Mechanism of export of organic material from the developing fruits of pea

Pisum sativum L. fruits export a small quantity of radiolabeled substances to other plant parts after the fruits are allowed to photosynthesize in the presence of ¹⁴CO₂. Export was uninhibited by peduncle girdling suggesting an apoplastic route for transport of material, presumably by `reverse' flow in the peduncle xylem. To determine if any diurnal water potential gradient formed between pea leaves and fruit might be responsible for the observed export, the water potentials of the various organs were monitored over 24 hours. Water potential differences of up to 7.5 bars existed between leaves and fruit in long photoperiods, and up to 2.5 bars in short photoperiods. Pulses of <sup>14</sup>CO<sub>2</sub> labeling indicated that initial delivery of exported label was to `transpirational sinks,' with subsequent redistribution of label to metabolic sinks. Export to the apical bud appeared to be direct via the xylem. Application of membrane-impermeable inulin to a surgically opened seed coat `cup' resulted in export mainly to the subtending leaf with little redistribution. Simultaneous application of sucrose to the seed coat resulted in more extensive distribution of the sucrose, consistent with reloading of the sucrose into mature leaf phloem. Thus, export of material from fruits appears to occur via a xylem pathway in response to transpirationally derived water potential gradients.