Abstract
Granulation is a physiological disorder of juice sacs in citrus fruits, which develops through secondary cell wall formation. However, the synergistic changes in the cytoplasm of juice sac cells remain largely unknown. This study investigated the dynamic ultrastructure of juice sacs of “Guanxi” pummelo fruits by transmission electron microscopy and determined their cell wall material, soluble sugar, and organic acid contents. The results showed that lignin and hemicellulose are accumulated in juice sacs isolated from dorsal vascular bundles, while lignin and cellulose contribute to the granulation of juice sacs isolated from septal vascular bundles. The significant differences in lignin, cellulose, and hemicellulose contents between the two types of juice sacs began to be observed at 30 days of storage. Fructose levels were elevated in juice sacs isolated from the dorsal vascular bundles from 10 to 60 days. Sucrose contents significantly decreased in juice sacs isolated from the septal vascular bundles from 30 to 60 days. Meanwhile glucose, citric acid, and malic acid contents exhibited no apparent changes in both types of juice sacs. Based on the comprehensive analysis of the ultrastructure of both types of juice sacs, it was clearly found that plasma membrane ruptures induce cell wall material synthesis in intracellular spaces; however, cell wall substance contents did not significantly increase until the number of mitochondria sharply increased. In particular, sucrose contents began to decrease significantly just after the mitochondria amount largely increased in juice sacs isolated from the septal vascular bundles, indicating that mitochondria play a key role in regulating carbon source sugar partitioning for cell wall component synthesis.
Highlights
Citrus is an important fruit tree crop around the world
Lignin and hemicellulose are the major cell wall components synthesized in dorsal juice sacs, and the transient increases in their levels indicate that their degradation reaction should not be ignored
Lignin and cellulose are the two kinds of cell wall compounds contributing to cell wall substance accumulation in septal juice sacs. e significant increases in their levels begin to be observed at 40 days and prolong to the end of storage
Summary
Citrus is an important fruit tree crop around the world. Juice sac granulation of citrus fruits is a severe physiological disorder during the late growing season and postharvest storage of citrus fruits. Is disorder is characterized by an increase in firmness and a decrease in the flavour quality of juice sacs. A high rate of juice sac granulation leads to severe decline in fruit quality and in the economic effectiveness of the citrus industry. Granulation rates of citrus fruits vary widely. Studies have demonstrated that a secondary cell wall is formed in juice sacs during citrus fruit granulation [3, 4]. Sugar contents decrease in granulated juice sacs [5, 6]. The abundance of organic acids varies strongly
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