Microautoradiographic localization of imported (14)C-photosynthate in induced sink leaves of two dicotyledonous C4 plants in relation to phloem unloading.

Pruned source-sink transport systems from predarkened plants of Amaranthus caudatus L. and Gomphrena globosa L. were used to study the localization of (14)C-labeled photosynthate imported into experimentally induced sink leaves by microautoradiography. During a 6-h (Amaranthus) or a 4-h (Gomphrena) transport period, (14)C-assimilates were translocated acropetally from a mature source leaf provided with (14)CO2, into a younger induced sink leaf (dark/-CO2). In addition, a young still-expanding source leaf exposed to (14)CO2 exported (14)C-assimilates basipetally into a mature induced sink leaf (dark/-CO2). Microautoradiographs showed that imported (14)C-photosynthate was strongly accumulated in the sieve element/companion cell complexes of midveins, secondary veins, and minor veins of both the mature and the expanding sink leaf. Some label was also present in the vascular parenchyma and bundlesheath cells. In petioles, (14)C-label was concentrated in the sieve element/companion cell complexes of all bundles indicating that assimilates were imported and distributed via the phloem. Moreover, a considerable amount of radioactivity unloaded from the sieve element/companion cell complexes of petiolar bundles, was densely located at sites of secondary wall thickenings of differen-tiating metaxylem vessels, and at sites of chloroplasts of the vascular parenchyma and bundle-sheath cells. These observations were more striking in petioles of Gomphrena than Amaranthus.