BC2 FRONT COVER REV2.indd

Abbreviations and notes: N = nitrogen; Cu = copper; Fe = iron, Mn = manganese; Mo = molybdenum; Ni = nickel; Se = selenium; Zn = zinc. Sulfur is an essential macronutrient for plants and animals, and is required for many important metabolic functions. Plants are able to convert sulfate (SO 4 ) into organic compounds, but animals must consume S-containing amino acids (methionine and cysteine) for their dietary requirement. The need for S in crops has taken a higher profi le in recent years as many farming systems have fewer S inputs than previously. Higher crop yields, slower organic matter turnover, reduced use of S-containing crop inputs, and changing crop patterns have also contributed to the need for additional S fertilization. While most S in soils is present in organic matter, soluble sulfate is present in most soils and is the primary source of S nutrition for plants. It is actively transported into the root, especially in the root hair region, and moves into plant cells through a variety of sulfate transporters. Within the plant, sulfate moves in the transpiration stream until it is stored in cell vacuoles or participates in a variety of biochemical reactions. Leaves are also able to assimilate sulfur dioxide (SO 2 ) from the atmosphere, but this amount is usually no more than 1 kg S/ ha/yr. Plant leaves can also emit hydrogen sulfi de (H 2 S) gas, which is assumed to be a type of detoxifying mechanism after exposure to high SO 2 . Most of the sulfate taken up by roots is converted to cysteine in leaf chloroplasts. Cysteine is the primary starting point from which most other organic S compounds in plants are formed. This synthesis process begins with sulfate reduction to adenosine phosphosulfate and ultimately to various S-containing organic compounds (Figure 1). Sulfate reduction requires considerable plant energy. Other important S amino acids include the amino acids cystine (a linkage of two cysteine molecules), and methionine (Figure 2). Smaller amounts of S are incorporated into important molecules such as coenzyme A, biotin, thiamine, glutathione, and sulfolipids. Once sulfate is converted to organic compounds, they are exported through the phloem to the sites of active protein synthesis (esp. root and shoot tips, fruits and grains) and then become largely immobile within the plant. The symptoms of S defi ciency occur fi rst in the younger tissues and are seen as leaves and veins turning pale green to yellow. These chlorosis symptoms look similar to those that occur with N defi ciency, but because of its higher internal mobility a low N supply becomes fi rst visible in the older leaves. When S defi ciencies are fi rst observed, some crops may not entirely recover the lost growth following S fertilization. There are a large number of secondary S compounds that provide biochemical benefi t to specifi c plant species. Some crops (e.g. brassicas such as canola and mustard) have a relatively high S requirement and produce glucosinolate compounds. Members of the Allium species (e.g. garlic and onions) produce alliin compounds that may contain >80% of the total plant S. The characteristic fl avor and smell of onions and garlic related to these volatile S compounds are enhanced when plants are grown in high S soil. These and other S-containing compounds are linked with resistance to various pests and environmental stress.