Effects of Sulfur from H2S and S02 on Spruce and Cabbage

The intended project would be a part of the COST Action 829: "Fundamental, Agronomical and Environmental Aspects of Sulfur Nutrition and Assimilation in Plants". Sulfur is an essential element for plant growth, but also component of relevant air pollutants. Deficiency may result in reduced yield and quality of crops. The sulfur requirement of plants depends on the developmental stage and it varies between species. Plants are able to use different sulfur compounds as a sulfur source; among these, sulphate is the most important. In the last two centuries, as the result of industrialisation, also other sulfur containing compounds, such as SO 2 , H2 S or sulfur containing fertilisers became a significant factor for plant nutrition, but, on the other hand, these airborne sulfur compounds are known to be phytotoxic, in particular to forest trees. Sulfur dioxide is regarded a "classical" air pollutant responsible for forest damages and crop yield losses in the vicinity of industrial regions. Although the phytotoxic effects of SO 2 have been studied for many years, the mechanisms through which it is damaging plants are still largely unclear. The sulphite ion formed by the solution of SO 2 in water is highly toxic and inhibits photosynthesis effectively. However, only upon relatively high S02 levels sulphite was detected in cells, indicating effective detoxification mechanisms. The physiological cause of H2 S induced growth reduction as well as H2 S induced growth stimulation is largely unknown. But also physiological consequences of enhanced levels of thiol compounds in plants in the presence of H2 S are unknown. For this purpose the contents of sulphate, total sulfur and thiols will be measured. The goal of the present application is to follow the pathways of sulfur introduced as H 2 S or SO 2 into the leaves by labelled H2 35S and 35SO 2 . Since differences between trees and herbs can be expected, it is intended to use spruce trees (Picea abies (L.) Karst., as a representative for a tree species with rather low sulfur demand and high susceptibility to sulfur damages) and cabbage (Brassica oleracea L., as a herbaceous species with high S demand). This research contributes to fill knowledge gaps with respect to the origin of accumulating sulfur compounds, the relative proportion of different detoxification pathways and the redistribution and transport of reduced sulfur compounds both in herbaceous plants and trees.

The project was designed to evaluate the contributions of sulfur sources from the atmosphere (SO2 and H2 S) and the nutrient solution (sulfate) to the sulfur nutrition of plants. The possible differential behavior of shoots and roots and the sulfate uptake regulation were paid particular attention. These investigations were conducted using radiolabelled sulfate in the nutrient solution and various fumigations of Brassica and Picea (to compare trees and herbs) with H2 S and SO 2 . The results show a pronounced impact of H2 S and SO 2 fumigations on the sulfur metabolism of the plants. The questions listed in the proposal could be answered sufficiently: Detoxification through sulfate formation Detoxification processes through sulfate formation take place both upon H2 S and SO 2 exposure, mainly in the shoots. In the roots, effects are marginal. The sulfur source of the roots is largely the sulfate in the nutrient solution. Responses of trees (Picea) and herbs (Brassica) are comparable, although effects on Picea are less pronounced due to longer exposure times (necessary because of lower uptake rates). Formation of sulfur containing metabolites after fumigation This question can be answered for both experimental plants in the same way (even though spruce responded less pronouncedly due to lower uptake and metabolisation rates). In shoots, a strong increase of sulfur containing compounds was observed upon H2 S fumigation, this increase was lower upon SO 2 exposure. Upon H2 S exposure, the sulfur in thiols (cysteine, glutathione, etc) was only to a small extent sulfate from the nutrient solution (e.g. 10% in Brassica), upon SO 2 to a greater extent (50% for Brassica). In the roots, increases in thiols are lower, more pronounced in spruce, where the main sulfur source for these thiol pools is the nutrient solution. Hence, there are significant differences between shoot and root in the utilization of different sulfur sources. Trees (Picea) and herbs (Brassica) respond in the same way, but to a different extent. Redistribution of reduced sulfur compounds By an evaluation of the specific radioactivity it was possible to detect the origin of sulfur pools. An increase of unmarked thiols in the roots and a simultaneous decrease in the specific label of the thiol pool therein point toward a transport from shoot to root. Such a transport could be observed in both experimental species and after exposure to both atmospheric sulfur sources. H2 S always had a stronger effect than SO 2 . The following additional results were found: There is sulfur reduction going on the roots. The transport root-shoot is regulated through xylem loading. Sulfate uptake of the roots is only marginally inhibited by exposure to H 2 S or SO 2 .