Many commercial apple (Malus x domestica Borkh.) cultivars follow a biennial bearing cycle, alternating between an “on” year with a heavy crop load and an "off" year with few if any fruit. It is commonly thought that biennial bearing is conditioned by repression of floral initiation by developing fruit. Although various studies have provided some insights into the regulation of flowering and biennial bearing by fruit load, the associated molecular-genetic pathway involved remains unclear. The objective of this study is to evaluate the influence of fruit load on the expression of specific, presumed flowering genes in the spur leaves and shoot apex, focusing on the potential role of sugars in this process. The hypothesis is that the potential accumulation of photosynthates in the leaf or shoot apex related to the absence of a strong sink (developing fruit) promotes floral initiation by directing the expression of floral-promotive genes. To test this, physiologically similar ‘Royal Red Honeycrisp’ trees were manipulated to influence fruit load. The accumulation of soluble sugars (fructose, sorbitol, glucose, and fructose) in the spur leaf and shoot apex was evaluated using a gas chromatograph. RNAs were extracted and gene expression patterns in the leaf and apex were evaluated using quantitative RT-PCR and direct sequencing; results will be presented. This study should provide new insights into the fundamental mechanisms of biennial bearing in apple. This may lead to approaches to limit the impact of biennial bearing in existing orchards, and genetic information useful for apple cultivar improvement.
