Summary• Functional-structural plant models simulate the development of plant structure, taking into account plant physiology and environmental factors. The L-PEACH model is based on the development of peach trees. It demonstrates the usefulness of Lsystems in constructing functional-structural models.• L-PEACH uses L-systems both to simulate the development of tree structure and to solve differential equations for carbohydrate flow and allocation. New L-systembased algorithms are devised for simulating the behavior of dynamically changing structures made of hundreds of interacting, time-varying, nonlinear components.• L-PEACH incorporates a carbon-allocation model driven by source-sink interactions between tree components. Storage and mobilization of carbohydrates during the annual life cycle of a tree are taken into account. Carbohydrate production in the leaves is simulated based on the availability of water and light. Apices, internodes, leaves and fruit grow according to the resulting local carbohydrate supply.• L-PEACH outputs an animated three-dimensional visual representation of the growing tree and user-specified statistics that characterize selected stages of plant development. The model is applied to simulate a tree's response to fruit thinning and changes in water stress. L-PEACH may be used to assist in horticultural decisionmaking processes after being calibrated to specific trees.

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Carbohydrate partitioning is closely coupled with plant growth and architecture, and therefore constitutes an important aspect of the functional-structural modelling of plants. L-PEACH is an L-systembased tree simulation model that combines supply/demand concepts of carbon partitioning with a developmental model of tree architecture. The model is expressed in terms of modules that represent plant organs. An organ may correspond to one or more elementary sources or sinks for carbohydrates. The whole plant is modelled as a branching network of these sources and sinks, connected by conductive elements. An analogy to an electric network is used to calculate the flow and partitioning of carbohydrates between the individual components. It can simulate multiple years of tree growth while capturing the effects of irrigation, crop load and pruning on carbon partitioning and the dynamics of architectural development. The growing tree can be visualized in a schematic or semi-realistic manner, while quantitative data characterizing individual organs, organ types or the whole tree can be output for visualization and analysis to an external program, such as MATLAB.

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