| Salicylic acid (SA) is a simple phenolic compound involved in regulation of various processes in plant, including stomatal movement, seed germination, ion absorption, sex polarization, and induction of disease resistance. SA has also been showed to delay the ripening of many fruits. However, the mechanism of SA regulating fruit ripening, especially the relationship between the change in endogenous SA level and the extent of fruit ripening is still not clear.'Bruno' kiwifruit (Actinidia deliciosa) was used to investigate the effects of SA on the postharvest ripening and its mechanism, with the aim of providing a better understanding of the role of SA in fruit ripening in the present experiment. Results were as following:1. A platform for extracting and detecting of S A from kiwifruit flesh tissue has been built. Endogenous SA was extracted with ethylether and determined by high-performance liquid chromatography (HPLC) with fluorescence detector. The results showed that the recovery of the method was 96.42+3.01%. higher than that of the reported methods, and the lowest limit of determination was 0.9 ng/g.FW. Compared with the other methods, this protocol was simpler and easier, which could shorten the operation time by 50%.2. The change in free SA level in fruit flesh tissues during postharvest ripening and softening implies the existence of SA in senescence regulation. In fruit stored at 20 ℃, SA level declined with the kiwifruit ripening and softening; however, it stood at relatively high level at 0℃. A close positive relationship was found between change in free SA level in fruit tissues and the extent of fruit ripening and softening (r = 0.9067**).3. Kiwifruit flash discs taken at different ripening stages were used to study the role of SA on postharvest senescence. It was showed that SA level in the fruit discs with application of acetylsalicylic acid (ASA) maintained higher than that in the control. ASA treatment inhibited LOX activity, slowed down O2- production rate, and suppressed ethylene biosynthesis in the fruit. These results were confirmed by theresults on whole fruit. Significant negative correlations existed between change of endogenous SA level in the fruit discs and changes in LOX activity, O2 production rate, ACC oxidase activity and ethylene production, respectively. It is suggested that SA probably acts as O2 "scavenger, thus inhibits LOX self-propagating reaction, retards the ethylene biosynthesis, and delays kiwifruit ripening.4. Both ASA+C2H4and C2H4+ASA treatments antagonised the physiological effect of ethylene on fruit ripening, decreased the LOX activity, reduced O2 "production rate, inhibited the biosynthesis of ethylene, and delayed the fruit ripening. It was also observed that the effect of ASA + C2H4 treatment on delaying fruit ripening was more effective than that of C2H4+ASA, which implied that SA might play a role in the ethylene action or signal transduction in senescence fruit.5. SA may also take part in the regulation of sugar metabolism in postharvest kiwifruit. During the fruit ripening, amylase activity increased at the initial stage, followed by a marked decrease in starch content and dramatic increase in hexose content. This was associated with an increase in sucrose phosphate synthase(SPS) activity, an decline in acid invertase (AI) activity and the accumulation of sucrose. ASA treatment inhibited the amylase activity, slowed down the hydrolysis of starch and the accumulation of hexoses, suppressed the rise of SPS activity and the decline of acid invertase activity, hence delayed the accumulation of sucrose. It is suggested that SPS could be activated by hexose and feedback-inhibited by sucrose. SA regulats sugar metabolism probably through influencing the SPS activity.
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