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柑桔是典型的亚热带常绿果树,对低温极为敏感。就温州蜜柑来说,-7℃低温对叶片有冻害影响,-9℃骨架受冻,-10℃以下有冻死威胁。安全越冬是柑桔生产最重要的气象条件。解放三十多年来,我国曾出现三次影响较大的严重冻害天气(1954～55年、1968～69年、1976～77年),致使不少省份柑桔产量大幅度下降。位于我国柑桔栽培北缘地区的上海也不例外,从上海市农科院园艺所柑桔课题组对上海郊区40多个桔园调查可知,1977年1月31日的低温,使上海郊区桔树产生冻害4级的占2.54％,3级占54.97％,2级占14.3％,1级占5.72％,0级占2.64％。但是长兴、横沙两岛柑桔冻害轻微,冻害指数仅1～2％,引起了不少生产单位和科研人员的关注。
Citrus is a typical subtropical evergreen fruit tree, very sensitive to low temperatures. In the case of Wenzhou mandarin, the low temperature of -7 ℃ has the effect of frost damage on the leaves, the frame of -9 ℃ is frozen and the threat of freezing and freezing below -10 ℃ is threatened. Safe winter is the most important weather conditions for citrus production. In the past 30 years since the liberation, China has witnessed three major severe freeze-harming weather (1954-55, 1968-69, 1976-77), resulting in a sharp drop in citrus production in many provinces. Shanghai is located in the northern margin of China’s citrus cultivation is no exception, from the Shanghai Academy of Agricultural Sciences horticultural citrus citrus subproject survey of more than 40 orange orchards in the suburbs of Shanghai, January 31, 1977 low temperature, the Shanghai suburbs orange Grade 4 trees account for 2.54%, grade 3 54.97%, grade 2 14.3%, grade 1 5.72% and grade 0 2.64%. However, Changxing, Hengsha two citrus cold damage slightly, frost damage index only 1 to 2%, causing a lot of production units and researchers attention.