A comparison of ‘Empire’ apple fruit size and anatomy in unthinned and hand-thinned trees

SummaryA stereological method was developed to analyze the anatomical features of fresh ‘Empire’ apple fruit sectors cut in the transverse equatorial plane. Fruits were from unthinned trees or trees hand-thinned to one fruit per cluster at –7, 0, 10, 20 or 40 d after full bloom. At final harvest (140 DAFB), fruits representing the size range within each treatment were analyzed for the effects of thinning on fruit size, weight and cortex anatomy, namely, parenchyma cell size, cell number and the proportion of cortex volume occupied by intercellular space (IS). A dissecting stereobinocular microscope fitted with a ten-by-ten reticule was used to count cells and proportion of IS in three fields in each of two cortex sectors per fruit. Cell volume in each field was derived by knowing only the grid area, a point-count for proportion of IS and a count of cell numbers within the grid. Fruit size and weight decreased as thinning was prolonged and unthinned trees had the smallest fruit. Within a thinning treatment, fruit size was positively correlated with cortex cell number, not with cell size or proportion of IS. This also held between treatments; unthinned trees had smaller fruit with fewer cells than did larger fruit from thinned trees, and fruit of trees thinned near bloom were larger with more cells than those of trees thinned later. For ‘Empire’, fruit thinning appeared to increase fruit size by allowing remaining fruits to continue cell division under less competition during the first weeks after bloom, and not by extending the cell division period, increasing cell size or increasing proportion of IS.