The mechanism of assimilate partitioning and carbohydrate compartmentation in fruit in relation to the quality and yield of tomato

The limitation of tomato yield was investigated in a number of cultivars with contrasting fruiting habits. Unless light is limiting, yield is mainly restricted by the number or the size of the fruit (i.e. the sink strength) rather than the supply of assimilate (i.e. the source strength). Fruit size is determined by both cell number and cell size. The rate of fruit expansion is affected by assimilate supply, temperature and water relations in the plant. The size or the growth rate of a tomato fruit is regulated by the import of assimilate and water. The sink strength for assimilate of a tomato fruit measured by the rate of assimilate import may be related to the routes of sugar transport into the sink cells during fruit development. Enzymic regulation of the hydrolysis of sucrose by sucrose synthase and the accumulation of starch by ADPG pyrophosphorylase may determine the rate of assimilate import in the young fruit. Vacuolar invertase activity may determine the sugar composition of a mature fruit, but may not affect the overall dry matter accumulation of a tomato fruit. While yield is determined by the balance between source and sink strengths of the plant, quality is determined by the transport and metabolism of sugars within the fruit.