Sulfur (S) is unique in having changed within just a few years, from being viewed as an undesired pollutant to being seen as a major nutrient limiting plant production in Western Europe. In East Asia, where, under current legislative restrictions, sulfur dioxide (SO2) emissions are expected to increase further by 34% by 2030 (1), considerations of sulfur pollution are a major issue. Similarly in Europe, sulfur is still associated with its once detrimental effects on forests which peaked in the

1970s (2), and which gave this element the name 'yellow poison.' With Clean Air Acts coming into force at the start of the 1980s, atmospheric sulfur depositions were reduced drastically and rapidly in Western Europe, and declined further in the 1990s after the political transition of Eastern European countries. In arable production, sulfur deficiency can be retraced to the beginning of the 1980s (3). Since then, severe sulfur deficiency has become the main nutrient disorder of agricultural crops in Western Europe. It has been estimated that the worldwide sulfur fertilizer deficit will reach 11 million tons per year by 2012, with Asia (6 million tons) and the Americas (2.3 million tons) showing the highest shortage (4).

Severe sulfur deficiency not only reduces crop productivity and diminishes crop quality, but it also affects plant health and environmental quality (5). Yield and quality in relation to the sulfur nutritional status for numerous crops are well described in the literature. In comparison, research in the field of interactions between sulfur and pests and diseases is relatively new. Related studies indicate the significance of the sulfur nutritional status for both beneficial insects and pests.

Since the very early days of research on sulfur in the 1930s, significant advances have been made in the field of analysis of inorganic and organic sulfur compounds. By employing genetic approaches in life science research, significant advances in the field of sulfur nutrition, and in our understanding of the cross talk between metabolic pathways involving sulfur and interactions between sulfur nutrition and biotic and abiotic stresses, can be expected in the future.

This chapter summarizes the current status of sulfur research with special attention to physiological and agronomic aspects.

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