK was higher than these of every treatment.With decreasing temperature,the difference gradually became more significantly.By significance analysis on all data,the difference reached very significant level among the SOD activity of each treatment pad.So,compared with CK,the treatment of inoculated with INA could rise the temperature 2℃on which the membrane was damaged.All of these showed that INA bacteria could decrease the SOD activity of \’Skiranui tangerine\’ fruits,and then,decreased the protective effect of SOD to membrane.Second,the POD activity of pericarp increased with decreasing temperature,whole stress of low temperatures process,POD activity of CK was higher than these of every treatment.With decreasing temperature,the change tendency of gradually became more significant.By significance analysis on all data,on 0℃the difference of POD activity was insignificant among each of treatment pad.But,with decreasing temperature,the POD activity of pericarp inoculated with INA gradually became significant lower than the CK, and these of P.syringae pvs were significant lower than these of P.viridiflava.Those showed that,INA bacteria could decrease the POD activity,made the hydrogen peroxide accumulation and aggravated the action which low temperance damaged membrane.The activity of CAT decreased with decreasing temperature.CAT activity of CK was always higher than these of every treatment,and the change tendency of CAT activity was different among each of treatment pad.In addition SOD activity of pericarp was higher than these of flesh.We could know that INA bacteria could decrease the CAT activity, made it not to clear effectively free radicals membrane,and then aggravated lipid peroxidation effect.6.Analyzing the factors influencing of ice nucleation activity of INA bacteria,we fond,the culture temperature of 20℃could make INA bacteria grow better,and its freezing percentage was the most,ice nucleation activity of INA bacteria was the biggest. The culture temperatures whether increase or decreased also damage the growth and ice nucleation activity of INA bacteria.Second,while bacterial concentration didn\’t change, freezing percentage increased significantly with decreasing treatment temperature.When it was less than 2℃,the ice nucleation activity of INA bacteria became significant.Third, while bacterial concentration reached 5×10~6cfu/ml,INA bacteria had ice nucleation activity,and freezing percentage increased with increasing bacterial concentration.There was obviously positive correlation between ice nucleation activity of INA bacteria and bacterial concentration.Forth,when the temperature and bacterial concentration didn\’t change,freezing percentage also obviously changed with changing pH of medium.We fond,the suitable pH scope of INA bacteria was pH 6.0～8.0,the medium of pH＜4 and pH＞10 also inhibited the growth of INA bacteria.

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