Anoxic conditions should hamper the transport of sugar in the phloem, as this is an active process. The canopy is a carbohydrate source and the roots are carbohydrate sinks. By fumigating the shoot with N-2 or flooding the rhizosphere, anoxic conditions in the source or sink, respectively, were induced. Volume flow, velocity, conducting area and stationary water of the phloem were assessed by non-invasive magnetic resonance imaging (MRI) flowmetry. Carbohydrates and δ¹³C in leaves, roots and phloem saps were determined.
Following flooding, volume flow and conducting area of the phloem declined and sugar concentrations in leaves and in phloem saps slightly increased. Oligosaccharides appeared in phloem saps and after 3 d, carbon transport was reduced to 77%. Additionally, the xylem flow declined and showed finally no daily rhythm. Anoxia of the shoot resulted within minutes in a reduction of volume flow, conductive area and sucrose in the phloem sap decreased. Sugar transport dropped to below 40% by the end of the N-2 treatment. However, volume flow and phloem sap sugar tended to recover during the N₂ treatment. Both anoxia treatments hampered sugar transport. The flow velocity remained about constant, although phloem sap sugar concentration changed during treatments. Apparently, stored starch was remobilized under anoxia. In order to test the hypothesis that anoxic conditions should hamper the transport in the phloem, we chose to subject the entire shoot (source) and the entire root (sink) of castor bean plants to anoxia, while monitoring non-invasively phloem transport by means of MRI-flowmetry. Moreover we analyzed carbon compounds in source and sink tissues as well as in phloem saps.
Both anoxia treatments reduced sugar transport, however, flow velocity stayed constant, even though sugar concentration in phloem saps changed. Our study overcomes the important shortcomings of recent studies as we provide direct information on the phloem transport, its change due to anoxia and the effect of such change on root carbohydrate supply.