Variability In Sweet Cherry Flower Bud Distribution In the UFO System Is Related to Genotype and Shoot Vigor

The Upright Fruiting Offshoots (UFO) architecture for sweet cherry (Prunus avium L.) was developed at Washington State University in collaboration with industry leaders. This novel planar system aims to improve production efficiency, utilize natural cherry growth habit, reduce environmental footprint, and facilitate the incorporation of technology. Though greatly simplified compared with other cherry training systems, the establishment and management of UFO system is still subject to research and development due it its infancy. The architecture is based upon vertical, unbranched fruiting wood that is established in the first two years and renewed regularly. Our observations are that vigor of current season shoots is variable and inversely related to shoot precocity and productivity.  In order to improve uniformity and system productivity, a better understanding of the physiology behind the variability in flower bud formation and distribution is needed. The aim of this study was to document the variability of flower bud distribution in two-year-old vertical uprights, related to upright vigor. We studied these relationships in a research orchard with five cultivars on two rootstocks. Dormant two-year-old wood was divided in three equal sections by length (proximal, medial and distal) and the number flower buds per spur were recorded for each node along with caliper and length of each upright. In addition, the shoots were segregated by vigor (caliper) to examine the influence of shoot vigor on the distribution and density of reproductive buds. Observations suggest that the distribution of spurs and flower buds within fruiting wood is cultivar dependent, but most exhibited more than 50% of the reproductive buds in the distal section. The relationships between upright vigor and the distribution and density of reproductive buds reveal that medium-low vigor uprights have more homogenous distribution and greater density (+5 – 40%) of fruit buds than upright shoots with high vigor. These results underscore the importance of moderating excessive vigor in annual vegetative growth in this system.