| Apple fruit sunburn, a common physiological disorder caused by higher temperature and intense sunlight, occurs everywhere in main apple-producing areas on the world every year, resulting in a severe loss of both fruit yield and quality. As new dwarfing rootstocks suitable to the Chinese climate and soil conditions are popularized, together with improvement of our environment and the influence of global warming-up caused by "greenhouse effect", this disorder will become a severe problem which undoubtedly affect the apple industry in China. Therefore, a study is needed urgently in this field.The prerequisite for fruit sunburn occurrence is that, fruit surface have to reach the "threshold temperature" which depend largely upon air temperature and solar radiation at the moment. However, the study on physiological aspect of fruit sunburn is very limited so far. The present test is undertaken to study the response and mechanism of apple fruit to higher-temperature and intense-light stress by means of application of basic principles and methods in modern biology. The main results are as follows:1. Under natural conditions, there is a considerable difference in physiological status of fruit on different exposures on a canopy. The higher-temperature and intense-light stress frequently happens on the southwest exposure. In a clear and sunny day, the light intensity received by the fruit on southwest canopy is 30,000~60,OOOLux more than those from northeast, east, north and northwest exposures, and maximum fruit surface temperature is 5"C~6.5*C higher. Under sub-lethal temperature stress, the contents of O2- and MDA as well as the activities of SOD and POD in the fruit on southwest canopy are significantly higher than those from other exposures.2. The changes of O2- SOD, POD and MDA in fruit peel are sensitive to higher-temperature and intense-light stress. Under sub-lethal temperature stress, O2-can be generated quickly and the peak occurs within Ih immediately after the stress. Moreover, it can increase the activities of SOD and MDA, which may attain themaximum values at about 3rd- and 5th h, respectively. If light stress happens, the protective enzymes could eliminate less amount of O2 ', and in this case no apparent changes will be found in the content of MDA. If severe stress happens, however, the protective enzymes will be deteriorated, resulting in a marked rise of MDA content.3. A difference of enzymatic action with SOD and POD is documented in different stages of fruit sunburn. Before sunburn symptoms appear, the activities of SOD and POD increase within a certain range of stress, but after sunburn symptoms are visual, the SOD activity decreases with the advance of fruit sunburn. In contrast, the activity of POD continues to increase during this period.4. Exogenous free-radical agents could change the metabolism of endogenous O2- And different types of exogenous free-radical agents can consistently increase endogenous O2- content and raise the activities of SOD and POD. As far as the present test is concerned, the O2- content in fruit peel increases by 14.52%61.70%, the SOD activity by 1.07-1.66 times and POD activity by 0.6-3.5 times. A differential effect of exogenous free-radical agents on endogenous O2- production may exist with Na2S2O4, showing a more powerful agent.5. In a clear day, the surface temperature of bagged fruit is always higher than the exposed fruit, with the treatments with four types of bags exhibiting 2.5癈~6.0癈 higher than that from the control. After bags are removed, the O2- contents in fruit peel increase by 16.21%~60.29%. However, the activities of SOD and POD as well as MDA content vary with various kinds of bags.6. The mechanism of fruit sunburn occurrence could be described as the following: under higher-temperature and intense-light stress, a disturbance of free radicals in fruit peel takes place and the protective enzymes are insufficient to eliminate the injury cause by free radicals. As a consequence, the membrane is damag... |
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