No abstract

Different field surveys have shown that sulphur (S) fertilization can increase the resistance of agricultural crops against fungal pathogens. The mechanisms of this sulphur-induced resistance (SIR) are, however, not yet known. Volatile S compounds are thought to play an important role because H(2)S is toxic to fungi. A field experiment was conducted to analyse the influence of S fertilization and the activity of H(2)S-releasing enzymes on fungal infections. Two levels of N and S fertilizers and two varieties of oilseed rape were investigated with respect to their potential to release H(2)S by the enzymatic activity of L-cysteine desulphydrase (LCD) and O-acetyl-L-serine(thiol)lyase (OAS-TL). LCD releases H(2)S during cysteine degradation, while OAS-TL consumes H(2)S during cysteine synthesis and free H(2)S is only released in a side reaction. All plots of the field trial showed an infection with Pyrenopeziza brassicae and leaf disc samples were taken from visibly infected leaf areas and apparently uninfected areas to investigate the reaction to the infection in relation to the treatments. Different S fractions and the activities of LCD and OAS-TL were measured to evaluate the potential to release H(2)S in relation to S nutrition and fungal infection. S fertilization significantly increased the contents of total S, sulphate, organic S, cysteine, and glutathione in the plants, but decreased LCD activity. Infection with P. brassicae increased cysteine and glutathione contents, as well as the activity of LCD. Therefore crops were able to react to a fungal infection with a greater potential to release H(2)S, which is reflected by an increasing LCD activity with fungal infection.

No abstract

Sulfur deficiency developed into a widespread nutrient disorder in the 1980s because of the drastic decrease of SO(2) emissions in western Europe after Clean Air Acts came into force. It was observed that not only the yield and quality of agricultural crops were negatively affected by sulfur deficiency but also their health status. Since the mid 1990s the physiological background of this latter phenomenon in the sulfur metabolism has been studied by different researchers. From 2001 until 2006, field trials with different varieties of oilseed rape were conducted in Germany, and also from 2001 until 2003 in Scotland, to investigate the underlying mechanisms of sulfur-induced resistance and to develop fertiliser strategies which increase the health status of crops and minimise the requirement for chemical fungicides. A comprehensive disease assessment was conducted and a range of different sulfur-containing metabolites and enzymes were analysed in relation to sulfur nutrition and fungal diseases. H2S emissions from field-grown crops under different sulfur nutritional status were studied for the first time and a positive relationship was observed. Besides S fertilisation, fungal infection increased H2S emissions, too. The studies deliver new insight into the complex of sulfur-induced resistance but many questions still remain open. This contribution will show different possible strategies to solve some of the open questions.

No abstract

Until the 1970's of the last century sulfur (S) was mainly regarded as a pollutant being the main contributor of acid rain, causing forest dieback in central Europe. When Clean Air Acts came into force at the start of the 1980's SO2 contaminations in the air were consequently reduced within the next years. S changed from an unwanted pollutant into a lacking plant nutrient in agriculture since agricultural fields were no longer “fertilized” indirectly by industrial pollution. S deficiency was first noticed in Brassica crops that display an especially high S demand because of its content of S-containing secondary metabolites, the glucosinolates. In Scotland, where S depositions decreased even faster than in continental Europe, an increasing disease incidence with Pyrenopeziza brassicae was observed in oilseed rape in the beginning 1990's and the concept of sulfur-induced-resistance (SIR) was developed after a relationship between the S status and the disease incidence was uncovered. Since then a lot of research was carried out to unravel the background of SIR in the metabolism of agricultural crops and to identify metabolites, enzymes and reactions, which are potentially activated by the S metabolism to combat fungal pathogens. The S status of the crop is affecting many different plant features such as color and scent of flowers, pigments in leaves, metabolite concentrations and the release of gaseous S compounds which are directly influencing the desirability of a crop for a variety of different organisms from microorganisms, over insects and slugs to the point of grazing animals. The present paper is an attempt to sum up the knowledge about the effect of the S nutritional status of agricultural crops on parameters that are directly related to their health status and by this to SIR. Milestones in SIR research are compiled, open questions are addressed and future projections were developed.

No abstract

The emission of gaseous sulfur (S) compounds by plants is related to several factors, such as the plant S status or fungal infection. Hydrogen sulfide (H(2)S) is either released or taken up by the plant depending on the ambient air concentration and the plant demand for S. On the contrary, carbonyl sulfide (COS) is normally taken up by plants. In a greenhouse experiment, the dependence of H(2)S and COS exchange with ambient air on the S status of oilseed rape (Brassica napus L.) and on fungal infection with Sclerotinia sclerotiorum was investigated. Thiol contents were determined to understand their influence on the exchange of gaseous S compounds. The experiment revealed that H(2)S emissions were closely related to pathogen infections as well as to S nutrition. S fertilization caused a change from H(2)S consumption by S-deficient oilseed rape plants to a H(2)S release of 41 pg g(-1) (dw) min(-1) after the addition of 250 mg of S per pot. Fungal infection caused an even stronger increase of H(2)S emissions with a maximum of 1842 pg g(-1) (dw) min(-1) 2 days after infection. Healthy oilseed rape plants acted as a sink for COS. Fungal infection caused a shift from COS uptake to COS releases. The release of S-containing gases thus seems to be part of the response to fungal infection. The roles the S-containing gases may play in this response are discussed.