Abstract
Tomato fruits accumulate most of their solid contents during rapid fruit development. The balance of water transport to assimilate transport into fruits during that stage is an important factor that determines the final solid content of fruits at harvest. In this study, the relationship of sucrose concentration of the phloem sap to the solid content of rapidly growing fruits was investigated. At the anthesis of the second truss, the stem just above the first truss was heat-girdled, while newly emerging leaves above the second truss were pinched regularly to diminish cross translocation of carbohydrates between the source-sink unit, consisting of the second truss and three leaves below, and other parts of the plant. The leaf to fruit ratio (LFR) as the number of leaves per fruit of this isolated source-sink unit was manipulated at the fruit set of the truss. When LFR was increased from 0.2 to 1, fruit dry weight increased linearly from 3.5 to 7 g, but it saturated beyond the LFR of 1. By contrast, the contents of dry matter and soluble sugars on a fresh weight basis increased linearly within the whole range of LFR from 0.2 to 3. The sucrose concentration of the phloem sap, collected from the pedicel by the modified EDTA method, also increased as LFR increased, and a positive correlation was observed between sucrose concentration of the phloem sap and the contents of dry matter and total soluble sugars. These data suggested that solid content can be increased in a wide range of LFR in proportion to the sucrose concentration of the phloem sap.
