Susan Dunford, University of Cincinnati, Cincinnati, OH, USA
There is no bidirectional transport in single sieve tubes, and solutes and water move at the same velocity
Researchers have investigated bidirectional transport by applying two different radiotracers to two source leaves, one above the other. Each leaf receives one of the tracers, and a point between the two sources is monitored for the presence of both tracers.
Transport in two directions has often been detected in sieve tubes of different vascular bundles in stems. Transport in two directions has also been seen in adjacent sieve tubes of the same bundle in petioles. This happens in the petiole of a leaf that is undergoing the transition from sink to source and simultaneously importing and exporting photosynthates through its petiole. However, simultaneous bidirectional transport in a single sieve tube has never been demonstrated.
Measured velocities for transport in the phloem are remarkably similar, whether measured using carbon-labeled solutes or using NMR techniques, which detect water flow. Solutes and water move at the same velocity.
Both observations—the lack of bidirectional transport in a single sieve element and similar velocities for solutes and water—support the existence of mass flow in the sieve elements of the phloem.
The energy requirement of the transport phloem is small in herbaceous plants
In herbaceous plants that can survive periods of low temperature, such as sugar beet (Beta vulgaris), rapidly chilling a short segment of the petiole of a source leaf to approximately 1°C does not cause sustained inhibition of mass transport out of the leaf (Web Figure 12.5.A). Rather, there is a brief period of inhibition (minutes to a few hours), after which transport slowly returns to the control rate. Chilling reduces respiration rate and both the synthesis and the consumption of ATP in the petiole by about 90%, at a time when translocation has recovered and is proceeding normally. These experiments show that the energy requirement for actual transport through the pathway of these herbaceous plants is small, consistent with mass flow. Many of the effects of chilling treatments have in fact been attributed to loss and retrieval mechanisms along the path, rather than to the transport mechanism itself. It should be noted that extreme treatments that inhibit all energy metabolism do inhibit translocation even in herbaceous plants.
It is technically challenging to do chilling experiments in trees. The methods used to evaluate transport, such as radial growth rates below the treatment zone or soil CO2 efflux, do not permit short-term, transient changes in transport to be observed.
Geiger DR, Sovonick SA (1975) Effects of Temperature, Anoxia and Other Metabolic Inhibitors on Translocation. In MH Zimmermann, JA Milburn, eds, Transport in Plants I: Phloem Transport. Springer Berlin Heidelberg, Berlin, Heidelberg, pp 256–286.