This study with poplar (Populus tremula ϫ Populus alba) cuttings was aimed to test the hypothesis that sulfate uptake is regulated by demand-driven control and that this regulation is mediated by phloem-transported glutathione as a shoot-toroot signal. Therefore, sulfur nutrition was investigated at (a) enhanced sulfate demand in transgenic poplar over-expressing ␥-glutamylcysteine (␥-EC) synthetase in the cytosol and (b) reduced sulfate demand during short-term exposure to H 2 S. H 2 S taken up by the leaves increased cysteine, ␥-EC, and glutathione concentrations in leaves, xylem sap, phloem exudate, and roots, both in wild-type and transgenic poplar. The observed reduced xylem loading of sulfate after H 2 S exposure of wild-type poplar could well be explained by a higher glutathione concentration in the phloem. In transgenic poplar increased concentrations of glutathione and ␥-EC were found not only in leaves, xylem sap, and roots but also in phloem exudate irrespective of H 2 S exposure. Despite enhanced phloem allocation of glutathione and its accumulation in the roots, sulfate uptake was strongly enhanced. This finding is contradictory to the hypothesis that glutathione allocated in the phloem reduces sulfate uptake and its transport to the shoot. Correlation analysis provided circumstantial evidence that the sulfate to glutathione ratio in the phloem may control sulfate uptake and loading into the xylem, both when the sulfate demand of the shoot is increased and when it is reduced.

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 Dry deposition of NH$ and NO x (NO and NO # ) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH $ dissolves to form NH % + , whereas NO # dissolves to form NO $ − and NO # − . The latter compound can also be formed after exposure to NO. There is evidence that NH $ -N and NO x -N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO $ − and NO # − can be reduced, and NH % + can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase\glutamate synthase (GS\GOGAT) cycle. Fumigation with low concentrations of atmospheric NH $ increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH $ and in vitro GDH activity. The contribution of atmospheric NH $ and NO # deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates (s). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small. Key words : Apoplastic storage, gaseous nitrogen deposition, glutamate dehydrogenase, glutamine synthetase, plant nitrogen demand, sulphur dioxide root\shoot interaction, ammonia, nitrogen oxides. Anthropogenic emissions of nitrogen-containing air pollutants far exceed natural emissions in Europe and North America. These emissions cause two types of effect : generation of tropospheric O $ , and excess N deposition, which can result in direct phytotoxic effects, eutrophication, acidification and stimulation of greenhouse gas production. Anthropogenic emissions of N-containing air pollutants have resulted in atmospheric N deposition 5-20 times higher than in natural conditions. Indications of N excess in the field can be seen from : (1) changes in the species composition of indigenous vegetation types, with an increase in nitrophilous species and a decrease in others, and (2) increases in foliar N * To whom correspondence should be addressed. E-mail : g.stulen!biol.rug.nl. Abbreviations : GDH, glutamate dehydrogenase ; GS, glutamine synthetase ; GOGAT, glutamate synthase ; , net nitrate uptake rate ; NR, nitrate reductase ; P95, 95 percentile ; , plant nitrogen content ; , relative growth rate.

Brassica juncea seedlings contained a twofold higher glucosinolate content than B. rapa and these secondary sulfur compounds accounted for up to 30% of the organic sulfur fraction. The glucosinolate content was not affected by H2S and SO2 exposure, demonstrating that these sulfur compounds did not form a sink for excessive atmospheric supplied sulfur. Upon sulfate deprivation, the foliarly absorbed H2S and SO2 replaced sulfate as the sulfur source for growth of B. juncea and B. rapa seedlings. The glucosinolate content was decreased in sulfate-deprived plants, though its proportion of organic sulfur fraction was higher than that of sulfate-sufficient plants, both in absence and presence of H2S and SO2. The significance of myrosinase in the in situ turnover in these secondary sulfur compounds needs to be questioned, since there was no direct co-regulation between the content of glucosinolates and the transcript level and activity of myrosinase. Evidently, glucosinolates cannot be considered as sulfur storage compounds upon exposure to excessive atmospheric sulfur and are unlikely to be involved in the re-distribution of sulfur in B. juncea and B. rapa seedlings upon sulfate deprivation.

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