Effect Of Temperature And Light Intensity On Fruit Antioxidant Capacity And Its Acting Mechanism In Yali Pears

Temperature and light are very important environmental factors affecting fruit growthand development. However, fruit output and quality are influenced, more or less, due tofrequent stresses by excessive temperature and light during fruit growth （even during coldstorage）. Therefore, it is meaningful to reveal the effect of temperature and light stresses onfruits, so as to raise the ability to resist temperature and light stresses and achieve higheryield and top quality.In the present experiment, the effect of different temperature and light on fruit ROSgenerating rates, antioxidant enzymes activity, AsA contents as well as Ca²⁺distribution inpeel cells was examined under laboratory and field conditions with Yali pears. The response ofantioxidant system to different temperature and light levels and the pattern of Ca²⁺distributionin peel cells were preliminarily expounded. The main results are as follows:1. The hydrogen peroxide（H₂O₂）content, ascorbate peroxidase （APX） andperoxidase（POD） activity were improved under high temperature stress conditions.However, the susceptibility of APX and POD to high temperatures was quite different.During the incipient treatment by high temperature, APX activity was obviously increasedbut POD activity was not obviously varied. The POD activity was enhanced with time. Itwas concluded that APX was the primary H₂O₂–scavenging enzyme that could maintainH₂O₂content at a lower level. Whenever the stress became severe, POD wouldimmediately take action.2. AsA-GSH cycle was the main antioxidant pathway to scavenge ROS in fruits. APX,GR and MDHAR were the important components in the cycle and their susceptibility totemperature stress was relatively different. APX activity reached the peak at1hr, next to itwas MDHAR at3hr and the last one was GR at5hr.3. During the incipient treatment by high temperature, the contents of total ascorbicacid were significantly enhanced. The changing trend of reductive ascorbic acid （AsA） andMDHAR activity was consistent. It could be inferred that AsA scavenged ROS largelythrough the AsA-GSH cycle.4. Oxidant damage could be enhanced by double stresses from both temperature andlight. Under high temperature conditions, light could induce the ROS contents to rise, which also increased the antioxidant contents and the antioxidant enzymes activity. Butfruit injury would occur as fruit antioxidant ability declined and LOX was accelerated.5. Under temperature and light stresses，the different exogenous chemical applicationcan alleviate fruit harm. Before stresses, the different exogenous chemical application（AsA, oxalic acid, SA, ABA） significantly improved H₂O₂content. The abundant H₂O₂infruits can signale molecules to regulate the AsA-GSH cycle. Under temperature and lightstresses,the APX,GR activity are enhanced, and the reductive ascorbic acid content andthe ratio of reductive ascorbic acid of oxidative ascorbic acid （AsA/DHA） are increased,which ensure the AsA-GSH cycle to successfully run, so that the LOX activity can remainat a low level to slow down the progress of fruit injury6. Under temperature and light stresses, the effect of calcium regulators on fruitresistance was examined. At the primary stage of temperature and light stresses, theaccelerance of Ca²⁺（CaCl2） increased the H₂O₂content, which improved the activity ofAPX, GR and the AsA content. As a result, the LOX activity was inhibited, and the fruitresistibility is enhanced. The inhibitor of Ca²⁺（EGTA、LaCl3）significantly reduced theAPX and GR activity and AsA content, which affected the AsA-GSH cycle. Moreover, theactivity of LOX was rapidly improved, which aggravated the cell membrane oxidation.Compared to EGTA, LaCl3showed stronger inhibiting effect.7. Temperature could markedly affect Ca²⁺distribution within fruit peel cells. When fruitswere stressed at45℃, a great deal of Ca²⁺was transferred from vacuolar into cytosol. After fruitswere treated for7h, the cytosol was damaged and the Ca²⁺moved from cytosol into vacuolar.8. Light could also affect the Ca²⁺distribution. The Ca²⁺existed mainly in vacuolar andintercellular spaces under dark conditions. When fruits were stressed by intense light, agreat deal of Ca²⁺was transferred from vacuolar and intercellular spaces into cytosol.There was a little of Ca²⁺left in intercellular spaces and there was no Ca²⁺in vacuolar. So itwas shown that the Ca²⁺in cytosol mainly came from vacuolar.9. Under temperature and light stresses, the ROS content and Ca²⁺content in cytosolincreased. Higher concentration of ROS and Ca²⁺functioned as the signal molecules toimprove the activity of antioxidant enzymes（SOD, APX, MDHAR or GR）, protecting fruitmembranes from injury by ROS to some extent. However, when the stress aggravated,especially when the ROS generating rate exceeded the clearance rate from the antioxidantsystem, total Ca²⁺moved from vacuolar into cytosol, as a result, the cells lost theautomatically adjustable capacity, resulting in that the antioxidant system could not make acorresponding response （or the scavenging capability was insufficient to eliminate theinjury caused by free radicals）. In such a case, the membrane was destroyed, which couldcause fruit physiological injury to some extent.