Effect Of High-temperature And Intense-light Stresses On Antioxidative Properties In Apple Peel Tissues

Apple fruit sunburn, a common physiological disorder, happens widely in main apple-producing areas on the world and results in a huge loss annually. With gradual popularization of dwarf rootstocks adaptable to native conditions in China, dwarf and compact culture will become a general trend of apple production inevitably. Coupled with the seriously prevailing tendency of global warming day by day, it could be expected that the injury caused by high temperatures and/or intense light will be aggravated in the near future, and this disorder will become a severe problem which undoubtedly affects the apple industry in China, and thus it is necessary to intensify the studies on the prevention and the mechanism of fruit sunburn in advance.Using the principles of resistance physiology and advanced technologies in both laboratory and field trials, the project was undertaken to reveal the patterns of antioxidative status, the responses of Fuji apples to the stresses caused by high temperature and/or intense light, as well as the efficacy of exogenous substances on adjustment of fruit antioxidative capability. As a consequence, the features of sunburn occurrence and the effect of different exogenous chemical application on antioxidative capability have been documented. The main results are as follows:1 A tender(?)towards antioxidative capability in fruit peel was found that the contents of O_2^- and MDA increased and the activities of SOD and POD decreased under the stress of high temperature, intense light and low relative humidity, but the activity of PPO increased under such stresses and the content of GSH and the activity of 5'-nucleotidase increased under the stresses of lower temperature, intense light and high relative humidity or under the stresses of higher temperature, intense light and low relative humidity .2 Within a certain range, the activity of antioxidative enzymes increased with the severity of fruit sunburn but decreased sharply beyond the limit, However, the activity of POD rose with the severity of fruit sunburn but no increase was found when fruit symptoms reached a certain degree. Similarly, MDA and O_2^-: contents rose with the severity of fruit sunburn but no increase was found when fruit symptoms reached a certain degree. As far as the content of GSH was concerned, it increased with the severity of fruit sunburn but decreased sharply beyond the limit.3 A sensitive response of O_2^- and MDA contents as well as antioxidant contentsand antioxidative enzymatic activities in apple fruit peel was found to the stress of high temperature and intense light, and 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 fruit.4 Exogenous antioxidants could effectively raise the activities of antioxidative enzymes and the contents of the antioxidants and lower the contents of C>2: and MDA and the relative electrolyte leakage in fruit peel. Treatments with 15mmol/L ascorbic acids added with 6-BA or treatments with 0.2mmol/L salicylic acid resulted in a significant improvement in membrane integration to heat, but treatments with 20mmol/L calcium chloride showed an insignificant improvement to heat.5 Application of exogenous active oxygen formulations could alter the levels of endogenous active oxygen species. In fact, all the treatments with exogenous active oxygen formulations were presumed to cause membrane injury, which was reflected by abrupt accumulation in O2: contents, although the activities of protective enzymes were, more or less, stimulated. Nevertheless, if fruit were subjected to a severer stress and the protective enzymes could not scavenge the excessive amount of activated oxygen free radicals at this moment, membrane damage would be caused, showing a significant increase in MDA contents. As far as MDA contents were concerned, there was a similar effect with the treatments of two endogenous active oxygen species, and a significant increase in MDA contents was found with apples treated by 'O2 before bag removal, and a significant increase in MDA contents was shown with apples treated by OH under different high temperature stresses.