Food systems need to produce enough of the essential trace element Se to provide regular adult intakes of at least 40 microg/d to support the maximal expression of the Se enzymes, and perhaps as much as 300 microg/d to reduce risks of cancer. Deprivation of Se is associated with impairments in antioxidant protection, redox regulation and energy production as consequences of suboptimal expression of one or more of the Se-containing enzymes. These impairments may not cause deficiency signs in the classical sense, but instead contribute to health problems caused by physiological and environmental oxidative stresses and infections. At the same time, supranutritional intakes of Se, i.e. intakes greater than those required for selenocysteine enzyme expression, appear to reduce cancer risk. The lower, nutritional, level is greater than the typical intakes of many people in several parts of the world, and few populations have intakes approaching the latter, supranutritional, level. Accordingly, low Se status is likely to contribute to morbidity and mortality due to infectious as well as chronic diseases, and increasing Se intakes in all parts of the world can be expected to reduce cancer rates.

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Selenium in global food systems.

Mineral Nutrition of Higher Plants, 2nd edition, H. Marschner. Academic Press, London (1995), 889, (ISBN 0-12-473543-6). Price: 29.95 Pound Sterling

Onion (Allium cepa L.) is botanically included in the Liliaceae and species are found across a wide range of latitudes and altitudes in Europe, Asia, N. America and Africa. World onion production has increased by at least 25% over the past 10 years with current production being around 44 million tonnes making it the second most important horticultural crop after tomatoes. Because of their storage characteristics and durability for shipping, onions have always been traded more widely than most vegetables. Onions are versatile and are often used as an ingredient in many dishes and are accepted by almost all traditions and cultures. Onion consumption is increasing significantly, particularly in the USA and this is partly because of heavy promotion that links flavour and health. Onions are rich in two chemical groups that have perceived benefits to human health. These are the flavonoids and the alk(en)yl cysteine sulphoxides (ACSOs). Two flavonoid subgroups are found in onion, the anthocyanins, which impart a red/purple colour to some varieties and flavanols such as quercetin and its derivatives responsible for the yellow and brown skins of many other varieties. The ACSOs are the flavour precursors, which, when cleaved by the enzyme alliinase, generate the characteristic odour and taste of onion. The downstream products are a complex mixture of compounds which include thiosulphinates, thiosulphonates, mono-, di- and tri-sulphides. Compounds from onion have been reported to have a range of health benefits which include anticarcinogenic properties, antiplatelet activity, antithrombotic activity, antiasthmatic and antibiotic effects. Here we review the agronomy of the onion crop, the biochemistry of the health compounds and report on recent clinical data obtained using extracts from this species. Where appropriate we have compared the data with that obtained from garlic (Allium sativum L.) for which more information is widely available.

Onions—A global benefit to health

The chemical and physical resemblance between selenium (Se) and sulfur (S) establishes that both these elements share common metabolic pathways in plants. The presence of isologous Se and S compounds indicates that these elements compete in biochemical processes that affect uptake, translocation and assimilation throughout plant development. Yet, minor but crucial differences in reactivity and other metabolic interactions infer that some biochemical processes involving Se may be excluded from those relating to S. This review examines the current understanding of physiological and biochemical relationships between S and Se metabolism by highlighting their similarities and differences in relation to uptake, transport and assimilation pathways as observed in Se hyperaccumulator and non-accumulator plant species. The exploitation of genetic resources used in bioengineering strategies of plants is illuminating the function of sulfate transporters and key enzymes of the S assimilatory pathway in relation to Se accumulation and final metabolic fate. These strategies are providing the basic framework by which to resolve questions relating to the essentiality of Se in plants and the mechanisms utilized by Se hyperaccumulators to circumvent toxicity. In addition, such approaches may assist in the future application of genetically engineered Se accumulating plants for environmental renewal and human health objectives.

Selenium uptake, translocation, assimilation and metabolic fate in plants.

Glucosinolates (GLSs) are found in Brassica vegetables. Examples of these sources include cabbage, Brussels sprouts, broccoli, cauliflower and various root vegetables (e.g. radish and turnip). A number of epidemiological studies have identified an inverse association between consumption of these vegetables and the risk of colon and rectal cancer. Animal studies have shown changes in enzyme activities and DNA damage resulting from consumption of Brassica vegetables or isothiocyanates, the breakdown products (BDP) of GLSs in the body. Mechanistic studies have begun to identify the ways in which the compounds may exert their protective action but the relevance of these studies to protective effects in the human alimentary tract is as yet unproven. In vitro studies with a number of specific isothiocyanates have suggested mechanisms that might be the basis of their chemoprotective effects. The concentration and composition of the GLSs in different plants, but also within a plant (e.g. in the seeds, roots or leaves), can vary greatly and also changes during plant development. Furthermore, the effects of various factors in the supply chain of Brassica vegetables including breeding, cultivation, storage and processing on intake and bioavailability of GLSs are extensively discussed in this paper.

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Glucosinolates in Brassica Vegetables - the Influence of the Food Supply Chain on Intake, Bioavailability and Human Health

Sixty-two onion (Allium cepa L.) entries grown under high (4.0 mequiv.liter-1) and low (0.1 mequiv.liter−1) S fertility were evaluated for foliar and bulb S utilization, and pungency. Significant differences were detected among entries at both S fertility levels for foliar and bulb %S, and enzymatically developed pyruvate (pungency) in bulb tissue. Correlations between foliar %S and enzymatically developed pyruvate and between bulb %S and enzymatically developed pyruvate were poor suggesting that S is differentially partitioned into flavor and non-flavor compounds in the bulbs of onion germplasm. Significant S by entry interactions were found for all variables tested indicating that selection for S utilization and bulb pungency in onions be conducted at S fertility levels indicative of targeted production areas.

Onion germplasm interacts with sulfur fertility for plant sulfur utilization and bulb pungency

Glucosinolates (GSs) are S-containing compounds found in Brassica species and whose degradation products may provide protection against cancer. Sulphoraphane, a product of 4-methyl-sulphinylbutyl GS degradation, is a particularly potent inhibitor of anticarcinogenic detoxification enzymes. Selenium also has anticancer properties, and consumption of plants containing Se may be an effective way to increase dietary Se. Since plant uptake of Se and S is competitive, GS synthesis may be affected by Se fertilisation. The objective of this study was to determine how Se fertilisation of rapid-cycling B oleracea would affect Se and GS concentrations. Plants were grown in hydroponic solutions containing 0.0, 1.0, 2.0, 3.0, 6.0, 7.2 or 9.0 mg l -1 Na 2 SeO 4 . Mineral and glucosinolate concentrations were measured in shoots harvested just before anthesis. Total GSs decreased from 5.84 μmol g -1 (0.0 mg l -1 Na 2 SeO 4 ) to 1.90 μmol g -1 (9.0 mg l -1 Na 2 SeO 4 ). Levels of 4-methylsulphinylbutyl GS decreased 90% when Na 2 SeO 4 fertilisation was increased from 0 to 1 mg l -1 , and remained low at higher Na 2 SeO 4 concentrations. Shoot Se concentration was undetectable at 0.0 mg l -1 Na 2 SeO 4 and increased significantly with Na 2 SeO 4 fertilisation. Although B oleracea may not simultaneously deliver high levels of dietary 4-methylsulphinylbutyl GS and Se, levels of other GSs with anticarcinogenic benefits may be beneficial even with Se fertilisation. Des plants de Brassica oleracea ont ete cultives dans des solutions hydroponiques en presence de differentes concentrations de selenate de sodium. Les compositions minerales et en glucosinolate ont ete mesurees dans la plante. Le taux de glucosinolate de 4-methylsulphinylbutyle diminue de 90% quand la fertilisation avec NaSeO4 passe de 0a 1mg/l, a l'inverse d'autres glucosinolates a activite anticancereuse qui ne sont pas affectes par les doses de selenium lors de la fertilisation.

Sodium Selenate Fertilisation Increases Selenium Accumulation and Decreases Glucosinolate Concentration in Rapid-cycling Brassica Oleracea

Because sulfate (SO4 ‐2) accumulation patterns were unknown in onion, a study was conducted to determine how three onion varieties accumulated total bulb‐sulfur (S) and bulb‐SO4 ‐2 in response to increasing S‐fertility. Plants were greenhouse grown in pots containing washed river sand and fertilized with five different nutrient solutions possessing SO4 ‐2 concentrations of 5, 25, 50, 75, or 150 mg liter1 until the plants were mature. Bulbs were harvested and analyzed for total‐S, SO4 ‐2, and pungency, as measured by enzymatically developed pyruvic acid (EPY). Bulbs accumulated significant levels of SO4 ‐2 and S. The amount of bulb‐SO4 ‐2 and bulb‐S increased linearly as S‐fertility increased. The three varieties differed in total bulb‐S, bulb‐SO4 ‐2, the percent of total bulb‐S accumulated as SO4 ‐2, and EPY. Bulb‐SO4 ‐2 ranged from 0.047 to 0.318 % dry mass in response to S‐fertility level and variety, while total bulb‐S ranged from 0.154 to 0.535% dry mass. Diverse S and SO4 ‐2 accumulation and...

Total sulfur and sulfate accumulation in onion is affected by sulfur fertility

Intact cells of Allium spp have no odour, but when cells are disrupted, the enzyme alliinase (EC 4.4.1.4) hydrolyses the S-alk(en)yl sulphoxides (ACSOs) to produce pyruvate, ammonia and the many volatile sulphur compounds associated with flavour and odour. In onions, there are three main sulphoxides: methyl, propyl and 1-propenyl, which gives rise to onion's tear-producing effect. The relative proportions of these are a factor in determining the subsequent flavour. γ-Glutamyl peptides, which are biosynthetic intermediates to ACSOs, are also present in significant amounts, but their contribution to flavour is not known. There is little work on the reaction of alliinase in vivo. The hydrolysis of ACSOs by alliinase was studied in rapidly macerated bulbs at intervals between 5 s and 2 h. ACSO and γ-glutamyl peptide levels were measured by HPLC, and pyruvate levels were measured spectrophotometrically. Although the hydrolysis of propenyl cysteine sulphoxide (PrenCSO) was immediate and almost 100% between 5 and 20 s after bulb maceration, the hydrolysis of propyl cysteine sulphoxide (PCSO) and methyl cysteine sulphoxide (MCSO) was incomplete. After 5 s maceration, about 50% PCSO and MCSO remained and thereafter no further hydrolysis occurred. The levels of ACSOs and the extent to which they were hydrolysed were dependent on the cultivar and sulphur environment in which it was grown. The addition of pyridoxal phosphate cofactor enhanced the extent of MCSO and PCSO hydrolysis. Additional purified alliinase per se had no effect. Substantial hydrolysis of γ-glutamyl peptides also occurred after maceration, and this was unexpected. Levels of pyruvate product were between 15 and 25% of the expected amount from ACSO hydrolysis assuming a stoichiometric relationship. The incomplete hydrolysis of MCSO and PCSO, enhancement of activity by additional pyridoxal phosphate and non-stoichiometric production of pyruvate are evidenced that reaction inhibition of alliinase may be occurring in onion macerates. © 1998 Society of Chemical Industry.

Differential hydrolysis of alk(en)yl cysteine sulphoxides by alliinase in onion macerates : Flavour implications

Brassica oleracea L. are important economic vegetables, and are capable of selenium (Se) enrichment to enhance human nutrition and health. Because Se enrichment may influence the nutrient balance of this crop, a study was done to test the effects of selenate‐Se on plant nutrients. Plants of a rapid‐cycling B. oleracea population were grown in nutrient solutions amended with Na2SeO4at 0.0, 3.0, 6.0, and 9.0 mg L‐1. Leaf tissue was then analyzed for nutrient content. Boron (B) (P=0.01), iron (Fe) (P=0.01), and phosphorus (P) (P=0.01) content decreased, while Se (P=0.01), sulfur (S) (P=0.01), and potassium (K) content (P=0.01) increased with increasing selenate‐Se treatments. Significant quadratic responses were found for magnesium (P=0.01) and molybdenum (P=0.01). No significant differences in leaf fresh or dry weight were detected. Changes in plant nutrient content can be expected when Brassicas are enhanced for delivery of beneficial organic Se.

William M. Randle

Papers

Onion germplasm interacts with sulfur fertility for plant sulfur utilization and bulb pungency

Sodium Selenate Fertilisation Increases Selenium Accumulation and Decreases Glucosinolate Concentration in Rapid-cycling Brassica Oleracea

Total sulfur and sulfate accumulation in onion is affected by sulfur fertility

Differential hydrolysis of alk(en)yl cysteine sulphoxides by alliinase in onion macerates : Flavour implications

Nutrient accumulation in leaf tissue of rapid‐cycling brassuca oleracea responds to increasing sodium selenate concentrations