Walnut (Juglans regia L.) is an important crop that is cultivated extensively worldwide for nut production. China is one of the major producers of this crop. Traditional Chinese medicine purports that the nutmeat of walnut is nutrient-dense and beneficial for brain health. Similarly, many tissues of the walnut besides the nut, including the husk, shell, shoots, wood, and leaves, are widely used in medicine, industry, and many other applications (Pei and Lu, 2011). The shell is usually ignored and thrown away, despite belonging to a class of highly insoluble species that can be used to produce chemicals, such as activated carbons, natural fibers, and powders, which are extensively used in the defense industry, space industry (Khantwal et al., 2016), chemical industry (Yuan and Liu, 2007), and in environmental protection (Ding et al., 2013).
Walnut pericarp development can be divided into three stages (Xiao et al., 1998). 1) There is an early phase during which there is no boundary between the three layers of the pericarp, namely, the exocarp, mesocarp, and endocarp. Then the exocarp appears with densely arranged epidermal cells with numerous glandular hairs and corneous layer cells and stomata (Taylorteeples et al., 2014). Under the epidermis, there are multilayer parenchymal cells containing chloroplasts and phenols in vesicles, which are rich in phenolic compounds. 2) Subsequently, the boundaries between the exocarp, mesocarp, and endocarp become clearer, along with the emergence of two layers of sclereids under the epidermis. The volume of parenchymal cells in the mesocarp increases rapidly (by a dozen or dozen-fold). 3) Finally, 4 to 5 vascular bundles develop in the exocarp, and the endocarp transforms into a hard shell composed of lignified sclereids. The pattern of shell-hardening is basipetal in the longitudinal plane and centripetal in the transverse plane. The first cells to become sclerified are those at the micropylar end (the upper part of the fruit), with sclerification occurring in early June (Pinney and Polito, 1983; Zhao et al., 2016). Sclerification proceeds basipetally from this point, with the cells along the seal lines becoming sclerified rapidly. Completion of sclerification from this point occurs in less than a week. Only the outermost one-half to two-thirds of this tissue actually contributes to the shell, and the remainder contributes to the packing tissue (Pinney and Polito, 1983).
With grafting and budding being extensively applied in commercial nurseries over the past few decades, new thin-shelled cultivars have been propagated and planted across China. Most of these cultivars are derived from precocious Xinjiang walnuts (lateral-fruiting type) (Pei and Lu, 2011; Xi, 1990) and are accustomed to a Mediterranean climate, which is associated with high light, dry air, and low planting density. However, the climate of central and eastern China is hot and humid in summer. Due to various factors, such as the selected walnut cultivars, climatic conditions, high planting density, extensive cultivation, and poor management, the ratios of dehiscent nuts, incomplete hulls, and dark kernels have rapidly increased after harvest, which has influenced the quality of the kernel in recent years. Additionally, recent studies have shown that shell thickness is significantly positively correlated with nut weight, which itself is significantly negatively correlated with kernel percentage (Arzani et al., 2008; Zhao et al., 2007). Shell thickness was one of the main variables accounting for kernel percentage and should be considered in breeding (Amiri et al., 2010). In addition, shell structural features such as shape, thickness, and texture, were the most important parameters affecting kernel extraction quality in nuts (Amiri et al., 2014; Ghafari et al., 2011). Many walnut-producing countries use relevant walnut shell features in breeding objectives (Avanzato et al., 2014).
Here, shell differentiation and microstructure were analyzed using paraffin sections and cryosections, and the relationship among the structure, components, and physical properties were illuminated. The aim of this study was to ascertain the influence of shell structure and components on the physical properties of the walnut and provide a theoretical basis for cultivar breeding, rational planting density, and regulation of shell development.
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