Sulfur is an essential element for growth and physiological functioning of plants. The total sulfur content in the vegetative parts of crops varies between 0.1 and 2% of the dry weight (0.03 to 0.6 mmol S g-1 dry weight). The uptake and assimilation of sulfur and nitrogen by plants are strongly interrelated and dependent upon each other, and at adequate levels of sulfur supply the organic N/S ratio is around 20:1 on a molar basis (6-9). In most plant species the major proportion of sulfur (up to 70% of the total S) is present in reduced form in the cysteine and methionine residues of proteins. Additionally, plants contain a large variety of other organic sulfur compounds such as thiols (glutathione; ~1 to 2% of the total S) and sulfolipids (~1 to 2% of the total S); some species contain the so-called secondary sulfur compounds such as alliins and glucosinolates (7,8,10,11). Sulfur compounds are of great significance in plant functioning, but are also of great importance for food quality and the production of phyto-pharmaceuticals (8,12).
The sulfur requirement varies strongly between species and it may fluctuate during plant growth. The sulfur requirement can be defined as 'the minimum rate of sulfur uptake and utilization that is sufficient to obtain the maximum yield, quality, and fitness,' which for crop plants is equivalent to 'the minimum content of sulfur in the plant associated with maximum yield' and is regularly expressed as kg S ha-1 in the harvested crop. In physiological terms the sulfur requirement is equivalent to the rate of sulfur uptake, reduction, and metabolism needed per gram plant biomass produced over time and can be expressed as mol S g-1 plant day-1. The sulfur requirement of a crop at various stages of development under specific growth conditions may be predicted by upscaling the sulfur requirement in �mol S g-1 plant day-1 to mol S ha-1 day-1 by estimating the crop biomass density per hectare (tons of plant biomass ha-1). When a plant is in the vegetative growth period, the sulfur requirement (Srequirement, expressed as �mol S g-1 plant day-1) can be calculated as follows (11):
Srequirement = Scontent x RGR
where Scontent represents the total sulfur concentration of the plant (�mol g-1 plant biomass) and RGR is the relative growth rate of the plant (g g-1 plant day-1). The RGR can be calculated by using the following equation:
RGR = (ln W2 - ln W1) / (t2 - t1)
where W1 and W2 are the total plant weight (g) at time t1 and t2, respectively, and t2 - t1 the time interval (days) between harvests. In general, the sulfur requirement of different crop species grown at optimal nutrient supply and growth conditions ranges from 0.01 to 0.1 mmol g-1 plant dry weight day-1. Generally, the major proportion of the sulfate taken up is reduced and metabolized into organic compounds, which are essential for structural growth. However, in some plant species, a large proportion of sulfur is present as sulfate and in these cases, for structural growth, the organic sulfur content may be a better parameter for the calculation of the sulfur requirement (see also Section).
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