Research Of Tissue Damage And Semi-lethal Temperature Of Cold Sensitive Fruits During Low Temperature Stress

Low temperature storage is one of the most effective ways to preserve post-harvest fruits and vegetables. Inappropriate storage temperature will easily have an effect on the quality of post-harvest fruits and vegetables, damaging normal physiological metabolism, forcing the internal damage of cell tissue structure. The researchers have studied chilling injury on post-harvest fruits and vegetables in resent years, mainly focusing the mechanism of chilling injury, but they couldn’t have reached an agreement.Therefore, further studies on the mechanism of chilling injury and controlling methods not only has theoretical significance, but also has important application value.By selecting cucumber, squash, eggplant as three typical cold sensitive fruits for the study, they were stored at different low temperatures. Choosing 2℃and 8℃ as low temperature stress group while taking 12℃and 20℃as critical temperature and control group. Cell wall structure, fruits’ quality, physiological and biochemical characteristics,active oxygen metabolism and antioxidant enzymes’ activity, thermal properties were measured. Meanwhile, from the aspect of chilling injury in cell membrane tissue resulting from the changes of cell membrane permeability, combined with variation of electrolyte leakage rate, which determined semi-lethal temperature of cold sensitive fruits to provide theoretical and experimental basis for further studies on the mechanism of chilling injury, reducing and preventing damage on cold-sensitive fruits. The main findings are as follows:1.Study on cold sensitive fruits’ cell tissue structure during low temperature stressUnder normal temperatures, cold sensitive fruits’ cell tissue was more closely connected and secondary cell wall was relatively intact. But under different low temperature stress, cold sensitive fruits’ inter-cellular layer thickness will vary with extension of storage time and the secondary cell wall thickness will decrease as well,the gradual degradation of the cell wall certainly caused the cell destruction andultimate death.Eggplant fruits’ PE and PG activity increased slowly at the beginning, the activity began to decline when they had been stored for 6 days. PE and PG activity of 20℃ and control group were always higher than 2℃ group. Eggplant fruits’ protopectin content at 2℃ group was significantly higher than 20℃ and control group, soluble pectin content was significantly lower than 20℃ and control group. When storage temperature was 2℃, the eggplant fruits’ cellulase activity initially increased and then decreased.Eggplant fruits’ PAL activity and lignin content increased with the extension of storage time while lignin content and PAL activity had a significant positive correlation（P<0.05）. When storage temperature was 2℃, PAL activity and lignin content increased.The fruits became stiff and had rough texture with deterioration. When storage temperature was 20℃, PAL activity had a slow growth, the lignin content remained at a low level without obvious woody corruption.2.Study on cold sensitive fruits’ quality, physiological and biochemical characteristics during low temperature stressWhen storage temperature was 2℃, the cold sensitive fruits’ firmness decreased with extension of storage time, consistently changing with chilling injury index, while showing the occurrence of browning phenomenon. When storage temperature was 12℃and 20℃, cold sensitive fruits had no obvious symptoms of chilling injury. The higher the ratio of free water and bound water, the more vigorous physiological metabolism of fruits’ tissue, the worse the resistance to low temperature stress. Squash fruits’ ratio of free water and bound water reached to 0.631 as max from the beginning, while reached to 0.596 as min when they had been stored for 12 days.3.Study on cold sensitive fruits’ antioxidant enzymes’ activity during low temperature stressWith extension of storage time, eggplant fruits’ SOD, POD, CAT, APX, GR and other antioxidant enzymes’ activity decreased, O₂^- production rate increased, H₂O₂ content increased and LOX activity increased. At 20℃ and control group, fruits’ SOD,POD, CAT, APX, GR activity were significantly higher than 2℃ group, but O₂^-production rate, H₂O₂ content and LOX activity were significantly lower than 2℃group.4.Study on cold sensitive fruits’ thermal properties and tissue damage during low temperature stressWhen storage temperature was 2℃, thermal conductivity of different varieties of cucumber fruits’ tissue increased with extension of storage time. Among them, thermal conductivity of Shenqing cucumber fruits’ tissue was significantly larger than the other two kinds of cucumbers. After comparing cucumber fruits mature 15 days with those mature 10 days and 5 days, their organizational structure was relatively loose, the cell gap was large, thermal conductivity was small, while cucumbers mature 5 days had some tightly bounded cells, the cell gap was small, and had a large thermal conductivity.At 2℃ and 20℃ group, thermal conductivity had a lesser fluctuations trend. With extension of storage time, cold sensitive fruits’ tissue cooling speed and the fall of curves varied, freezing temperatures of cucumber, squash and eggplant fruits were approximately around-2.14℃,-1.39℃,-2.85℃, having a rising trend. Fruits’ freezing temperature which they had been stored for 12 days were significantly higher,correspondingly fruits’ freezing temperature which they had been stored at 2℃ were always higher than those at 12 ℃.5.Study on cold sensitive fruits’ electrolyte leakage and semi-lethal temperature during low temperature stressCucumber fruits’ electrolyte leakage increased with falling of storage temperature and increased with extension of storage time. Different electrolyte leakage measurement units will lead to the differences in results, the methods of I and I’ had a better reflection on different varieties of fruits’ resistance to chilling injury, and identified cold resistance order, I’ with the observed results could maintain a high consistency. When the penetration time reached to 90 min, changes in cold sensitive fruits’ electrolyte leakage rate maintained stabilized, in this case the results were accurate and reliable. Also,semi-lethal temperature of Ruiqing, Shenqing and Jinyou cucumber fruits were respectively within a range of 5.35℃⁶.86℃, 6.69℃⁷.86℃ and 8.04℃⁸.96℃, cold resistance order was Ruiqing, Shenqing and Jinyou cucumber.By studying three kinds of typical cold sensitive fruits as cucumber, squash,eggplant and changes in cell wall structure under low temperature stress, PE and PG activity, PAL activity and lignin content, fruits’ firmness, the ratio of free water and bound water content, total phenol, soluble protein content, soluble sugar content, proline content, PPO activity, SOD, POD, CAT, APX, GR and other antioxidant enzymes’ activity, LOX activity, O₂^- production rate, H₂O₂ content, thermal conductivity, freezing and super-cooling point. The combining effects of parameters and analyzing electrolyteleakage rate caused by changes of membrane permeability has influenced variation of thermal characteristics of cell tissue, and explores the correlation between cold sensitive fruits’ tissue damage and thermal characteristics of their own adaptability and resistance to low temperature stress. This study provides a theoretical basis and reference data base.Meanwhile, applying electrolyte leakage rate with the Logistic equation for determination of semi-lethal temperature of cold sensitive fruits provides an important theoretical basis and practical value.