The mass transfer rate of "C-sucrose translocation from sugar beet (Beta vulgaris, L.) leaves was measured over a range of net photosynthesis rates from 0 to 60 milligrams of C02 decimeters2 hour-' under varying conditions of light intensity, CO2 concentration, and 02 concentration. The production (2, 3). Dry matter increases should be reflected in an increased mass transfer rate. In this study, the mass transfer rate of "C-sucrose translocation from sugar beet leaves was measured over a range of net photosynthesis rates which were varied by adjusting light intensity, CO2 concentration, and 02 concentration in an attempt to determine the independent effects of light and photosynthetic carbon assimilation on translocation.The individual roles of the source, sink, and path of active translocating systems are currently under investigation. According to the Munch pressure-flow theory of translocation, the source maintains the driving power for translocation through build up of a sucrose concentration gradient between source and sink by photosynthesis. Photosynthesis would be expected to limit the rate of translocation by limiting the availability of sucrose. Evidence for this direct control by photosynthesis in regulating the translocation rate is sparse (4, 10, 18, 24). Much discrepancy exists in the literature over the single and combined effects of light intensity and photosynthesis controlling translocation from the source. Some workers have indicated that light directly promotes translocation, independently of the assimilation of C02, by the production of ATP through cyclic photophosphorylation. The ATP would act either by increasing the rate of vein loading (11) 16-hr photoperiod with a 24 C day temperature and 17 C night temperature. Light intensity at leaf level was about 1200 ft-c during the 30-to 35-day growth period. Leaves of the experimental plants were removed except for a source leaf with a 10-cm long blade (0.5 dM2) and a sink leaf with a 3-to 4-cm long blade.Measurement of Net Photosynthesis Rate. The experimental system used permitted the simultaneous measurement of net photosynthesis and translocation rates and has been reported earlier (7,8). The day before the experiment, the source leaf was sealed in the leaf chamber and the system leak-tested. Be-fore the experiment, compressed air containing 400 Ml/I of CO2 was allowed to flow over the leaf. Just before the experiment the leaf was allowed to photosynthesize for at least 2 hr at a light intensity of 7200 ft-c. The experiment was started by allowing labeled CO2 of known specific radioactivity (40 moles/Ci) to be taken up by the leaf. The CO2 concentration of the system was regulated between set high and low levels (a 1 OO-ul/I span) on the recorder monitoring the infrared gas analyzer output. As the CO2 concentration of the system was reduced by photosynthesis to the low level, a solenoid valve opened allowing labeled CO2 to enter the system and return the CO2 concentration to the set high level. Net photosynthesis was calculated as the chan...