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Calcium in different forms was compared in the pericarp of cracked and healthy fruit in the cracking-susceptible litchi ( Litchi chinensis Sonn.) cultivar ‘Nuomici’. Incorporation of 45 Ca into the cell walls of the pericarp and effects of sprays of calcium chloride (0.2% or 0.5%) on endogenous calcium and cracking incidence were examined. The results showed that calcium concentration in cracked fruit was significantly lower than in healthy fruit. Structural calcium as part of cell wall components contributed to the majority of the difference in calcium concentration between cracked fruit and healthy fruit. Surface spray of calcium chloride, either once or three times at different stages of fruit development exhibited no significant effect on fruit cracking and hardly increased calcium, especially structural calcium in the pericarp. X-ray microanalysis revealed that the applied calcium remained on pericarp surface. Radioisotope test showed that less than 0.1% of 45 Ca applied on fruit surface was incorporated into the cell walls. It is suggested that spraying calcium on fruit surface is not an effective way to prevent litchi fruit cracking due to the difficulty in uptake of the calcium sprayed on fruit surface.

SummaryConcentrations 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 DA...

SummaryChanges in structural calcium and galacturonan concentrations, the degree of methylesterification of pectins, the activities of pectin methylesterase (PME) and soluble and wall-bound peroxidases (POD), and amino acid compositions of structural proteins in the fruit pericarp (skin) were studied in cracking-resistant ‘Huaizhi’ and cracking-susceptible ‘Nuomici’ litchi. Structural calcium concentrations were higher in ‘Huaizhi’ than in ‘Nuomici’ and decreased from 22 d to 52 d after anthesis (DAA), then increased. Galacturonan concentrations increased over time and were higher in ‘Huaizhi’. Methylesterification of pectins increased from 50% at 22 DAA, to 55% at 78 DAA in ‘Huaizhi’, but fluctuated around 48% in ‘Nuomici’. PME activity was significantly higher in ‘Nuomici’ than in ‘Huaizhi’ in the later stages of fruit development (> 50 DAA). The activities of soluble (SPOD) and ionically wall-bound POD (IWBPOD) increased with fruit development. Maximum SPOD activity was similar in both cultivars, where...

Cultivar performance of litchi ( Litchi chinensis Sonn.) and roles of water loss and calcium in postharvest fruit senescence were studied. Desiccation and membrane breakdown were accelerated once litchi fruit were detached from the tree. Membrane breakdown began from the inner part (mesocarp) of the pericarp within 2 days after harvest (DAH) and within 5 DAH in the sclerenchyma exocarp cells. Membrane breakdown was not yet observed at 5 DAH in the epidermis cells, where a very high calcium concentration was revealed by X-ray microanalysis. Transfusion of water to the fruit through the stalk slowed fruit water loss and browning within 4 DAH, but the treated fruit tended to rot from the internal and deteriorated entirely by 6 DAH. There was no difference in response to transfusions of water and calcium chloride (18 mmol/l) solution. Among the five tested cultivars, ‘Nuomici’ had the shortest shelf-life, while ‘Guiwei’ had the longest. Desiccation rates of these cultivars were not consistent with their fruit deterioration rate. Concentration of structural calcium in the pericarp was negatively correlated to fruit deterioration rate and membrane leakage. The results suggest that calcium in structural form may influence fruit senescence through its role in tolerance to desiccation and in maintenance of membrane integrity. Preharvest spray of calcium chloride (40 mmol/l) at different stages of fruit development failed to increase structural calcium in the pericarp and was ineffective in slowing fruit senescence.

SummaryThe 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...

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 Ca 2 + in early fruit development signals PCD that leads to spongy tissue formation, a process essential for pericarp extension during aril growth.

Among the 5 tested litchi (Litchi chinensis Sonn.) cultivars ("Huaizhi", "Guiwei", "Nuomici", "Hongmili" and "Shuijingqiu", "Nuomici" became deteriorated much faster than other cultivars while "Guiwei" fruit was the slowest in the rotting process (Fig. 1A). Fruit deterioration was accompanied by fruit desiccation (Fig. 2B), but the speed of water loss was not significantly correlated to fruit deterioration rate, indicating that it was not the key factor causing the difference in postharvest performance among cultivars. Fruit deterioration rate was significantly positively correlated to membrane leakage (Fig. 2A), suggesting the capacity to maintain membrane integrity is closely related to the shelflife of litchi. Skin browning potential, uronic acid concentration, degree of methylation of pectin and soluble Ca content in pericarp as well as total Ca content in the pulp were not significantly correlated with fruit deterioration. Content of structural Ca (water-insoluble but acetic acid-soluble calcium, membrane or wall-bound Ca), the major form of Ca in the pericarp, was negatively correlated to fruit deterioration rate (Fig. 2E). The results proved that differences in fruit desiccation rate, browning potential, Ca other than structural form were not the major cause leading to difference in postharvest performance among different cultivars. "Guiwei" being more tolerant to desiccation than other cultivars is likely associated its higher structural Ca concentration in the pericarp.