Number of cells in tomato fruit depending on fruit position and source-sink balance during plant development

Fruit sink strength or its ability to attract assimilates depends both on sink activity and size. This study investigated one main component of sink size, that is the number of fruit cells during tomato plant development. Plants were grown in a controlled climate chamber under a limiting (LS, six fruits per truss) and non-limiting (NLS, two fruits per truss and CO2 enrichment) supply of carbon assimilates. Under NLS conditions, fruit cell number was homogeneous among successive trusses, and fruits contained on average 1.2 x 10(6) more cells than under LS conditions, though differences were not significant on the first truss which underwent the lowest competition. Under LS conditions, an ontogenetic increase in cell number was observed in proximal fruits of the upper trusses attributed to the enlargement of the apical meristem during plant development. The decrease of cell number from proximal to distal fruits within a truss, that was expected from the literature, was generally observed in the LS experiment, with an average significant difference of about 1.6 x 10(6) cells between the first and fifth fruits. Nevertheless, whereas the gradient in cell number from proximal to distal fruits was steep in the upper trusses, it was not significant on the lower trusses indicating that this gradient largely depended on the level of competition during floral development. Thus, under low assimilate supply, cell division is a main limiting factor for fruit growth, although cell enlargement during further fruit development is also affected, but was not measured in this work.