EXPANSION RATE OF YOUNG TOMATO FRUIT GROWING ON PLANTS AT POSITIVE WATER POTENTIAL

Changes in tomato fruit expansion rate and carbohydrate content have b een assessed during treatments designed to alter the carbon import rat e. Because fruit expansion is sensitive to plant water status, the rel ationship with carbon import is difficult to assess, and thus, the dia meter growth rate of young fruit was measured on plants maintained at positive water potentials. The detached top metre of a tomato plant wa s supplied with water, through the cut stem base, at a pressure of 0.0 8 MPa. Developing fruit on the stem continued to grow at high rates fo r up to 2d. Fruit diameter growth rate after plant detachment was dire ctly proportional to temperature. Plants acclimated to different conti nuous irradiances for 5d before detachment gave fruit growth rates aft er plant detachment which were directly proportional to the irradiance up to 7MJm(-2)d(-1) photosynthetically active radiation (PAR). In con tinuous darkness, fruit growth rate remained unchanged for 20h and the n declined to less than 40% of the original rate over the following 30 h. On re-exposure to light, about 5h elapsed before fruit growth rate increased but the growth rate stabilized at approximately 50% of the r ate in continuously illuminated plants. During darkness, both fruit st arch and hexose content decreased in comparison to illuminated control s, but on re-illumination, carbohydrate content increased before carbo n was allocated to structural growth. Heat-killing the phloem of the f ruit pedicel caused an immediate, but temporary, cessation of growth. After a partial recovery, expansion growth continued, but more slowly than in untreated fruit and at steadily declining rates. Starch ad hex ose sugars were not used to provide substrates for growth and starch s ynthesis was maintained. Continuing cell expansion was assumed to have been supported by water import via the xylem. Thus, fruit expansion m ay be related to carbon accumulation in most circumstances, but the ch anging allocation of imported carbon to storage and cell expansion may modify this relationship.