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

Abstract

A carbon-based model is described-of-the source-sink relationships of a stem bearing fruits in space and time and focusing on growth variability along the branch. The novelty of the model comes from the aggregation of physiological processes taking into account spatial aspects. The stem is represented as a set of compartments (metamers) connected to source (leafy shoots) and 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 considered are photosynthesis, respiration of fruits and leaves, translocation of assimilates and fruit growth. Assimilate production is regulated by sink strength. Carbon translocation between two compartments depends on the gradient of assimilate concentration. The gradient induces carbon translocation from the most to the least concentrated compartment, except for the vacuole compartment into which translocation is possible whatever the concentration gradient. Fruit growth, in terms of fresh weight, results from the phloem water supplied-to-the fruit according to the concentration gradient between the fruit and the stem. The model is calibrated for peach trees by comparing observed and simulated fruit dry and fresh weights for a shoot with normal fruit load. The model simulates variability between peach fruits and the effect of contrasting fruit loads. According to this model, photosynthesis increases and assimilate concentrations in leaves and phloem decrease with decreasing leaf:fruit ratio as reported in the literature. Simulated concentrations of assimilates in the phloem range from 2 to 14%. Simulated concentration 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 relative to the leaves.