| Pre-harvest fruit cracking is an important problem in citrus production. Peel pitting, also known as albedo breakdown, is not observed at the early stage of fruit development. It can cause orchard losses because the surface of peel pitting fruit is abnormal, which leads fruit peel toughness to decrease and the fruit not suitable for transporting and storing. In order to know the time and reason of peel pitting development, this study was conducted.Normal orchards and serious fruit peel pitting orchards of 8-10 years old trees Newhall navel orange (Citrus sinensis Osbeck cv. Newhall) and Cara cara navel orange (Citrus sinensis Osbeck cv. Cara Cara) on trifoliate orange (Poncirus trifoliata (L.) Raf.) root stocks, grown in the forth quaternary period red soil of An yuan County Jiangxi Province in China, were used in this study. Fruit peel thickness, peel albedo microstructure, nutrition element (N, P, K, Ca, Mg, S, Fe, Mn, Zn,Cu) contents in fruits in different stages after full bloom were tested or observed, and nutrition element contents in leaves,peel pitting rate and fruit quality characteristics were also tested in November.The main results were as follows:1. Comparation of peel pitting rate, the thickness of peel and the quality of fruitsThe pitting rate of Newhall and Cara cara peel pitting orchards were significantly higher than those of normal orchards, and the pitting rate of Newhall peel pitting orchards were significantly higher than those of Cara cara orchards. Newhall was more sensitive to peel pitting than Cara Cara.There was no significant correlation between fruit peel thickness and fruit peel pitting occurrence, and peel thickness rapid decreasing could be a promoting effect causing the pitting happen. Fruit quality analysis showed that there was hardly any difference between pitting fruits and normal fruits in the two varieties.2. Comparation of peel microstructure in the development of peel pittingThe development of pitting was a slow process of gradual change.The peel microstructure of pitting fruits and normal fruits were no difference 21 d after full bloom that the cells of peel albedo arranged regularly and closely, almost no intercellular space could be visible; Crevices or small pores began appearing in peel albedo of both Newhall and Cara cara fruits 80d after full bloom and then became larger and larger. Crevices or small pores were visible by naked eyes in peel cross-sections 140d after full bloom when pitting symptom occurred on fruit surfaces. Microstructure crevice became larger and extended to the surface of fruits, and leaded to split 217d after full bloom. The development of peel pitting had two kinds of forms:tear and cell apoptosis.3. Comparation of mineral nutrient element in fruits and leavesThe contents of mineral nutrient element such as N, P, K, Ca, Mg, S, Fe, Mn, Zn and Cu in peels and pulp of fruits were decrease with the growth and development of fruit. Ca contents in peel and pulp of pitting fruits were significantly lower than those of normal fruits on the 80th,140th and 217 th days after full bloom, while the contents of N, P, K, Mg, S, Fe, Mn, Zn and Cu had no obviously trends between normal and pitting fruits. Ca contents in leaves of fruit pitting orchards were also significantly lower than those of normal orchards, but S contents in leaves were significantly higher at pitting fruit orchards, while the contents of N, P, K, Mg, S, Fe, Mn, Zn and Cu had no obviously trends between normal and pitting fruit orchards. Those indicated that Ca deficient was related to fruit peel pitting.Finally, some of the factors related to pitting were discussed, especially the relationship of Ca and pectin components of cell wall. The different soil types may be the main reason of Ca deficiency in navel orange trees, and cultivation management measures for preventing fruit peel pitting were also discussed. |
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