Changes Of Two Types Of Citrus Cuticle And Their Impact On Penicillium Digitatum Infection During Fruit Storage

Cuticle as the first barrier against pathogen infection,plays an important role in host-pathogen recognizing and attacking host.In this paper,two types of citrus fruits were used as experimental materials,changes in crystal morphology of epicuticular wax and chemical composition of cuticle were observed or analyzed by scanning electron microscope(SEM)or gas chromatography-mass spectrometry(GC-MS)during fruit storage;in addition,effect of chemical composition of cuticle on Penicillium digitatum growth were also evaluated through in vitro and in vivo test.The results showed as follows:1.The main aliphatic components of epicuticular and intracuticular wax of two citrus cultivars were mostly composed of alkanes,fatty acids,and alcohols,while their proportions and carbon number distribution were depended on cultivars.Farnesol was exclusive in epicuticular wax of‘Bingtang’sweet orange.Moreover,triterpenoid compounds were detected only in intracuticular wax.The content of cutin of‘Satsuma’mandarin and‘Bingtang’sweet orange were 44.1μg/cm~2 and 70.1μg/cm~2,respectively,and significant difference was observed between of them.SEM showed that irregular platelets waxes were deposited on the surface of‘Satsuma’mandarin,while flattened platelets waxes were deposited on the surface of‘Bingtang’sweet orange.2.Except for the‘Bingtang’sweet orange fruit stored at 25℃,the content of epicuticular wax in the two types of citrus fruits increased as a whole,and the content of intracuticular wax and cutin showed a decreasing trend when the‘Satsuma’mandarin and‘Bingtang’sweet orange fruit were stored at 4℃and 25℃respectively.During storage,the proportion of terpenoids in the wax of the‘Satsuma’mandarin fruit was gradually reduced,and the proportion of alkanes was continuously increased.The opposite trend in the‘Bingtang’sweet orange fruit were observed.Although there were significant changes in the contents of each cutin monomer,their proportions in the cutin did not change significantly during storage of two types of citrus fruits.3.The epicuticular wax crystal morphology presented significant change during fruit storage.When the two types of citrus fruit were stored a t 4℃,the wax gradually transform into flate-like crystal structure and cover the entire fruit surface.When the two types of citrus fruit were stored at 25℃,the crystal morphology gradually turns into particles and begins to melt.4.Results of in vitro tests showed that the mycelial growth and spore germination of Penicillium digitatum could be inhibited by intracuticular wax and cutin,respectively.In vivo experiments showed that the fruit of removal of epicuicular wax or cutin,the spore germination rate and mycelial growth of Penicillium digitatum were significantly promoted.In conclusion,these results reveal the biochemical basis of the changes in the cuticle during storage and the effects on the growth of Penicillium digitatum,which would provide important evidence for improving the storage of‘Satsuma’mandarin and‘Bingtang’sweet orange fruit.