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 comme rcial benefit. To assess the main causes of variability in the potenti al and actual fruit weight of mature fruit, both were measured togethe r with the seed content on three tomato cultivars. Potential and actua l weight were appraised on plants with single-fruit and seven-fruit tr usses, respectively, until maturation of the fifteenth truss. Variabil ity in the potential weight was mainly related to the cultivar, wherea s differences between proximal and distal fruits were significant for beefsteak tomatoes only. In the long term, no truss effect on this pot ential could be detected. Under competitive growth conditions, the wei ght of distal fruits was reduced more than that of proximal fruits esp ecially for the beefsteak cultivar. All trusses were not equally affec ted, inducing a large variability along the stem. The relation between fruit weight and seed number was closer as the range of variability i n fruit weight was reduced, that is in increasing order: long life, ro und, beefsteak cultivars. The slope of the linear relation between fru it weight and seed number was reduced by the competition for assimilat es, except for the long life cultivar. This was attributed to its smal ler range of fruit weight, assuming that all sinks were affected in th e same proportion by limiting assimilate supply, and hence bigger frui ts lost a higher absolute weight than small fruits. Therefore a high r esidual variability in fruit weight is not explained either by the fru it potential or by its seed content, and may relate to internal regula tions 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.