Effects Of Cuticular Wax On Postharvest Senescence In Blueberry Fruit And The Possible Mechanisms

Cuticular wax is regarded as the first barrier of self-defense of plants.It plays a pivotal role in non-stomatal moisture retention,reduction of pathogenic and insect attacks,attenuating ultraviolet radiation,keeping plant surfaces clean,and protection against mechanical damage.Blueberry(Vaccinium spp.)is a new small berry with high nutritional and commercial value.The fruit is rich in bioactive compounds such as anthocyanins and phenolics,and is widely popular among consumers.The light-blue appearance of blueberry is because of its blue-black skin,which is covered with cuticular wax.However,this conspicuous wax layer is vulnerable to damage,and it might be removed during the harvesting,packaging,and transport of fruit,resulting in unattractive appearance and giving the impression of poor fruit quality.Until]now,there are few researches on cuticular wax of blueberry and postharvest fruit senescence.Here,on the one hand,mature fruit wax from major blueberry cultivars in Zhejiang Province were characterized from the perspectives of morphology,chemical composition,and their changes during fruit ripening and postharvest cold storage,to explore the biological functions of wax components.On the other hand,epiculticular wax layer was removed by slightly wiping with Blu-tack adhesive,and effects of wax removed on postharvest fruit softening and senescence were investigated to assess the roles of natural wax on the postharvest quality of blueberry fruit.The main results were showed as follow:1.Morphology and chemical composition of cuticular wax in mature fruit of nine blueberry cultivars were investigated using and scanning electron microscope and gas chromatography-mass spectrometry.The results of SEM showed that a large amount of tubular wax deposited on the surface of blueberry fruits.There was no apparent difference in wax morphology among the nine cultivars.However,even though the amount and chemical composition of cuticular wax varied considerably among blueberry cultivars,triterpenoids and P-diketones were the most prominent compounds,accounting for on average 64.2%and 16.4%of the total wax,respectively.Aldehydes,primary alcohols,fatty acids,and alkanes displayed relatively less amount.Ursolic or oleanolic acid was identified as the most abundant triterpenoids differing in cultivars.Twoβ-diketones,hentriacontan-10,12-dione and tritriacontan-12,14-dione,were detected in cuticular wax of blueberry fruits for the first time.Notablely,hentriacontan-10,12-dione and tritriacontan-12,14-dione were only detected in highbush(V corymbosum)and rabbiteye(V.ashei)blueberries,repectively.2.Changes in the content of cuticular wax of two blueberry cultivars during fruit ripening and postharvest cold storage were investigated.The results showed that wax content was related to the maturity of blueberry fruit.The wax compounds gradually deposited on the epidermis of blueberry fruit during fruit ripening.Both the amounts of total wax and main classes of wax constituents including triterpenoids,aldehydes,fatty acids,primary alcohol and alkanes reached maximum at complete mature stage(dark blue skin color).The total wax content of both blueberry cultivars declined with the storage time and the variational trend of cuticular wax composition was also cultivar-related.All the triterpenoid and aliphatic compounds contents decreased with different degrees in ’Britewell’ fruit while only β-diketones content declined significantly in‘Legacy’fruit after 30 days storage at 4 ℃.3.Wax removal accelerate postharvest fruit senescence.Natural wax plays an important role in maintaining the postharvest blueberry quality.The removal of cuticular wax on the fruit was found to accelerate the postharvest water loss and decay,reduce the sensory and nutritional qualities,and shorten the shelf-life.Wax removal decreased the activities of antioxidant enzymes(SOD,POD,CAT,and APX)and contents of antioxidants(total phenolic,total anthocyanin,and Vc),and accelerated accumulation of ROS(O2·-、H2O2)and lipid peroxidation,especially at the later period of storage.Moreover,the organellar membrane structure such as mitochondrial and plastids was disrupted in fruit with wax removed,while they were relatively intact in fruit with natural wax.4.Wax removal accelerate postharvest blueberry softening.Wax removal induced increase in activities of cell wall degrading enzymes including PME,PG,Cx and β-Gal,and decrease in contents of cell wall material including pectin,cellulose and hemicellulose.The results of transmission electron microscopy and atomic force microscopy showed that wax removal accelerated the dissolution and depolymerization of pectin,and relaxed the networks of hemicellulose-cellulose microfibril,which was favorable for cell wall degrading enzyme.At the end of storage,depolymerization of pectin main chains and degradation of pectin side chains in blueberry with wax removed were more obvious than those of the control fruit.Frequency of CSP and WSP chains with short length and narrow width was also greater than that of the control fruit.Schematic models for the degradation pathway of CSP and SSP the chain widths were preliminarily proposed.