| The mechanism of induction of chilling tolerance of mango by heat treatment was systematically investigated based on the observation on the changes in anatomy, physiology and the synthesis, contents and composition of proteins. The time duration of heat treatment required for inducing chilling tolerance in mango fruits varied with the temperature and the manners of treatment. While 3 min was enough for hot-water treatment (HWT) at 55～56℃, at least 12h was needed for hot air treatment (HAT) at 38℃, to induce chilling tolerance in mango, with any length between 12～72h of HAT being of almost the same effect, as far as the chilling tolerance is concerned. Heat treatment could make a difference of 15d from the control under cold storage at 2℃. 2～4d at 20℃ following 1～3d at 38℃ reduced little of the acquired chilling tolerance. Heated mango ripened normally after storage at 2℃ for 11d, but the control suffered from serious chilling injury and couldn't ripen normally. Heat treatment inhibited rotting and remarkably enhanced external quality of mango fruits. 3～5min of hot water soaking at 55℃ is recommended for extension to farmers as an alternative to reduce chilling injury and rotting. Observation through Trans-electronic Microscope showed that after cold storage for 15d the membrane system of the cell of mango was seriously damaged, especially the chloroplasts, while heat pretreatment prevented such damages from happening. This was coincided with the levels of ion leakage, which was lower in heat-treated mango than in control after 12d at 2℃. However, during heat treatment, different trends in ion leakage were revealed for the two different types of heat treatments. While 38℃ hot air treatment increased the ion leakage level, 55℃ hot water treatment reduced it. In order to understand the physiological mechanism of heat treatment in inducing chilling tolerance, the physiological effects of HAT on mango were studied. The results showed that heat treatment induced a series of physiological changes in the cells of mango fruits, which helped to induce chilling tolerance. The induction of the chilling tolerance in mango was correlated with high levels of SOD, POD and PAL activities and high rates of IAA and ABA during cold storage at 2℃. The investigation on the physiological pattern of mango during heat treatment and cold storage showed that the physiological changes during heat treatment were the bases and the causes of the physiological differences between heat treatment and control during cold storage at 2℃. The peel and the flesh of mango, although closely linked physically, displayed different trends physiologically. Physiological observation of both peel and flesh is necessary for the precise understanding of the inducing mechanism of chilling tolerance. Among all the changes caused by heat treatment, the heat-shock proteins (HSPs) were most closely and most directly connected with the induction of chilling tolerance. Within 24h of HAT, the rate of protein synthesis in mango peel was much higher than that at normal temperature, which indicated that the high-speed synthesis of HSPs could last at least 24h. During HAT, the content of proteins increased continuously, while that at room temperature or low temperature showed no obvious change. Among the proteins accumulated during HAT, heat-stable proteins took a larger part. This indicated that HSPs newly synthesized were mainly made up of heat-stable proteins. And it was probably these heat-stable proteins that were correlated with the induction of chilling tolerance. The results of the electrophoresis analysis further supported the close link between the HSPs and the chilling tolerance.in mango fruits. The most direct proof acquired for the link was found in fruits partially exposed to HWT, with the exposed part ripened normally and the other part showed black symptoms of chilling injury. The two-dimensional electrophoresis diagrams of proteins extracted from the two different parts were on the whole similar to each other. However, in the diagram from the exposed part there were 6 new proteins of molecular weights (MW) of 13.7, 15.7, 28.5, 38.1, 59.1 kD, respectively (including two proteins of the same MW and of different iso-electric point), which didn't exist in the non-heated part. Another proof for the link between new HSPs and chilling tolerance came from a set of mango fruits with degrees of chilling injuries ranging from very serious to very slight as a result of varying duration of exposure of fruits to HAT. The HAT duration of 3h, 6h and 24h induced 2, 5 and 7 new HSPs, respectively, though these proteins were mostly different in MW from those induced by HWT. The extent of chilling tolerance increased with the number of new HSPs. The persistence of the HSPs in mango fruits also accounted for the chilling tolerance. 2～4d at 20℃ following 1～3d at 38℃ affected little of the acquired chilling tolerance of mango fruits. This coincided with the fact that the HSPs decayed very little during the short period at 20℃. Moreover, most of the HSPs induced by HAT continued to exist after cold storage at 2℃ for 11d. The big difference between HAT and HWT in the length of time needed for inducing chilling tolerance and in the compositions of newly produced HSPs indicated that the mechanisms of new HSPs induction and chilling tolerance formation were possiblly different for the two kinds of heat treatments. |
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