Characteristic And Regulation On Lignification Of Kiwifruit Stored At Low Temperature

Kiwifruit is climacteric fruit and low temperature storage helps to prolong the shelf life.However,kiwifruit is sensitive to chilling condition which leads to chilling injury(CI),including lignification of the outer pericarp,subcutaneous tissue and core tissue,grainy and brown regions in the outer pericarp.This has severly influenced fruit interior and exterior value and wide spoilage out of the warehouse.The effects of 5 mmol·L-1 oxalic acid(OA),0.01 ?L·L-1 methyl jasmonate(MeJA)and 0.1 ?L·L-1 methyl salicylate(MeSA)treatments on lignin accumulation and lignin-associated enzymes and genes of postharvest fruit,respectively in Actinidia chinensis cv.Huayou by dipping and Actinidia delicious cv.Xuxiang by fumigation were studied.The main results are as follows:1.The research combined hand-cut samples and paraffin sections indicated that lignin accumulation of ‘Huayou' kiwifruit mainly occurred in outer pericarp,inner pericarp and vascular bundles.OA treatment significantly reduced lignin content and total phenolics content.Meanwhile,OA treatment inhibited the key enzymes involved in the lignin biosynthesis,including 4-coumarate coenzyme A ligase(4-CL)activity,peroxidase(POD)activity and polyphenol oxidase(PPO)activity.2.Lignin accumulation of ‘Xuxiang' kiwifruit exists in inner pericarp and core tissues,and only cell walls were lignified.Occurrence of lignification in kiwifruit began from the stem end and gradually spread to the calyx end.The lignified cells mostly occurred around vascular bundles first and then the inner pericarp of kiwifruit.At the end of cold storage,sclerenchyma under the outer pericarp became thicker than those observed on the harvest day and intercellular space of the inner pericarp shrunk.Lignified cells emerged around vascular bundles and were smaller than surrounding parenchymal cells,occurring in clusters.This illustrates that the process of lignin accumulation is discrepant in different tissues.3.MeJA treatment significantly retarded the decrease of firmness and lowered the lignification incidence in ‘Xuxiang' kiwifruit.Both MeJA and MeSA treatments markedly inhibited the increase of lignin content in fruit core tissue,with no dramatic differences in pulp tissue.4.Both MeJA and MeSA treatments markedly inhibited the increase of lignin content in fruit core tissue,but enhanced its PAL enzyme activity.Similarly,though there is no significant differences between control fruit and two treatments,MeJA and MeSA treatments respectively declined and improved its PAL enzyme activity.Meanwhile,MeSA treatment promoted expression of AdPAL in fruit core,making it different in pulp and core tissues.These results demonstrated PAL may not be the direct factor in lignin synthesis.CAD enzyme activities and expression patterns of Ad CAD in MeJA-and MeSA-treated fruit core and pulp tissues showed no obvious correlation with lignin content,suggesting CAD may not affect the lignin content directly.MeJA treatment enhanced expression of Ad POD1 in fruit pulp,follwed by no remarkable differences with control during later storage.This trend is similar to that of lignin content,which illustrated MeJA treatment promoted lignin accumulation in fruit pulp probably via regulating AdPOD1.