Regulation of Assimilate Translocation Between Leaves and Fruits in the Tomato

Simultaneous measurement of export from leaves and import to fruits were made on tomato plants reduced to one fully expanded leaf and one fruit. Experimental leaves were exposed to six light flux densities (0.5–100 W m−2) for 24 h while rapidly growing fruits were kept in the dark at 22 °C. The rates of export of assimilate from these leaves varied from 70 to 120 mg C leaf−1 day−1 corresponding with rates of carbon fixation from 3 to 290 mg C leaf−1 day−1. Export from leaves with the lowest carbon fixation rates were maintained by a loss of up to one-sixth of their initial carbon. In contrast, leaves with the highest carbon fixation rates exported only half the newly fixed carbon. The rates of import of assimilate to similar-sized fruits (c. 16 cm3) were between 80 and 110 mg C fr−1 day−1 but differed from the export rates of the source leaves. The specific growth rates and the specific respiration rates of the fruits were related to their initial carbon content at the beginning of the experiment. Thus, over 24 h, the rate of import was predetermined by the developmental stage of the fruit unaltered by the rate of current carbon fixation in the source leaf. Translocation of assimilate was regulated by sink demand under both source- and sink-limiting conditions in this short-term situation. The dynamic relationship between assimilate production in leaves and its utilization in fruits is discussed together with the role of sucrose concentration in these organs in regulating transport.