Influence of cultivar, fruit position and seed content on tomato fruit weight during a crop cycle under low and high competition for assimilates

The control of tomato fruit weight during a crop cycle may be of commercial benefit. To assess the main causes of variability in the potential and actual fruit weight of mature fruit, both were measured together with the seed content on three tomato cultivars. Potential and actual weight were appraised on plants with single-fruit and seven-fruit trusses, respectively, until maturation of the fifteenth truss. Variability in the potential weight was mainly related to the cultivar, whereas differences between proximal and distal fruits were significant for beefsteak tomatoes only. In the long term, no truss effect on this potential could be detected. Under competitive growth conditions, the weight of distal fruits was reduced more than that of proximal fruits especially for the beefsteak cultivar. All trusses were not equally affected, inducing a large variability along the stem. The relation between fruit weight and seed number was closer as the range of variability in fruit weight was reduced, that is in increasing order: long life, round, beefsteak cultivars. The slope of the linear relation between fruit weight and seed number was reduced by the competition for assimilates, except for the long life cultivar. This was attributed to its smaller range of fruit weight, assuming that all sinks were affected in the same proportion by limiting assimilate supply, and hence bigger fruits lost a higher absolute weight than small fruits. Therefore a high residual variability in fruit weight is not explained either by the fruit potential or by its seed content, and may relate to internal regulations of the plant in response to the source-sink balance during crop development. Dynamic models able to simulate this pattern may help in developing approaches for the control of fruit weight variability.