A disorder in pears that is known as ‘hard-end’ fruit affects the appearance, edible quality, and market value of pear fruit. RNA-Seq was carried out on the calyx end of ‘Whangkeumbae’ pear fruit with and without the hard-end symptom to explore the mechanism underlying the formation of hard-end. The results indicated that the genes in the phenylpropanoid pathway affecting lignification were up-regulated in hard-end fruit. An analysis of differentially expressed genes (DEGs) identified three NAC transcription factors, and RT-qPCR analysis of PpNAC138, PpNAC186, and PpNAC187 confirmed that PpNAC187 gene expression was correlated with the hard-end disorder in pear fruit. A transient increase in PpNAC187 was observed in the calyx end of ‘Whangkeumbae’ fruit when they began to exhibit hard-end symptom. Concomitantly, the higher level of PpCCR and PpCOMT transcripts was observed, which are the key genes in lignin biosynthesis. Notably, lignin content in the stem and leaf tissues of transgenic tobacco overexpressing PpNAC187 was significantly higher than in the control plants that were transformed with an empty vector. Furthermore, transgenic tobacco overexpressing PpNAC187 had a larger number of xylem vessel elements. The results of this study confirmed that PpNAC187 functions in inducing lignification in pear fruit during the development of the hard-end disorder.

‘Akizuki’ ( Pyrus pyrifolia Nakai) is a dominant Asian pear cultivar with gradually increasing cultivation area in Shandong province. However, this cultivar is found susceptible to cork spot disorder in recent years. In this study, we explored the physiological-biochemical mechanism of cork spot disorder in pear fruit, and investigated the effectiveness of spraying calcium (Ca), boron (B) solution or prohexadione calcium (P-Ca) on cork spot incidence. Cork spotted fruit had the characteristics of significantly larger fruit size with shorter fruit pedicels. Compared with normal fruit, cork spotted fruit had lower content of total soluble solids, soluble and reducing sugar, and vitamin C. In addition, cork spotted fruit accumulated much higher levels of N and Mg, and lower levels of K and P. However, Ca deficiency was not observed in cork spotted fruit, on the contrary, we determined high concentrations of Ca and free Ca ²⁺ in disordered fruit. At the same time, the ratios of K/Ca, Mg/Ca, and (K+Mg)/Ca were significantly lower in cork spotted fruit as compared with normal fruit. Among all treatments, spraying with 3500 times dilution of P-Ca at 15-day intervals from 30 to 90 days after full bloom showed promise for reducing cork spot incidence in ‘Akizuki’ pear without affecting fruit quality attributes. This research herein reveals the physiological-biochemical characteristic of cork spot disorder, and implicates P-Ca as a potential tool to reduce cork spot incidence in Asian pear cultivar Akizuki.

Fruit pedicel is the bridge linking the parent tree and the fruit, which is an important channel for water and nutrients transport to the fruit. The genetic specificity determines the characteristics of the pedicel and the fruit, but the relationship between the pedicel structure and the fruit characteristics is unexplored. Combining the investigation of fruit characteristics, the statistical analysis of the pedicel structural properties, and the 2D and 3D anatomical observation of the pedicel, this study found distinctive contributions of the pedicel elements to the fruit characteristics in four pear species. The European pear (Conference) showed distinct fruit shape index and pedicel structural properties compared with the oriental pears (Akizuki, Yali, and Nanguoli). The fruit size positively correlated with pedicel length, fiber area, pedicel diameter, the area percentage of the cortex, and the area percentage of phloem; however, fruit firmness and soluble solids concentration are showed a stronger positive correlation with xylem area, pith area, the area percentage of xylem, the area percentage of sieve tube, and the area percentage of pith. Pedicel elements, including pith, fiber, and cortex, likely play a certain role in the fruit growth due to the variations of their characteristics demonstrated in the four pear species. The porosity, the ratio of the surface area to the volume, and the spatial arrangement of the vessels showed significant variations across the pear species, indicating the distinction of the hydraulic conductance of the pedicels. Our findings provided direct evidence that pedicel structural elements contributed distinctively to the fruit characteristics among pear species.

Cork spot is one of the most damaging physiological disorders in pear fruit, causing considerable economic loss every year. However, the mechanism of cork spot occurrence requires further examination. In this study, X-ray CT scanning was applied to analyze the microstructure of pear fruit “Akizuki” (Pyrus pyrifolia), a cultivar susceptible to cork spot disorder, to elucidate the fruit texture alteration between healthy and cork spotted fruit. Results showed that cork spotted fruit had much higher porosity (9.37%) than healthy fruit (3.52%). Reconstructed three-dimensional (3D) network skeleton models showed highly branched pore channels in cork spotted fruit and a low degree of pore connectivity in healthy fruit. Even in areas of disordered fruit without cork spot, the pore throat diameter, pore length, and coordinated core number (i.e., 77, 160, and 16, respectively) were much higher than that of healthy fruit. The structure analysis of fruit core showed that core deformation only occurred in cork spotted fruit. A much more highly branched network was observed in cork spotted fruit cores compared with healthy fruit cores. High-resolution observation of flesh tissue directly demonstrated that pore size in cork spotted fruit (87 μm) was four times larger than that of healthy fruit (22 μm). Altered expression of genes related to Ca²⁺ transport and the uneven distribution of intracellular Ca²⁺ were also shown to associate with the development of cork spot disorder. Our results suggest that flesh tissue damage likely occurred prior to the initiation of cork spot. The dysfunction of long-distance and transmembrane Ca²⁺ transport channels could be responsible for the imbalanced distribution of Ca²⁺ inside the fruit, thus resulting in the development of cork spot.

‘Akituki’ (Pyrus pyrifolia Nakai.) is a very popular and profitable pear cultivar in China. However, its high susceptibility to cork spot disorder has limited its expansion of cultivated area. The mechanisms of cork spot disorder have been discussed extensively, focusing on Ca²⁺ deficiency, yet no consensus has been made. In this study, we applied nitrendipine (NI) as a Ca²⁺ uptake inhibitor to explore the role of calcium in cork spot disorder occurrence. Results showed that NI treatment on the fruit remarkably increased the incidence of cork spot disorder; alteration of mineral contents happened at the early developmental stage of the fruit, especially on the outer flesh and the peel of the fruit; and this gap was filled gradually along with the expansion of the fruit. Significant differences in the expression levels of Ca²⁺ transport-related genes were found in the inner flesh, outer flesh and peel during the fruit growth period. The observation of free Ca²⁺ localization indicated the intracellular imbalance of Ca²⁺ in the NI-treated fruit. In conclusion, NI treatment reduced the calcium content in the fruit at an early developmental stage, altered the related expression of genes and influenced the cellular Ca²⁺ balance in the fruit, which prompted the occurrence of cork spot disorder. Measures for the prevention and control of cork spot disorder should be taken at the early stage of the fruit development in the field.

Cork spot is a common physiological disorder in pear fruits, which has been found in some pear cultivars. Mineral nutrition imbalance in fruit is regarded as the principal influence factor for disorder incidence, with some ongoing confusion and controversy. In our research, we explored the cork spot characteristics in Japanese pear ‘Akizuki’ (Pyrus pyrifolia Nakai), adopted metabolome and mineral content analysis for healthy and disordered fruits, and made a correlation analysis of mineral and metabolites. Cork spots are mainly distributed on the outer flesh beneath the fruit peel. In cork spotted tissues, superoxide (SOD) and peroxidase (POD) activities, as well as malondialdehyde (MDA) content, increased. A total of 1024 known metabolites were identified from all the samples and more changes in metabolism were detected between normal and cork spotted flesh tissues. Correlation analysis displayed that Ca, especially the Mg/Ca in fruits, could be used to predict whether an orchard will develop cork spot disorder; Mg and B were associated with the appearance of symptoms, and the contents of Zn, Fe, and Mg, as well as Mg/B and Zn/B, might be strongly tied to the formation of cork spots in pears. This research provides insights into the occurrence of pear cork spot disorder and clarifies the role of minerals.