Abstract
[Objective] ‘Cuiguan’ pear (Pyrus pyrifolia Nakai) is a precocious cultivar with many excellent characteristics,such as high yield,larger fruit,crisp flesh,less stone cells and delicious favor.However,the fruit surface is covered by large-scale irregular russet,which impairs its appearance quality and consumer acceptance.Many studies to prevent russet formation have been conducted,such as fruit bagging and protective cultivation.However,these measurements were seldom effective.The reason for this may be that we do not understand the mechanism underlying fruit russet formation.In this study,we observed the dynamic process of ‘ Cuiguan’ pear fruit russet formation in order to provide a theoretical evidence for the prevention and reduction of fruit russet formation.[Methods] Pear fruits were sampled once a week from the 14 d after full blooming in the orchard of Nanjing Agricultural University until fruits naturally matured.The fruit photos were taken by a digital camera in the field or under a stereoscopic microscope to determine when the russet began to occur and the degree of fruit russet formation.Then,the structural changes of epidermis,sub epidermis and the near skin were observed by an optical microscope or fluorescence microscope combined with different chemical staining methods,or a transmission electron microscope.The lignin content of fruit skin was detected by an ultraviolet spectrophotometry to compare the difference between russetting and non-russet parts of fruits.[Results](1) During 14-49 d after full blooming,the ‘ Cuiguan’ pears grew slowly and no visible russet could be found on the surface.(2) After 49 d after full blooming,fruits began to grow rapidly.At this time,the cell sizes of the inner flesh expanded outwards,while the epidermis,sub-epidermis and the near skin cells could not expand synchronously,which led to cuticles broken,and the walls of epidermic and subepidermic cells were stimulated to secondarily lignified-thicken firstly,but then distorted,ruptured and died.Meanwhile,the wall of tannin cells about 3-5 layers under the sub epidermis was also thickened,distorted,ruptured and then died.These may be the results of the mechanical pressure from the rapid expanding cells of the inner flesh.The remains from the dead epidermis,sub epidermis and tannic cells became the major source of the fruit russet.Additionally,another interesting phenomenon was firstly reported.During fruit russet formation,about 3-5 layers of long parenchyma cells under the tannin cell layer,whose cell wall was also greatly thickened,but not broken.It is assumed that the sclerenchymatous cells formed a protective tissue to protect normal growth and fruit maturity.(3) Fruit dots were often assumed to be related with fruit russeting.However,in this work,the author observed that ‘Cuiguan’ pear fruit dots and fruit russet were coincidently formed by the pressure stimulation from the cell expansion of inner flesh to epidermal or subepidermal cells,tortuosity,rupture and death of epidermal layer cells induced by internal cell extrusion beside the fruit dots.Therefore,fruit dots and russet formation may be a parallel event during fruit rapid growth stage of the ‘Cuiguan’ pear without causal relationship between them.(4) A comprehensive consideration from the observations in the work,the authors proposed that at 49-56 d,the cuticular membrane between fruit dots was disrupted,and the lignified cell wall residues of epidermal and sub-epidermal cells were tightly attached to the skin surface to form so-called "fruit russet".[Conclusion] ‘Cuiguan’ pear russet is mainly composed of the broken cuticular membrane,the lignified epidermis and sub-epidermis cell walls and the exfoliated cell remains beside fruit dots.Moreover,fruit russet formation is due to epidermis,sub-epidermis and the cells near the layers cannot synchronously enlarge with the inner cells,which impels to stimulate wall lignification and leads to the distorted cell dead.When this mechanical resistance can not counteract the interual swelling,the cells near the peel ruptured and died.Thus,how to reduce the degree of lignification of epidermal cells and improve cell flexibility and extensibility may be important for prevention of the fruit russet formation.In addition,the main time of lignification thickening of epidermal cells is about 49-56 d after full blooming.In other words,this period may be critical to the prevention of ‘Cuiguan’ pear russet formation.关键词
‘翠冠’梨/角质膜/果点/厚壁细胞/木质化/果锈Key words
‘ Cuiguan’ pear/Cuticle/Fruit dot/Thickened-walled cells/Lignification/Fruit russet分类
农业科技
