Analysis of the impact of carbon source-sink relationships on flowering patterns reveals that apple tree growth and functioning are determined by mechanisms occurring at the tree and shoot scales

In apple trees, external (temperature and water stress) and internal (hormonal signaling, carbon source-sink relationships) factors are assumed to affect floral induction and, consequently, to determine the regular or biennial fruiting behavior. Nevertheless, no clear consensus exists on the role of each factor and on the level of plant organization at which the production patterns are determined. This study thus is aimed at analyzing the C source-sink relationships from shoot to tree scales, in a set of genotypes displaying a large variability in flowering and production patterns. Within an apple tree population issued from a bi-parental cross, three groups of genotypes were identified: biennial flowering genotypes, regular flowering genotypes resulting from a high production of reproductive growth units over years (“bourse over bourse” genotypes), regular flowering genotypes displaying, each year, an almost equal proportion of vegetative and reproductive growth units (“desynchronized” genotypes). Photosynthesis and growth measurements were carried out in 2014 and 2015, during spring and summer. No difference in photosynthesis was observed between “bourse over bourse” compared to biennial genotypes in “ON” flowering years. For biennial genotypes, a higher leaf photosynthesis rate was observed for shoots of “ON” trees compared to “OFF” ones, suggesting a positive feed-back of fruit presence (sink activity) on sources (leaves). All shoots (vegetative or reproductive) of “desynchronized” genotypes reached the same photosynthesis activity, suggesting a regulation of photosynthesis at the tree scale. Conversely, in those genotypes, vegetative monopodial shoots were longer than reproductive sylleptic shoots and reached the same length as the vegetative shoots of biennial genotypes in “OFF” years. These results give new insights into the within-tree variability of source-sink relationships in apple depending on either local or global fruiting conditions. They provide evidence that shoot growth seems to be determined by the local context whereas photosynthesis is regulated by the carbon demand, at the plant scale.