Conversion of mature leaves into sinks on the dicotyledonous C4 plants Amaranthus caudatus and Gomphrena globosa

Abstract

The effect of an artificially created sink leaf on translocation of 14C-labeled assimilates from a source leaf of the C4 plants Amaranthus caudatus L. and Gomphrena globosa L. was studied with autoradiography. Anatomical studies showed that the vasculature of two successive leaves within a given orthostichy is directly connected by stem bundles. Therefore, in an intact plant, much of the acropetally translocated 14C-labeled assimilates from a mature source leaf moved into the next younger leaf directly above. When such a young leaf on an otherwise illuminated plant was kept under sink conditions (dark, CO2-free air), its increased sink demand resulted in a clearly higher 14C-import, compared with a control. Even a mature leaf of Gomphrena subjected to the similar sink conditions developed sufficient sink demand to import a certain amount of basipetally transported 14C-labeled assimilates from a younger source leaf. In Amaranthus, there was no basipetal translocation from a younger source leaf and, consequently, no 14C-import into a mature leaf under sink conditions occurred. A complete conversion of a mature leaf into a sink was attained only with a pruned source–sink transport system of both plants under source and sink limitation. This conversion from the source to the sink status caused a change in the direction of assimilate flow within the petiolar and stem bundles. The data indicate that a mature leaf under sink conditions can be characterized as a sink with regard to assimilate translocation, although no growth or storage processes are involved in controlling this sink activity.