A space-time model of carbon translocation along a shoot bearing fruits

A carbon-based model is described of the source-sink relationships of a ste m bearing fruits in space and time and focusing on growth variability along the branch. The novelty of the model comes from the aggregation of physiol ogical processes taking into account spatial aspects. The stem is represent ed as a set of compartments (metamers) connected to source (leafy shoots) a nd sink (fruits) compartments. Each leafy shoot forms one compartment. The fruit consists of three compartments involved in translocation (cytoplasm), structure (cell wall) and storage (vacuole). Physiological processes consi dered are photosynthesis, respiration of fruits and leaves, translocation o f assimilates and fruit growth. Assimilate production is regulated by sink strength. Carbon translocation between two compartments depends on the grad ient of assimilate concentration. The gradient induces carbon translocation from the most to the least concentrated compartment, except for the vacuol e compartment into which translocation is possible whatever the concentrati on gradient. Fruit growth, in terms of fresh weight, results from the phloe m water supplied to the fruit according to the concentration gradient betwe en the fruit and the stem. The model is calibrated for peach trees by compa ring observed and simulated fruit dry and fresh weights for a shoot with no rmal fruit load. The model simulates variability between peach fruits and t he effect of contrasting fruit loads. According to this model, photosynthes is increases and assimilate concentrations in leaves and phloem decrease wi th decreasing leaf: fruit ratio as reported in the literature. Simulated co ncentrations of assimilates in the phloem range from 2 to 14%. Simulated co ncentration gradients and specific mass transfer for peach trees range from 0.05 to 0.17 g cm(-3) m(-1) and from 0 to 3 g cm(-2) h(-1), respectively, and are of the same order of magnitude as those reported for various other tree species. The model is used to analyse the effect of fruit position rel ative to the leaves. (C) 1999 Annals of Botany Company.