Morphological and anatomic observation on fruit russet formation of pear (Pyrus pyrifolia Nakai ‘Cuiguan’)

Abstract:【Objective】‘Cuiguan’pear (Pyrus pyrifolia Nakai) is a precocious cultivar with many excel⁃lent 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 andconsumer acceptance. Many studies to prevent russet formation have been conducted, such as fruit bag⁃ging and protective cultivation. However, these measurements were seldom effective. The reason for thismay 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 evi⁃dence for the prevention and reduction of fruit russet formation.【Methods】Pear fruits were sampled oncea 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 micro⁃scope 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 mi⁃croscope. The lignin content of fruit skin was detected by an ultraviolet spectrophotometry to compare thedifference between russetting and non-russet parts of fruits.【Results】(1) During 14-49 d after full bloom⁃ing, the‘Cuiguan’pears grew slowly and no visible russet could be found on the surface. (2) After 49 d af⁃ter 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, whichled to cuticles broken, and the walls of epidermic and subepidermic cells were stimulated to secondarilylignified-thicken firstly, but then distorted, ruptured and died. Meanwhile, the wall of tannin cells about3-5 layers under the sub epidermis was also thickened, distorted, ruptured and then died. These may bethe results of the mechanical pressure from the rapid expanding cells of the inner flesh. The remains fromthe 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 oflong 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 andfruit 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 pres⁃sure stimulation from the cell expansion of inner flesh to epidermal or subepidermal cells. tortuosity, rup⁃ture 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 observationsin the work, the authors proposed that at 49-56 d, the cuticular membrane between fruit dots was disrupt⁃ed, and the lignified cell wall residues of epidermal and sub-epidermal cells were tightly attached to theskin surface to form so-called“fruit russet”.【Conclusion】‘Cuiguan’pear russet is mainly composed ofthe broken cuticular membrane, the lignified epidermis and sub-epidermis cell walls and the exfoliatedcell remains beside fruit dots. Moreover, fruit russet formation is due to epidermis, sub-epidermis and thecells near the layers cannot synchronously enlarge with the inner cells, which impels to stimulate wall lig⁃nification and leads to the distorted cell dead. When this mechanical resistance can not counteract the internal swelling, the cells near the peel ruptured and died. Thus, how to reduce the degree of lignificationof epidermal cells and improve cell flexibility and extensibility may be important for prevention of the fruitrusset formation. In addition, the main time of lignification thickening of epidermal cells is about 49-56 dafter full blooming. In other words, this period may be critical to the prevention of‘Cuiguan’pear russetformation.