Influence of increased zinc levels on phloem transport in wheat shoots

Elevated zinc levels may cause various symptoms in plants by interfering with ion uptake in the roots, transport processes within the plant or metabolic activities. It has been reported previously that the symplastic transport via the phloem is inhibited under zinc toxicity. In general, an increased zinc supply causes higher zinc contents in the vegetative plant parts of wheat, while the zinc content in the grains is not or only slightly affected. Zinc can be transferred from the xylem to the phloem in the peduncle. In the work reported here, the influence of high zinc levels on phloem transport in maturing wheat was investigated by feeding increasing zinc concentrations via a stem flap below the flag leaf node of field-grown plants. Simultaneously fed strontium and rubidium served as markers for xylem and phloem transport, respectively. At the highest zinc quantities fed (above 1µmol per plant in 1mL), dry matter accumulation in the grains was markedly, and rubidium redistribution to a lesser extent, affected in this system. Our results led to the conclusion that the phloem transport was strongly inhibited above a critical zinc level and that this effect was most likely due to interference with phloem loading or with the mass flow in the sieve tubes and not primarily by affecting phloem unloading or metabolism in the sinks.