CULTIVAR-SPECIFIC APPLE FRUIT-GROWTH RATES INVIVO AND SINK ACTIVITIES INVITRO

Absolute and relative fruit growth rates (AGR and RGR) of apple (Malus domestica Borkh.) were calculated from the fruit dry weights of several cultivars harvested periodically following June drop during 1988-90. AGRs were constant or varied slightly, and RGRs generally declined as the season progressed. Generally, both AGR and RGR values were higher for relatively large fruit of several cultivars with similar days to maturity, e.g., 'Mclntosh' vs. 'Jonathan' and for summer- vs. fall-ripening cultivars, e.g., 'Stayman' vs. others. An exception was observed in 1990, when 'Golden Delicious' exhibited a higher AGR but lower RGR than 'Rome Beauty', yet ripened 1 month earlier. 'Golden Delicious' AGR and RGR values were lower for both fruit of a pair on a spur than the values for a single fruit on a spur, and the dominant fruit of the pair exhibited higher growth rates than the inferior fruit. Rates of sorbitol accumulation (SAR) by cortex disks incubated in C-14-labeled sorbitol solutions in vitro declined as the season progressed. Within a cultivar, SARs were not related to fruit size, nor were differences found between cortex disks from competing fruit on a spur, although SARs were higher for both competing fruit on a spur as compared to that of a single fruit per spur. Due to a positive correlation between RGR and SAR values, the SAR of cortex cells may be regulated in such a manner as to be a physiological constraint on fruit sink strength and growth rate.