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Zinc(Zn)is an exceptional micronutrient regarding its relevance in biological systems because it is the only trace metal represented in all classes of enzymes.Zinc sprays at critical stages can improve apple fruit quality,but its physiological reason is unclear.The study was initiated to determine the response of apple(Malus domestica)fruit quality to sprays of zinc sulfate(ZnSO4)and sugar alcohol zinc.The trees were sprayed with ZnSO4 and sugar alcohol zinc separately during four different developmental stages:2 weeks before bud break(P1),3 weeks after bloom(P2),the termination of spring shoot growth(P3),and 4 weeks before harvest(P4).The fruits were harvested at these four stages and analyzed for sorbitol dehydrogenase(SDH),sorbitol oxidase(SOX),fruit quality and Zn concentration.The results showed that the treatments at stages P1 and P4 increased the fruit firmness of'Gala',while the treatments at stages P1 and P2 increased the fruit firmness of 'Fuji'.The treatments at all the stages increased the soluble sugar and vitamin C of'Fuji',and the effects of sugar alcohol zinc were equal and more pronounced than those of ZnSO4.Thus,Zn sprays at critical periods can improve fruit quality of apple trees,which show no Zn deficiency symptoms.Why these effects exist? The physiological reason may be: the activity of sorbitol dehydrogenase(SDH)was significantly higher at the above four stages than the control,but the sorbitol oxidase(SOX)activity of all the treatments was not significantly different compared with the control.Our results showed Zn treatments enhanced the biofortification of fruit Zn,which resulted in the higher activities of carbohydrate metabolism-related enzymes and the accumulation of sugar in fruits,then improved the fruit quality.