Abstract:Juice sac granulation is a universal physiological disorder of citrus fruits, which is characterized by increased firmness and lower fruit quality in juice sacs. In order to reveal the characteristics of physiological metabolism of other fruit segments during juice sac granulation, we stored ripe ‘Guanxi’ pummelo fruits at room temperature for 60 days in this study. The total cell wall material contents of juice sacs isolated from the dorsal vascular bundles (dorsal juice sacs), juice sacs isolated from the septal vascular bundles (septal juice sacs), segment membrane and pericarp, as well as the soluble solid contents of the two types of juice sacs were determined, while the dynamic ultrastructure of the dorsal and the septal vascular bundles were investigated by transmission electron microscopy. The results showed that: (1) at 10 days of storage, secondary cell wall apparently thickened in sieve tube and companion cells, while the number of mitochondria and vesicles began to increase and secondary cell wall obviously thickened in phloem parenchyma cells. At 20 days of storage, the number of mitochondria and vesicles continuously increased and golgi apparatus appeared (disappeared at the following stages) in the two types of vascular bundles. In the meantime, total cell wall material contents began to significantly increase in segment membrane and pericarp. At 40 days of storage, the number of mitochondria continuously increased only in the septal vascular bundles, and the total cell wall substance levels began to statistically increase in the septal juice sacs. At 60 days of storage, secondary cell wall continuously thickened in the two types of vascular bundles, and the total cell wall component contents continuously increased in segment membrane, pericarp and septal juice sacs, but did not show distinct changes in the dorsal juice sacs during storage. (2) In the whole storage period, the total cell wall material contents of segment membrane were significantly higher than that of pericarp, while the total cell wall component contents of pericarp were distinctly higher when compared with the two types of juice sacs. At the late stages of storage, the total cell wall material contents of septal juice sacs were significantly higher compared to dorsal juice sacs. (3) Soluble solid contents remained constant in the dorsal juice sacs during storage, but significantly decreased in the septal juice sacs from 40 to 60 days of storage. The results in this study suggest that changes in cell wall component metabolism of fruit vascular bundles, segment membrane, and pericarp are earlier than that of juice sacs during storage. The increases in mitochondrial number in phloem parenchyma cells are accompanied with obvious secondary cell wall thickening in fruit vascular bundles, and the significant decreases in soluble solid contents are paralleled by distinct increases in total cell wall material contents in septal juice sacs during storage. The results in this study may facilitate to reveal the comprehensive mechanism of citrus fruit granulation.