Carbon Translocation in the Tomato: Carbon Import and Fruit Growth1

The rates of carbon import by fruits were measured over 48 h as the sum of the change in the total organic carbon content of the fruit and the respiratory loss of carbon. Over a range of fruit sizes from 20–90 per cent of the maximum volume the smaller fruits imported carbon at an absolute rate (mgC fruit−1 h−1) nearly twice that of the larger fruits. The imported leaf assimilates, identified as the 14C-compounds along the pathway between a 14CO2-fed leaf and a young fruit, comprised 90 per cent sucrose and 10 per cent glutamic acid, aspartic acid and malic acid. Within the fruit the imported 14C-sucrose was hydrolysed into hexoses. The changes in the levels of starch and insoluble residue in the fruit were positively correlated with the carbon import rates. In the largest fruits with the lowest import rates, there was breakdown of insoluble residue and less accumulation of starch, but a significant increase in the level of sucrose. The sink strength of a tomato fruit is dependent more on sink activity than on sink size.