figure 7.5 Chemical structure of alliin. (From Watzl, B., Bioaktive Substanzen in Lebensmitteln, Hippokrates Verlag, Stuttgart, Germany, 1999.)

S-methyl- and S-propyl-L-cysteine sulfoxides (107). Iso-alliin is the main form in onions, whereas alliin is the predominant form in garlic (108) (Figure 7.5). Alliins supposedly contribute to the defense of plants against pests and diseases. In vitro and in vivo experiments revealed a bactericidal effect against various plant pathogens (109).

The characteristic flavor of Allium species is caused after the enzyme alliinase hydrolyzes cys-teine sulfoxides to form pyruvate, ammonia, and sulfur-containing volatiles. In the intact cell, alliin and related cysteine sulfoxides are located in the cytoplasm, whereas the C-S lyase enzyme alliinase is localized in the vacuole (110). Disruption of the cell releases the enzyme, which causes subsequent a,P-elimination of the sulfoxides, ultimately giving rise to volatile and odorous LMW organosulfur compounds (111). The cysteine sulfoxide content of Allium species is an important quality parameter with regard to sensory features, since it determines the taste and sharpness.

Alliin acts as an antioxidant by activating glutathione enzymes and is regarded as having an anticarcinogenic and antimicrobial effect (86). On average, 21% of sulfur, but only 0.9% of nitrogen, are present as (iso)alliin in onion bulbs at the start of bulb growth (101). The ratio between protein-S and sulfur in secondary metabolites of the Allium species is, at between 1:4 and 1:6, much wider than in members of the Brassica family (between 1:0.3 and 1:2). The reason for this difference is supposedly the fact that glucosinolates may be reutilized under conditions of sulfur deficiency whereas alliins are inert end products. Interactions between nitrogen and sulfur supply exist in such a way that nitrogen and sulfur fertilization has been shown to decrease total sulfur and nitrogen concentration, respectively, in onion (101).

7.2.4 Interactions between Sulfur and Other Minerals

Interactions between sulfur and other minerals may significantly influence crop quality parameters (5,113,114). Sulfur and nitrogen show strong interactions in their nutritional effects on crop growth and quality due to their mutual occurrence in amino acids and proteins (see Section 7.2.3). Further examples of nitrogen-sulfur interactions that are not mentioned in previous sections of this chapter are shown below. Nitrogen-Sulfur Interactions

Under conditions of sulfur starvation, sulfur deficiency symptoms are expressed moderately at low nitrogen levels but extremely with a high nitrogen supply. This effect explains the enhancement of sulfur deficiency symptoms in the field after nitrogen dressings (114). The question of why sulfur deficiency symptoms are more pronounced at high nitrogen levels is, however, still unanswered. For experimentation, these results are relevant as the adjustment of the nitrogen and sulfur nutritional status of plants is essential before any hypothesis on the effect of a nitrogen or sulfur treatment on plant parameters can be stated or proved.

The use of the nitrogen/sulfur ratio as a diagnostic criterion is problematic because the same ratio can be obtained at totally different concentration levels in the tissue. Surplus of one element may therefore be interpreted falsely as a deficiency of the other (see Section Clear relationships between nitrogen/sulfur ratios and yield occur only in ranges of extreme ratios. Such ratios may be produced in pot trials but do not occur under field conditions. The effect of increasing nitrogen and sulfur supply on crop seed yield with increasing nitrogen supply is more pronounced with protein than with carbohydrate crops (Table 7.5).

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