S16 - Session O2 - A model of water and carbohydrate transport in fruit-bearing apple tree branches: effect of pruning-induced modifications in architecture

Authors: Gerhard Buck-Sorlin *, Aleksi Tavkhelidze, Winfried Kurth

The canopy of apple ( Malus domestica Borkh.) in an orchard production system consists of the trunk and several fruit-bearing branches (FBB). The architecture of a FBB is the result of the rhythmic activity of terminal and lateral buds. A FBB during the growing season is made up of source (leaves) and sink (fruits) organs connected to each other via internodes containing phloem and xylem vessels serving to transport water, sugar and minerals. A number of models have been proposed to simulate transpiration-driven water flow in the xylem as well as Münch flow and related sugar transport in the phloem. However, with few exceptions, none of these models have considered complex branch architectures beyond simple continuous axes, or the coupled xylem-phloem flux including direct water exchange between xylem and phloem in each internode. The coupled model proposed here, aims at providing insights into the qualitative assessment of water and carbon distribution patterns as well as their pruning-induced shifts by means of simulating, in real time, the coupled water/sap flow for arbitrary architectures of apple tree branches. Pruning is a management technique involving the removal of purely vegetative shoots, with the aim to improve light interception and leaf photosynthesis, and to induce a source-sink ratio favourable for fruit growth. The proposed framework unites the numerically solved differential equations for the flow with a composite sun/sky radiation model enabling a precise simulation of light interception and photosynthesis. All 3D branch architectures used were taken from experimentally-based models. For all simulations, the interactive modelling platform GroIMP was used. In the future, we will enhance the dynamic aspect of the model by including growth processes as well as improve the computational performance of the code in order to support more complex branch architectures and longer simulations.