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Sorbitol is the primary photosynthate and translocated carbohydrate in apple ( Malus  × domestica ), and most of it is converted to fructose by sorbitol dehydrogenase (SDH) in sink tissues. We studied the expression of nine  SDH  genes, SDH activity, and sorbitol content of apple 1) in buds and floral tissues from dormancy to bloom, 2) in leaves and shoot tips of trees on two rootstocks, the moderately vigorous ‘Malling Merton 111’ (MM.111) and the dwarfing ‘Malling 9’ (M.9), and 3) in shoot tips in response to application of prohexadione-Ca to suppress shoot growth and defoliation and girdling (D/G) to deprive the shoot tip of sorbitol. In mature, orchard-grown trees, sorbitol was the main soluble carbohydrate in expressed xylem sap from dormancy to bloom at levels over 3- to 6-fold those of glucose and fructose, the other major sugars present. Sorbitol levels there increased from dormancy to its highest concentration at the half inch green stage and declined by bloom, while those of the other sugars increased. SDH activity per milligram of protein increased over 4-fold from dormancy to flowering. Three of the nine known  SDH  genes ( SDH1, SDH2 , and  SDH3 ) were expressed in immature and mature leaves and all buds from dormancy to bloom, as well as in all floral organs, except that only  SDH3  transcript was found in stamen tissue. Two genes,  SDH6  and  SDH9 , were floral-tissue specific;  SDH6  transcript was detected in all floral organs except stamens at full bloom, and  SDH9  was only expressed in anthers with pollen. In buds and leaves of young, container-grown trees,  SDH1  and  SDH2  generally accounted for the majority of total  SDH  expression. There were generally no effects of rootstock on  SDH  expression, SDH activity, or sorbitol concentration in leaves, while apical shoot tips on M.9 rootstock exhibited greater SDH activity than those on MM.111 or lateral shoot tips on either rootstock, though  SDH  expression of apical and lateral shoot tips on M.9 was lower than on MM.111. Prohexadione-Ca reduced apical but not lateral shoot growth, increased apical but not lateral shoot tip sorbitol content, had no effect on SDH activity, and increased  SDH1  expression of all shoot tips. D/G treatment reduced shoot growth, sorbitol content, and SDH activity, but increased  SDH1  expression of apical shoot tips only and  SDH2  expression of lateral shoot tips only. This work indicates that sorbitol and other sugars are abundantly available from dormancy to bloom, that SDH activity increased during this period, and that  SDH  expression is at least in part developmentally regulated within the individual floral and leaf tissues. In shoot tips and leaves of young trees,  SDH  transcript level was not correlated with sorbitol availability or SDH activity, suggesting that other factors have significant regulatory effects after  SDH  expression on SDH activity.

journal of the american society for horticultural science

Sorbitol Dehydrogenase Gene Expression and Enzyme Activity in Apple: Tissue Specificity during Bud Development and Response to Rootstock Vigor and Growth Manipulation