| Apple fruit sunburn, a common physiological disorder, happens widely in main apple-producing areas on the world and results in a huge loss annually. The project was undertaken to study the ecological conditions, antioxidative properties and prevention of fruit sunburn by means of advanced technologies in both laboratory and field trials from 2001 to 2004. As a consquence, the patterns of sunburn occurrence and contributing factors were documented and a computer model for apple fruit sunburn prediction was developed. Moreover, some pathways or measures to control fruit sunburn were suggested. The main results were as follows:1. Fruit sunburn in relation to main ecological factorsA significant relationship was found between daily maximum fruit surface temperatures and main meteorological factors including air temperatures, solar radiation, wind velocity and relative humidity. High fruit temperatures (>45℃ ) resulted mainly from both high air temperatures and intense solar radiation. Fruit temperatures could be influenced by many parameters and besides air temperature, the intensity and duration of solar radiation had an irreplaceable effect. The results indicated that, the daily changes of fruit temperatures on different exposures of a tree showed distinct dynamics, with maximal fruit temperatures appearing from southwest side of the canopy. Maximum fruit temperatures, regardless of exposures, occured in the afternoon, which may imply the contribution of ambient temperature to fruit temperature increase. Sunburn happened mainly on the fully exposed fruits and surface temperatures of these fruits originated from two sources: both air temperature and light energy received by the fruit.2. Changes of antioxidative capability in fruit stressed by high temperatures Under natural conditions, there was a considerable difference in antioxidative status among fruits on different exposures of a canopy. Under sub-lethal temperature stress, the contents of O2- and MDA as well as the activities of SOD and POD in the fruits on southwest canopy were significantly higher than those from other exposures. A tendency towards antioxidative capability in fruit was found that the contents of O2- and MDA increased and the activities of SOD and POD decreased during high temperature treatment. A sensitive response of O2- and MDA contents as well as SOD and POD activities in apple fruit peel was found to the stress of high temperature, intense light and low relative humidity. The responsive extent of fruit to intense light stress depended closely upon the acclimation to which fruit were subjected before the stress. It was proved that gradual temperature rising was beneficial to rendering heat tolerance to fruits. The effect of relative humidity on membrane integration and functions in fruit peel tissues was examined with Fuji apples.The results showed that under high-temperature stress, LOX activity and membrane leakage decreased but 5'-nucleotidase activity increased as environmental relative humidity rose.3. Effect of exogenous substances on antioxidative capability in fruitsA great effect of exogenous growth regulators, including ASA, SBN, CaCl2and SA, was found on SOD and POD activities as well as O2 and MDA contents. Four kinds of regulators could consistently raise the activity of protective enzymes and lower MDA level. The effect of different types of exogenous antioxidants on the activities of 5'-nucleotidase, LOX and APX in fruit peel tissues under high-temperature stress was investigated with Fuji apples. The results indicated that, LOX activity increased but APX and 5'-nucleotidase activities decreased as temperatures stresses were accentuated, showing an enhancement of oxidative stress that may cause a decline in membrane intergration. Treatments with ascorbic acids, especially added with 6-BA or citric acids, resulted in a significant improvement in membrane stabilization to heat.4. Influence of bagging on fruit sunburn and antioxidative propertiesIn clear days during growing season, the surface temperatures of fruits enclosed in paper bags varied with different exposures on a tree canopy. Generally, the daily maximum fruit temperatures occurred at different times in a day with exposed fruits in response to the daily changes of solar radiation, rather than in a relatively concentrated time period in the afternoon with bagged fruits. With proper use of qualified bags and reasonable bagging techniques, only a minor difference was found in relative humidity between inside and outside bags, suggesting that the inside humidity was mainly dominated by the outside one. In addition, a very significant positive correlation was found between either solar radiation or air temperature, and daily maximum fruit temperature. Because different types of bags created particular microenvironments for individual fruits, a difference in antioxidative capability could be certainly expected in fruit peel tissue. Enclosed fruits became more sensitive to the new environment soon after bag removal. By means of fruit bagging and treatments with exogenous activated oxygen species and agents or under controlled conditions of both fruit temperature and relative humidity, the 5'-nucleotidase activities were compared with the corresponding controls. The results indicated that, a considerable effect of microenvironments was found on 5'-nucleotidase activities in fruit peel tissue. The highest enzymatic activity appeared in fruits on southwest canopy, regardless of bagged or non-bagged fruits, significantly higher than those from any other exposures. Fruits with bags had a significantly higher 5'-nucleotidase activity than the exposed ones. In sunny and clear days during growing season, a severer oxidative stress frequently occurred on the fruits from the southwest exposure of the canopy, regardless of bagged or exposed fruits. Generally, fruits became more sensitive shortly after bags were removed to harsh fluctuation of ambient temperatures and intense light because of the long-term adaptationto the "relatively stable and dark environment" within the bags. A marked influence of bagging techniques was found On the surface temperatures of fruits enclosed in paper bags. Generally, the inferior ventilation inside the bag and tight contact of a fruit with bag wall could result in a significant rise of fruit temperatures, thus increasing the risk of heat injury. In addition, an insignificant effect of different methods for disclosing bags was detected on fruit temperatures but a substantial effect was shown on light intensity. For example, the fruit covered merely with interior red bag could receive as much as 64.54% of the natural light, which may be beneficial to fruit acclimation to intense light. Removal of bags at different times in a clear day could bring fruit into different status of environmental transition between inside and outside the bag. Therefore, the fruit bags on southwest periphery of tree canopy should be disclosed after 15:00 in a high sunburn risky day, and removal around noon was not acceptable because of the huge potential of environmental stress.5. Fruit sunburn prediction and preventionThere existed some differences among 9 cultivars examined in resistance to thermal injury, ranging from 45.9 to 48.5 ± 0.5°C. Light was extremely important in inducing fruit sunburn because at the same "threshold temperature", fruit got sunburned under natural light but didn't necessarily without sunlight. A computer model for sunburn prediction was developed on the basis of determination of the "threshold temperature" for different cultivars and establishment of correlation between daily maximum fruit temperature and main meteorological factors, which include average air temperature, solar radiation, wind velocity and relative humidity. Meanwhile, the study on difference of fruit susceptibility to sunburn in cultivars, fruit growing stages and tree vigor has made it possible to incorporate these parameters into the model. By means of comparing predicted values with in-situ readings, the fitting rate of predicted results reached 85.22%. The model has been tried by apple growers. A special sensor, which can simulate the surface temperature changes as a real apple, was developed and used to control the evaporative cooling system. It has been proved that with the new system, substantial amount of water could be reserved and fruit sunburn could be suppressed more effectively and economically. Compared with the control, the sunburn rates of Gala, Golden Delicious and Fuji were reduced by 10.55%, 9.95% and 15.12%, respectively. Paper cone shading could significantly reduce light reception by fruit, resulting in a decrease of fruit surface temperature, so that sunburn rate was lowered dramatically. In areas where sunburn has been a problem, it is recommended that partly or totally shaded fruit should be remained for the first priority during fruit thinning. In addition, paper cone shading should be conducted in late May or early June for the fruits on the periphery of southwest canopy. Spraying with Raynox (a kind of particle film formulations) could reduce fruit sunburn significantly. This material was able to reflect infralight and provided a layer of protective coating on the fruit surface. From June...
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