Rootstock effects on growth and cell size of 'Gala' apple fruit

The effect of rootstock on apple fruit growth was investigated on fruit from 'Gala' trees growing on M.27 EMLA, P.2, M.9 EMLA, and M.26 EMLA rootstocks. These rootstocks resulted in a wide range of tree size, varying from 4.8 to 16.7 cm2 trunk cross- sectional area for M.27 EMLA and M.26 EMLA respectively. Trees bore crop loads that were not expected to negatively affect fruit growth. Rootstock did not affect fruit growth rate or final fruit size. Fruit growth rate ranged from 1.0 mm- day-' soon after flowering and decreased to 0.2 mm- day''' near harvest. Rootstock had no affect on cortical cell size. Fruit size was only slightly affected by cortical cell size, suggesting that differences in fruit size were primarily due to changes in cell number rather than cell size. It would appear that any effect of rootstock on fruit size is via modification of the cell number in the fruit cortex, either by affecting the number of cortical cells present in the flower at anthesis or by altering the dynamics of cell division.