Résumé
Beyond the well-described architectural diversity between apple tree cultivars, a functional variability is suspected, still scarcely investigated, which could affect their productivity and adaptation potential. In other respects, it has been assumed that functional variability of different apple cultivars considered at an integrated scale could depend on the architectural organization of the aerial system and on the variability of foliage functions. The comparison which was here carried out between genotypes included 4 apple cultivars: Fuji and Braeburn, on the one hand, and Ariane and X3305, on the other hand. The trees were respectively 7-years-old and 2-years-old at the beginning of this study. Trees were cultivated in an experimental orchard which was irrigated and submitted to standard professional practice. Between-trees study of the architectural variability revealed different dynamics of foliage setting between younger and older trees. Some differences in shoot composition were also revealed between trees of the same age, resulting in different light interception efficiency which was quantified by the STAR parameter (Silhouette to Total leaf Area Ratio). A method for tree 3D-structure description was validated, which used a mixed approach of foliage digitalization and reconstruction. Functional comparison at the leaf scale showed that parameters of stomatal regulation (Jarvis model) differed between the 4 genotypes, while photosynthetic parameters (biochemical model of Farquhar) showed little variation. Architectural and functional characteristics of each cultivar were taken into account to parameterize and apply the RATP functional-structural model, and their coupling made it possible to simulate light interception capacity, transpiration and carbon gain at the branch scale. A sensitivity analysis of the model was applied to the Braeburn cultivar; it showed that architectural traits predominate when functional variability is considered at the branch scale. The research prospects opened by this ecophysiological approach applied to the apple tree by means of a functional-structural model are discussed.