Consequences of air pollution on shoot-root interactions

The impact of SO2, NO2 and O-3 on physiological processes in plants and their consequences at the whole-plant level are discussed in the present: paper. Ozon interacts with carbon allocation most likely by inhibiting sucrose export. This causes an accumulation of carbohydrates and starch in leaves and results in a reduction of photosynthesis. Thus, O-3-exposure can diminish the availability of photosynthetate for growth and development and result in an increased shoot to root ratio and an overall reduction in biomass. By contrast, SO2 and NO2 can act as nutrients. SO2 affects the sulfate and the organic sulfur pools of the leaves and will cause an enhanced export of sulfur. As a consequence, plants fumigated with SO2 contain enhanced amounts of reduced sulfur, mainly glutathione, in the roots. Glutathione acts as a signal to control sulfate uptake from the soil and inhibits the process of xylem loading. Apparently, sulfur from atmospheric pollution can interact with the sulfur nutrition of plants. NO2 may interact with the nitrogen nutrition of plants in a similar way. The absorbed NO2 is used to synthesize amino acids which are translocated in the phloem to the roots. Since amino acids transported in the phloem can decrease nitrate uptake by roots, it is feasible that nitrogen taken up via the leaves can interact with whole nitrogen nutrition of plants as described for sulfur. The significance of SO2, NO2 and O-3 in affecting root-shoot interactions, will depend on the availability of defence systems, the size of internal storage pools and the actual growth rate of the plant.