Calcium content and its correlated distribution with skin browning spot in bagged Huangguan pear

Skin browning spot (SBS) is an important physiology disorder that often occurs in bagged fruit at the mature stage in the Huangguan (Pyrus bretschneideri x Pyrus pyrifolia) pear. Using atomic absorption spectrometry, X-ray microanalysis, and the potassium-pyroantimonate precipitation method, the water-soluble and total Ca2+, Mg2+, and K+ contents, their microdistribution, and the Ca2+ localization were investigated in bagged Huangguan pear fruit in the presence and absence of SBS. Our results show that the water-soluble and total Ca2+ contents in both the skin and flesh tissue and the total Ca2+ content only in the skin tissue of the fruits with SBS were significantly lower compared to those of the fruits without SBS. However, a higher K+ content in the skin tissue was found in the fruits with SBS. There were no significant differences in the water-soluble and total Mg2+ contents in the skin and flesh tissue between the fruits with and without SBS. In addition, the results of the X-ray microanalysis were consistent with changes in the total Ca2+, Mg2+, and K+ contents in the skin and flesh tissue of the pear fruit that were affected by SBS. Compared to the skin tissue of pear fruit without SBS and the healthy part near the lesion zone of SBS, the lesion zone of SBS exhibited a high accumulation of Ca2+ grains in the cell membrane of the epidermis cells, while fewer Ca2+ grains were found in the vacuoles and cell walls. Altogether, these results indicate that Ca2+ deficiency and the cellular Ca2+ distribution in skin tissue contributed to the occurrence of SBS in bagged Huangguan pear fruit.