Apple rootstocks can modulate the chilling requirements of grafted scions

Apple rootstocks modify scions at the molecular level, modulating gene expression and hormone flux. Some Geneva (R) apple rootstocks show increased productivity in low chill environments by causing more floral and vegetative buds to break in the top 1/3 of high chill scions like Gala (similar to 890) and 'Fuji' (similar to 1150). We used an existing, mature (8th leaf) 'Gala' orchard featuring a full-sib population of rootstocks. To measure the effect of rootstocks on bud break, we harvested 30 cm one-year growth shoots for replicated experiments in 2 consecutive years (2017-2018). In 2017 shoots were harvested when the trees had accumulated 800 h (North Carolina model) of chilling were then placed directly into a greenhouse environment (20 degrees C day/17 degrees C night). In 2018 we tested three chilling hour accumulation levels (585 h, 702 h, and 906 h). In 2018 shoots were harvested on November 22, when the trees had accumulated 585 chill hours. A set was placed directly in the greenhouse at 20 degrees C day/17 degrees C night while the rest were stored at 5 degrees C until they had reached the desired chilling hour accumulation (702 and 906 h) to be placed in the same greenhouse environment. Bud break was recorded at 15 and 30 days after move in a greenhouse. Rootstock genotype and chilling treatment significantly affected apical and lateral bud opening in both years. Normally, apical buds open first, followed by laterals. In 2017 approximately 50% of the rootstock genotypes had one or more buds open after 30 days, indicating a significant rootstock effect on the scion chilling requirement. In 2018 the proportion of rootstock genotypes displaying open buds after 30 days increased with increasing chilling hours (585 h =10%; 706 h =20%; 906 h =38%). These preliminary results in the germplasm tested to show a definite genetic effect of apple rootstocks on scion chilling requirement.