SORBITOL DEHYDROGENASE ACTIVITY AND CULTIVAR-SPECIFIC APPLE FRUIT GROWTH RATES

Abstract

The activity of sorbitol dehydrogenase (SDH) may be the primary determinant of apple fruit carbohydrate accumulation, perhaps defining cultivar-specific fruit growth rates. During two intervals following June drop, fruit SDH activity of 5 cultivars was assayed along with measurements of fruit relative growth rate (RGR) and total soluble carbohydrate (TSC) and sorbitol levels of fruit cortex tissue. SDH activity varied over 2-fold among cultivars in interval 1 and nearly 7-fold in interval 2. SDH activity was not correlated to fruit RGR or cortex TSC level, but it was negatively correlated to cortex sorbitol level. Defoliation and girdling to restrict sorbitol import slowed fruit growth and reduced SDH activity to nondetectable levels. Growth resumption, presumably following phloem re-establishment in the girdled zone, was accompanied by substantial SDH activity. SDH activity appears to be cultivar-specific and may be influenced by substrate, i.e., sorbitol, availability. While SDH likely plays a key role in apple fruit carbohydrate accumulation, its activity alone may not determine cultivar-specific fruit growth rate.