| Citrus is a very important fruit tree not only in the world but also in china. The fruits ripen mainly from October to December. So, there is very important commercial value to delay harvesting via cultivator or culture practices. In this thesis, the study was carried out by using plant growth regulators to delay the harvest time of Cara Cara (Citrus sinensis Osbeck cv.) and Frost (C. sinensis Osbeck cv.) navel orange and to analysis the effects of late harvest on the blossom, yield and nutrient contents of leaves in the coming year. The main results were as follows:1. After delay harvesting, the blossom and yield in the next year of Cara Cara navel orange was not be affected. Hanging fruits on tree by 100%, 75%, 50% for 90-120 days, did not affect flowers number and fruit yield among the treatments. The flower numbers were 400-450 per 1/8m~3.2. The nutrient contents of leaves in the coming year could not be affected by keeping fruits on tree either. After the analysis of the nutrient contents including soluble sugar, soluble protein, starch and total nitrogen, the same trend existed among different treatments; however, it affected the time of nutrient accumulation.3. Fruit quality could be improved by delay harvesting for Cara Cara navel orange. Total soluble solids (TSS) increased by one Brix and the maximum was 15.5%, titrable acidity (TA) decreased to 0.55%. The ratio between TSS and TA is from 14.4 to 18.0, the percentage of juice (PJU) and vitamin C (Vc) was constant at 41-44% and 45-50mg/100mg respectively, but there is no differences existed among the treatments.4. A protocol for delay harvest of navel orange was made for practice. During the last three years delayed harvest was tested at Zigui and Xingshan of Hubei and Gannan of Jiangxi, covering several cultivars like Frost, Red Flesh navel orange, Newhall and Jincheng sweet orange. The content of the protocol was comprehensive with detailed measures that could be manipulated easily.
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