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Annals of Botany 79: 585-591, 1997
© 1997 Annals of Botany Company

The Pathway of Assimilate Flow from Source to Sink in Urtica dioica L., Studied with14C under Ambient Atmospheric Conditions

MASRESHA FETENE, CHRISTINA BENKER and ERWIN BECK+

Addis Ababa University, Department of Biology, P.O. Box 1176, Addis Ababa, Ethiopia Universität Bayreuth, Lehrstuhl für Pflanzenphysiologie, Universitätstrasse 30, 95440, Bayreuth, Germany

Received August 13, 1996 ; Accepted January 8, 1997

The contribution of individual vascular bundles of the stem to the flow of assimilates from a selected source leaf to the sink regions was investigated inUrtica dioica L., a plant with a decussate leaf arrangement. Two homologous sets of eight vascular strands were recognized, arranged in mirror symmetry in the stem internodes. In each set, three of the bundles were identified as traces of one leaf merging into the vascular system of the stem one node below the origin of the leaf. The main bundle of a stem-half bifurcates at each end of the internode into two subdominant bundles, which combine in the next but one node to form the dominant bundle again. Each set of vascular strands also contains two minor bundles which pass more or less without interruption through the whole stem.

The uppermost mature source leaf (leaf number 5 as counted from the tip) was exposed to14CO2in a closed gas circuit. The concentration of the carbon-labelled CO2was maintained at the ambient CO2level to maintain the natural source strength of the leaf. By the end of the usual nocturnal dark phase, carbon from the source leaf had been imported predominantly by sink leaves of the same orthostichy. Lesser, but significant amounts of radiocarbon were also incorporated into the sink leaves of the adjacent two orthostichies via the marginal leaf traces. In spite of the junction of the vascular strands in the nodes and an interfascicular connection of the stem bundles, randomization of the photosynthates from individual leaves was minimal in the vascular system of the stem in the upward direction, and also low in the flux to the roots. Substantial amounts of radioactivity were also found in the lately-formed xylem elements of the vascular strands and their interfascicular connections, indicating active secondary growth.

Assimilate distribution; source–sink connections; Urtica dioica ; vascular architecture


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