COMPETITION FOR ASSIMILATES AND FRUIT POSITION AFFECT FRUIT-SET IN INDETERMINATE GREENHOUSE TOMATO

Localization and characterization of fruit set in winter tomato crops was investigated to determine the main internal and external controlli ng factors and to establish a quantitative relationship between fruit set and competition for assimilates. Individual fruit growth and devel opment was assessed on a beef tomato cultivar during the reproductive period (first nine inflorescences). A non-destructive photograph techn ique was used to measure fruit growth from very early stages of their development and then calliper measurements were made on big fruits. Fr om these measurements we determined the precise developmental stage at which fruit growth stopped. Fruit potential growth, which is defined as the growth achieved in non-limiting conditions for assimilate suppl y, was also assessed by this method on plants thinned to one flower pe r inflorescence. The latter was used to calculate the ratio between ac tual and potential growth, which was found to be a good index of the c ompetition for assimilates. Time lags of fruit set were observed mainl y on distal organs. When more than three flowers were left on each inf lorescence, distal organs developed at the same time as proximal organ s of the following inflorescence. Consequently they were submitted to a double competition within one inflorescence and among inflorescences . It was shown that, what is commonly named 'fruit set failure', is no t an irreversible death of the organ and that a small fruit could resu me growth after a delay of several weeks as soon as the first fruits r ipened and thus ceased to compete for assimilates. In that case proxim al fruits resumed growth before distal ones. The delayed fruits contai ned only few seeds but a germination test confirmed that fertilization took place before fruit set failed. Competition for assimilates was c alculated during plant development by the ratio between actual and pot ential fruit growth. Potential growth of proximal fruits was strongly dependent on the position of the inflorescence on the stem, whereas po tential growth of distal fruits was lower than or equal to that of pro ximal fruits of the same inflorescence and it was independent on the i nflorescence position. We took into account both inflorescence and fru it positions to establish a quantitative relationship between fruit se t of individual inflorescences and the ratio between actual and potent ial fruit growth.