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Concentrations of oxalate and calcium in soluble or structural forms, as calcium oxalate in the pericarp, or as oxalate in the fruit pedicel, and the distribution of calcium in the pericarp and fruit pedicel were studied in litchi (Litchi chinensis Sonn.) using cracking-resistant cv. 'Huaizhi' and cracking-susceptible cv. 'Nuomici'. Oxalate in the pericarp existed mainly in a soluble form and was significantly (P < 0.05) higher in 'Huaizhi' than in 'Nuomici'. The concentration of oxalate was higher than the amount of calcium oxalate. Calcium was first mobilised for the construction of cell walls in the pericarp, despite the high concentration of oxalate in the tissue. 'Huaizhi' fruit accumulated more structural calcium and calcium oxalate in the pericarp than 'Nuomici' fruit. There was no difference in soluble calcium between the two cultivars. Calcium oxalate was located chiefly in the epidermis of the pericarp, and increased in concentration within 50 d after anthesis (DAA) in 'Nuomici' and 64 DAA in 'Huaizhi', then decreased as structural calcium increased, indicating that calcium sequestered by oxalate could be remobilised. The fruit pedicel had higher concentrations of oxalate than the pericarp. X-ray microanalysis showed abundant calcium in pedicel tissue adjoining the fruit base, and less calcium in tissues at the fruit base and in pedicel tissue 1 cm from the fruit, suggesting that the pedicel serves as a bottleneck to calcium movement. This bottleneck is not likely to be caused by oxalate in the pedicel. Oxalate in the pericarp and in the fruit pedicel is not linked to a shortage of fruit calcium or to an increase in cracking incidence in litchi. [ABSTRACT FROM AUTHOR]

Changes in the micro-distribution of calcium in litchi (Litchi chinensis Sonn.) pericarp (fruit skin) were studied with an X-ray dispersive spectrometer (electron probe) and the structures of the spongy tissue-forming cells were observed with a transmission electron microscope (TEM) in litchi cvs. 'Huaizhi' (Wai Chee) and 'Nuomici' (No Mai Chee), which differ in cracking susceptibility. Before spongy tissue was visible, calcium had accumulated at the sites of spongy tissue formation, which formed calcium-rich zones in the inner mesocarp. These zones, which included groups of cells, were larger in cracking-resistant 'Huaizhi' than in susceptible 'Nuomici'. 'Huaizhi' pericarp also had a thicker spongy tissue. Cells forming the spongy tissue started to die, with a breakdown of the plasma membrane and tonoplast, plasmolysis, loss of cell contents, swelling and autolysis of the walls. The spongy tissue was initially formed by localized cell death, and later by an increase of intercellular spaces due to the breakdown of middle lamellae. The early accumulation of calcium in spongy tissue-forming cells might have signalled this localized cell death, which falls into the category of programmed cell death (PCD). Calcium-rich zones at the site of spongy tissue formation largely disappeared as spongy tissue was formed, with the endocarp becoming "calcium rich", indicating that intercellular re-localization of calcium may have occurred within the pericarp. The relations between calcium accumulation, spongy tissue formation, and fruit cracking resistance are discussed. We conclude that, in addition to its structural role, calcium also plays a non-structural role in conferring cracking resistance. The accumulation of Ca2+ in early fruit development signals PCD that leads to spongy tissue formation, a process essential for pericarp extension during aril growth. [ABSTRACT FROM AUTHOR]

The structure and mechanical properties (e.g. strength and extensibility) of the pericarp, the structure of the cuticle layer, and fruit desiccation rate were compared in two litchi (Litchi chinensis Sonn.) cultivars, the cracking-resistant cv. 'Huaizhi' and the cracking-susceptible cv. 'Nuomici'. Pericarp strength and extensibility were higher in 'Huaizhi' than in 'Nuomici' prior to rapid aril expansion. Based on changes in pericarp structure, we propose a "Zig-zag Unfolding" model for pericarp extension driven by the expanding aril. In this model, the cristate structure of the pericarp, and the spongy tissue in the mesocarp, provide extensibility for the pericarp. We suggest that the spongy tissue contributes to cracking resistance, as it permits pericarp extension/expansion. This hypothesis is supported by the observation that 'Huaizhi' has thicker spongy tissue than 'Nuomici'. The model also reveals that endocarp and mesocarp tissues beneath the 'cracks' are the main contributors to the tensile strength of the pericarp. However, no significant difference in thickness of these tissues was observed between the two cultivars. Post-harvest, 'Nuomici' lost water twice as fast as 'Huiazhi', suggesting that the thicker spongy tissue in the pericarp does not necessarily mean a faster desiccation rate. This study showed large differences in cuticle structure between the two cultivars, which may result in differences in desiccation rate. We suggest that the greater tendency for 'Nuomici' to lose water results in its higher sensitivity to drought, which in turn induces fruit cracking as well as a shorter shelf-life compared with 'Huaizhi'. [ABSTRACT FROM AUTHOR]