| The effects of rootstock on apple fruit growth, size, quality, cell number, and cell size were investigated over three consecutive seasons (2000--2002) on fruit from apple trees (Malus domestica Borkh cv. Gala) growing on M.26 EMLA, Ottawa-3, Pajam-1 and V-605-1 rootstocks at the Peninsular Agricultural Research Station near Sturgeon Bay, WI. These rootstocks provided a range of tree sizes, varying from 21.2 to 50.9 cm2 trunk cross-sectional area (TCA) for Ottawa-3 and M.26 EMLA, respectively. Because trees bore a range of crop loads, analysis of covariance was used to adjust fruit size for crop load. There was no interaction between seed count, light interception, specific leaf area (SLA) or fruit size. Fruit growth in length and diameter was recorded at weekly intervals from 4 weeks after full bloom (WAFB) to harvest. Fruit fresh weight and volume were estimated by quadratic regression. Fruit weight and volume were closely and positively correlated suggesting that fruit size can be estimated by either metric. Cell number and cell size were monitored from full bloom until harvest using scanning electron microscopy (SEM) and, generally, rootstock did not affect either parameter. Although trees on M.26 EMLA yielded slightly more during 2000, rootstock did not affect yield efficiency in any year. Rootstock had no influence on fruit quality attributes during 2001; however, in 2002, fruit obtained from trees on Pajam-1 tended to be less firm. Lower soluble solid concentration (SSC) was associated with fruit from trees on V-605-1 and Pajam-1. Rootstock had no effect on cortical cell number or cell size, except for mature fruit from trees on Pajam-1 which tended to have slightly larger cells than fruit from trees on Ottawa-3 in 2002. Cell division ceased about five to six weeks after full bloom. Fruit enlargement was from cell expansion for the rest of the season. Fruit size was positively correlated with cell size, suggesting that differences in fruit size were primarily due to changes in cell volume rather than cell number or intercellular space (IS). It would appear that apple fruit size could be influenced more by fruit load and factors other than rootstock. |
