A COMPARISON OF EMPIRE APPLE FRUIT SIZE AND ANATOMY IN UNTHINNED AND HAND-THINNED TREES

A stereological method was developed to analyze the anatomical feature s of fresh 'Empire' apple fruit sectors cut in the transverse equatori al plane. Fruits were from unthinned trees or trees hand-thinned to on e fruit per cluster at -7, 0, 10, 20 or 40 d after full bloom. At fina l harvest (140 DAFB), fruits representing the size range within each t reatment were analyzed for the effects of thinning on fruit size, weig ht and cortex anatomy, namely, parenchyma cell size, cell number and t he proportion of cortex volume occupied by intercellular space (IS). A dissecting stereobinocular microscope fitted with a ten-by-ten reticu le was used to count cells and proportion of IS in three fields in eac h of two cortex sectors per fruit. Cell volume in each field was deriv ed by knowing only the grid area, a point-count for proportion of IS a nd a count of cell numbers within the grid. Fruit size and weight decr eased as thinning was prolonged and unthinned trees had the smallest f ruit. Within a thinning treatment, fruit size was positively correlate d with cortex cell number, not with cell size or proportion of IS. Thi s also held between treatments; unthinned trees had smaller fruit with fewer cells than did larger fruit from thinned trees, and fruit of tr ees thinned near bloom were larger with more cells than those of trees thinned later. For 'Empire', fruit thinning appeared to increase frui t Size by allowing remaining fruits to continue cell division under le ss competition during the first weeks after bloom, and not by extendin g the cell division period, increasing cell size or increasing proport ion of IS.