Apple Rootstock-Induced Dwarfing is Strongly Influenced by Growing Environment

Despite the extensive use of dwarfing rootstocks in commercial apple cultivation worldwide, it is presently unknown how different growing environments may modify the first expression of rootstock-induced dwarfing of the scion. To address this, we measured the growth and architectural development of compound ‘Royal Gala’ apple trees grafted onto three different vigour clonal rootstocks (‘M27’, ‘M9’ and ‘M793’), grown in three locations of New Zealand. Detailed architectural measurements were made over the first year of growth after grafting and the study was repeated for a second year. Within each site and year, trees on ‘M793’ had the greatest primary axis length and node number, trunk cross-sectional area, number of sylleptic shoots and final tree dry weight, while those on ‘M27’ had the smallest. Trees on dwarfing rootstocks generally had a higher frequency of flowering than those on ‘M793’. Our results demonstrate that the effect of dwarfing rootstocks on sylleptic shoot growth, flowering, and dry weight is highly plastic in response to the growing environment. Across all rootstock genotypes, the scion bud type had a strong effect on tree growth: trees that developed from a vegetative scion bud (monopodial primary axis) had a final dry weight 15-45% greater than those with a floral scion bud (sympodial primary axis). One rootstock-induced effect that was consistent between sites and years was that dwarfing rootstocks caused earlier termination of primary axis growth. This provides novel information about the physiological processes that apple dwarfing rootstocks modify.