| Abstract Cracking-susceptible litchi (Litchi chinensis Sonn.) cv. 'Nuomci'and cracking-resistant cv. 'Huaizhi'were used as the materials for studying the role of calcium in fruit cracking. Contents of calcium in different forms and calcium micro-distribution in litchi pericarp were analyzed. Potential factors (eg. galactornic acid, degree of methylation in pectin, pectin esterase and oxalic acid in pericarp, oxalic acid in fruit pedicle, seed development) influencing calcium absorption and utilization were studied. Effects of girdling and calcium spray with (0.5% CaCl2) on cracking and calcium content in pericarp were also studied. The study has yielded the following results: 1. Calcium content in the cracked fruit was found significantly lower than that in the normal fruit. There was no difference in soluble calcium, and the difference came chiefly from structural (HOAc-extractable) calcium. The results suggested that calcium in structural form in the pericarp played an important role in maintaining the strength of pericarp and thus in cracking resistance. 2. The total calcium content in the pericarp of cracking-susceptible 'Nuomici'was significantly lower than that of 'Huaizhi'. However, there was no significant difference in soluble calcium between the two cultivars. Structural calcium was the major calcium form in the pericarp of in 'Nuomici', which was lower than 'Huaizhi'prior to and after rapid aril expansion. In both cultivars, structural calcium increased during the rapid fruit (aril) growth in the late fruit development stage. Calcium oxalate was found abundant in the pericarp of 'Huaizhi'but much less in that of 'Nuomici'. Content of calcium oxalate tended to increase in the early stage and decrease in late stage of fruit development. Calcium oxalate might serve as a calcium pool in litchi pericarp. The results provided further support for the important role of structural calcium in cracking resistant. 3. The same tendency of changes in calcium content in the pericarp was observed with x-ray micro-analysis. During the early stage of fruit development, calcium accumulated abundantly in endocarp and mesocarp, forming calcium rich bodies at the location of spongy tissue formation. The calcium-rich bodies largely disappeared after spongy tissue was formed, while calcium started to accumulate in epidermis cells, suggesting calcium oxalate was located chiefly in the epidermis. During rapid aril expansion in late stage, calcium signal was slightly reduced in epidermis but increased in endocarp and mecocarp, suggesting relocation of calcium from epidermis to endocarp and mesocarp had taken place. Therefore, calcium accumulated in the epidermis served as calcium reserve for future use. In general, calcium signal in the pericarp of 'Huaizhi'were stronger than 'Nuomici'. The above results suggest that 'Huaizhi'fruit (pericarp) has a significantly stronger capability to accumulate calcium. 4. In both cultivars, content of polygalacturonic acid increased with fruit development. The increase was faster in 'Huaizhi', therefore, 'Huaizhi'pericarp had a higher polygalacturonic acid content than 'Nuomci'after 36 days after blooming (DAB). The degree of methyl esterification was higher than 'Nuomici'which was related to lower activity of pectin esterase. 5. Fruit pedicle of both cultivars contained rich oxalic acid and calcium.
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