Carbohydrate analyses indicate that fruit-bud competition for assimilates is not the primary trigger of biennial bearing in apple

Biennial bearing is one of the major challenges in the commercial production of apples (Malus × domestica Borkh.). Unless a considerable portion of flowers in apple orchards is removed every year, naturally occurring high crop load (ON-year) strongly suppresses flowering in the following year, leading to low yields (OFF-year). This ON-OFF bearing cycle significantly diminishes the profitability of apple orchards. This phenomenon generally occurs in all apple varieties, but is much more pronounced in some genotypes (biennial-bearing) than in others (regular-bearing). Although apple fruits of the current season and flower buds for the next season develop simultaneously, it remains unclear whether biennial bearing is triggered by signaling compounds from the fruits or results from carbohydrate competition between growing fruits and buds. To test the carbohydrate competition hypothesis, we analyzed nine carbohydrates in bourse buds of the biennial-bearing cultivar 'Fuji' and the regular-bearing cultivar 'Gala'. Bud samples were collected from high-cropping (ON) and non-cropping (OFF) trees during the period of flower bud formation. Our results showed no evidence of carbohydrate deficiency in buds from ON-trees compared to those from OFF-trees. Contrary to the hypothesis, the concentrations of glucose and fructose in 'Gala' were higher in buds from ON-trees. Furthermore, we analyzed 15 carbohydrates in the leaves of nine regular-bearing and eight strongly biennial-bearing apple cultivars and found no clear connections between carbohydrates in leaves and bearing behavior of these cultivars. Our data therefore do not support the hypothesis that carbohydrate competition between fruits and buds is the primary trigger of biennial bearing in apple.