Euphytica 96: 385–390, 1997. 385c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

Short-day onion cultivars differ in bulb selenium and sulfur accumulationwhich can affect bulb pungency

Dean A. Kopsell & William M. RandleDepartment of Horticulture, The University of Georgia, Athens, GA 30602, U.S.A.

Received 20 August 1996; accepted 19 April 1997

Key words: Allium cepa L., cultivars, pungency, pyruvic acid, soluble solids content

Summary

The close chemical similarity of S and Se allow for antagonistic behavior between the two elements. To determinevariation for Se accumulation within onions, sixteen short-day cultivars were grown under high level (2.0 mgNa2SeO4 per litre) and no Se treatments. Selenium accumulation in bulb tissues was cultivar dependent, rangingfrom 60 to 113 �g Se per g dry wt. High Se fertility enhanced bulb S accumulation. Pungency was reduced insome cultivars grown under Se fertility when compared to the no Se treatment indicating that although S uptakewas enhanced, S metabolism in the flavour precursor biosynthetic pathway was affected. The existing variability inonion Se uptake and accumulation have important implications for Se intake in mammalian diets.

Introduction

Because of their similar chemistry, Se can substitute forS in plants allowing for the formation of Se analogs of Sorganic compounds (Mikkelsen et al., 1989). Selenate(SeO4

�2) is absorbed in the roots by the same carrierresponsible for sulfate (SO4

�2) absorption (Leggett &Epstein, 1956). The reduction of SeO4

�2 to selenite(SeO3

�2) occurs in the leaves (Brown & Shrift, 1982).Selenite then undergoes a series of reductions to formselenide, which is incorporated into selenocysteine (Ng& Anderson, 1978, 1979). Selenide and sulfide com-pete for binding sites on cysteine synthase (Leggett &Epstein, 1956). Selenide acts to inhibit the productionof cysteine by inactivating cysteine synthase,while sul-fide will inhibit the production of selenocysteine (Ng& Anderson, 1978). Competitive inhibition betweenSO4

�2 and SeO4�2 is believed to be more pronounced

when the two ions are at high levels (Milchunas et al.,1983). High Se concentrations in plant tissues can bephytotoxic (Mikkelsen et al., 1989). If ingested dailyat concentrations not exceeding 400 �g per day, Se hashealth benefits to humans. However, at higher levelsSe can cause mammalian toxicity (Combs & Combs,1986).

Onions are primarily consumed for their flavourwhich is dominated by S compounds. Flavour poten-tial in onions is genetically determined, but can bemodified through environmental manipulation (Land-caster & Boland, 1990). Among onion cultivars, Sfertility plays vital roles in determining flavour inten-sity and S utilization (Freeman & Mossadeghi, 1970;Randle et al., 1994). Onions normally take up S assulfate (SO4

�2). Sulfate is transported to the leaves,reduced to sulfide, and assimilated into cysteine. Glu-tathione, a cysteine tripeptide, is believed to be thestarting point in the proposed biosynthetic pathwayleading to flavour precursor synthesis (Landcaster &Boland, 1990). Glutathione may also act as a tem-porary storage compound of reduced S (Rennenberg,1984). Flavour intensity and pungency come from sev-eral nonprotein S amino acids collectively called S-alk(en)yl cysteine sulfoxides (ACSOs). The enzymealliinase acts on the ACSOs following cellular disrup-tion giving rise to the volatile S compounds responsiblefor onion flavour (Landcaster & Boland, 1990).

Variability exists among onion cultivars for bulbS, pungency, and soluble solids content (Randle &Bussard, 1993a). Onions also accumulate Se in bulbtissue (Ip & Lisk, 1994; Shane et al., 1988). Thisexperiment was undertaken to determine the effect of

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Table 1. List of short-day onion (Allium cepa L.) cultivars and their maturity dates

Cultivars Seed source Address Harvest date

Primavera Peto Seeds Saticoy, CA March 7, 1995

Super X Takii & Co., Ltd. Kyoto, Japan March 7, 1995

Rio Unico Rio Colorado Bakersfield, CA March 12, 1995

Rio Blanco Grande Rio Colorado Bakersfield, CA March 12, 1995

T-321 Takii & Co., Ltd. Kyoto, Japan March 12, 1995

Savannah Sweet Peto Seeds Saticoy, CA March 18, 1995

Sweet Dixie Rio Colorado Bakersfield, CA March 18, 1995

Dessex Sun Seeds Hollister, CA March 18, 1995

Rio Ringo Rio Colorado Bakersfield, CA March 23, 1995

Southern Belle D. Palmer Seeds Yuma, AZ March 23, 1995

Henry’s Special Sun Seeds Hollister, CA March 23, 1995

Dehydrator # 3 Sun Seeds Hollister, CA March 24, 1995

Granex 33 Asgrow Seeds Kalamazoo, MI March 24, 1995

Spring Star L. Wannamaker Seed St. Matthews, SC March 27, 1995

Silver Spring L. Wannamaker Seed St. Matthews, SC March 27, 1995

Spring Sun L. Wannamaker Seed St. Matthews, SC March 28, 1995

Na2SeO4 on: 1) the accumulation of Se in bulb tissues;2) S uptake and accumulation; and 3) pungency amongshort-day onion cultivars.

Materials and methods

On 10 October 1994, 16 short-day onion cultivars(Table 1) representing a wide range of S utilization(Randle et al., 1993) were seeded into growing cubes(Grodan A/S, Dk-2640 Hedehusene, Denmark). Threeto four seeds were placed into each cell. The cell holeswere filled with fine vermiculite and watered daily.Fer-tilization was applied with a 200 mg per litre solutionof Peters 20N-20P-20K (Grace-Sierra Co., Malpitas,CA) as needed once cotyledons emerged. Seedlingswere thinned to one plant per cell 3 weeks after sow-ing. The seedlings were greenhouse-grown (25 �C dayand 20 �C night set points) for 5 weeks under naturalphotoperiods (~ 34� N latitude). A broad spectrumfungicide (Bravo 720, Fermenta Plant Protection Co.,Mentor, OH) was applied as needed.

Four tanks (5.47 � 1.22 � 0.31 m) with pre-fabricated black polyethylene liners were each filledwith approximately 2,000 litres of a modified half-strength Hoagland’s nutrient solution (Hoagland &Arnon, 1950). The magnesium sulfate (MgSO4�7H2O)concentration was 493 mg per litre. The solution wascirculated and aerated using a Teel pedestal pump(Model 3P611E, Dayton Electric Mfg. Co., Chicago,

IL) attached to PVC piping. Solution levels in the bedswere maintained with a mechanical float controllingflow from the deionized water source. Styrofoam insu-lation boards were placed in the tanks to support theplants. The boards were supported by PVC pipe. Holeswere cut in the styrofoam boards at 2.2 cm diametersand spaced 10.2 cm on centre. Polyethylene fencingwith 6.35 cm openings was suspended 20.32 cm overthe beds to support plant foliage.

On 8 November 1994, the onions were transferredto the styrofoam boards. The experimental design wasa split-plot with 4 replications, ten plants per replica-tion. Sodium selenate treatments were the main plotswith onion cultivars as the sub-plots. On 3 January1995, 2.0 mg Na2SeO4 per litre (ICN Biochemicals,Cleveland, OH) were added to the nutrient solution asthe Se treatment. No Se was added to the control.

In each tank, a complete solution change was madeevery 2 weeks. On 7 January 1995, the natural photope-riod was extended to 12 h using incandescent 125 Wlights to stimulate bulbing. The lights were suspend-ed over the beds at a height of 1 m. On 28 January1995, the natural photoperiod was extended to 13 h.The 13 h photoperiod was maintained to the end of theexperiment and was sufficient to complete the bulbingprocess and plant maturation.

Each cultivar was harvested when 50% of the plantshad a soft pseudostem, indicating plant maturity (Table1). Upon harvest, the roots and leaves were removedfrom the bulbs. The bulbs were placed into mesh bags

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and allowed to cure at ambient greenhouse tempera-tures for 7 days before analysis. Three wedges were cutfrom each of eight bulbs per treatment replication. Onewedge grouping was used to determine total enzymat-ic pyruvic acid and soluble solids content, one wedgegrouping was used to determine background pyruvicacid, and one wedge grouping was used for total Seand S determination.

Onion pungency for each cultivar was determinedusing the method of Randle & Bussard (1993b).Onions get their distinctive flavour from a group offlavour precursors called S-alk(en)yl cysteine sulfox-ides. Upon cellular disruption, the flavour precur-sors are hydrolyzed by the enzyme alliinase and pro-duce pyruvate, ammonium, thiopropanal-S oxide, andseveral volatile S compounds (Landcaster & Boland,1990). Wedges from eight bulbs were juiced in a pneu-matic press. Immediately following juicing, a 0.5 mlsample was used to measure soluble solids content(SSC) with a hand-held refractometer (Kernco, Tokyo,Japan). Soluble solids content is highly correlated withcarbohydrate content in onions (Jones & Mann, 1961).

For total Se and S determination, the combinedwedge groups were placed into paper bags and allowedto dry at 60 �C in a forced air drying oven (Model630, National Appliance Co., Portland, OR) for noless than 48 h. The dried tissue was then ground topass through a 0.5 mm screen in a cyclotec samplemill grinder (Model 1093, Tector, Hoganas, Sweden).A wet acid digest was used for Se analysis. One gof onion powder was placed into a 125 ml flask with10 ml of concentrated nitric acid and placed on a hotplate (Type 2200, Thermolyne, Dubuque, IA) for 4 hat 165 �C. The flasks were allowed to cool to roomtemperature and then brought up to a final volume of50 ml with deionized water. The solutions were fil-tered through Whatman No. 1 filter paper (Maidstone,England). Total Se was measured by graphite furnaceatomic absorption spectrophotometry (GFAA; Mod-el 4100ZL, Perkin Elmer Corp., Norwalk, CT). Thedetection limit for GFAA was 0.004 mg per litre. TotalS in the plant tissue samples was determined on a LecoSulfur Determinator (Model SC-232, St. Joseph, MI).One g of onion powder was combined with vanadiumpentoxide (Leco Corp., St. Joseph, MI) accelerator andcombusted at 1371 �C with O2. Total S was measuredas SO2 with an infrared cell detector.

Data were analyzed by the GLM procedure ofSAS (Cary, N.C.). Duncan’s multiple range test (P =0.05) was used to separate cultivar means within eachSe treatment. Differences among Se treatments were

Table 2. Total bulb tissue Se accumulation (�g Se per gdry wt) for onion cultivars responding to 2.0 mg Na2SeO4per litre in nutrient solutions. Bulb Se with no Se treatmentwere below detection limits on GFAA

Cultivars Sez

Primavera 88.6 abcde

Super X 105.2 ab

Rio Unico 98.0 ab

Rio Blanco Grande 101.8 ab

T-321 104.0 ab

Savannah Sweet 60.3 f

Sweet Dixie 112.9 a

Dessex 71.1 def

Rio Ringo 65.7 ef

Southern Belle 69.5 def

Henry’s Special 92.3 abcd

Dehydrator # 3 83.8 bcdef

Granex 33 71.6 def

Spring Star 68.7 def

Silver Spring 73.1 cdef

Spring Sun 96.3 abc

Mean 85.2

z Mean separation within column by Duncan’s multiplerange test P = 0.05.

detected by t-test (P = 0.05). A correlation matrix and aSpearman rank correlation was calculated for all vari-ables tested within each Se treatment.

Results and discussion

Bulb Se accumulation differed among Na2SeO4 treat-ments (P = 0.0001), cultivars (P = 0.0001) and for theinteraction among Na2SeO4 treatments and cultivars(P = 0.0001). With 2.0 mg Na2SeO4 per litre, cultivarsaveraged 85.2 �g Se per g dry wt (Table 2). With noadded Se, Se values were below the detection limitsfor graphite furnace atomic absorption (GFAA). Cul-tivars with the highest bulb Se accumulations were forPrimavera, Super X, Rio Unico, Rio Blanco Grande,T-321, Sweet Dixie, Henry’s Special, and Spring Sun,and the lowest Se accumulations were with SavannahSweet, Dessex, Rio Ringo, Southern Belle, Dehydra-tor # 3, Granex 33, Spring Star, and Silver Spring. Theability of onions to accumulate high levels of Se wasshown to be a function of Se concentration and the Seto S fertility ratio (Kopsell & Randle, 1997). Previous-ly, the maximum accumulation of Se in field grownonions under Na2SeO3 and Na2SeO4 fertilization was

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Table 3. Total bulb S (�g S per g dry wt) for short-day onion cultivarsgrown with 2.0 mg Na2SeO4 per litre or no Se in nutrient solutions

Cultivars Sez No Sez Significance

Primavera 5000 ef 4650 cd ns

Super X 5775 bcde 4275 d P = 0.04

Rio Unico 4750 f 5250 bcd ns

Rio Blanco Grande 6125 abcd 6025 ab ns

T-321 5650 bcde 4575 cd P = 0.03

Savannah Sweet 5300 def 5100 bcd ns

Sweet Dixie 5400 cdef 5275 bcd ns

Dessex 6950 a 5750 abc P = 0.03

Rio Ringo 5950 bcd 6025 ab ns

Southern Belle 5750 bcde 5850 ab ns

Henry’s Special 6200 abc 6250 ab ns

Dehydrator # 3 6450 ab 6525 a ns

Granex 33 5575 cdef 5450 abcd ns

Spring Star 5775 bcde 4575 cd ns

Silver Spring 6025 bcd 5525 abc P = 0.05

Spring Sun 6875 a 5200 bcd P = 0.01

Mean 5847 5394 P = 0.0001

ns Nonsignificant. z Mean separation within columns by Duncan’smultiple range test P = 0.05. Among Se differences determined byt-tests.

28 �g Se per g dry wt (Ip & Lisk, 1994). Similarly,supplementation with coal fly ash containing Se result-ed in only 0.138 �g Se dry wt accumulation in onionbulb tissues (Gutenmann & Lisk, 1996).

Total bulb S differed among Na2SeO4 treatments(P = 0.0001), cultivars (P = 0.0001) and for the Se bycultivar interaction (P = 0.0001).With 2.0 mg Na2SeO4

per litre, cultivars averaged 5847 �g S per g dry wt,while bulbs from the no Se control averaged 5394 �gS per g dry wt (Table 3). The ability of Se to enhance Suptake and accumulation in some cultivars was unex-pected since Se and S are competitive and are absorbedinto the plant by the same carrier (Leggett & Epstein,1956). Cultivars with the highest bulb S concentra-tions grown with 2.0 mg per litre Na2SeO4 treatmentwere Rio Blanco Grande, Dessex, Henry’s Special,Dehydrator # 3, and Spring Sun, and cultivars withthe lowest bulb S concentrations were Primavera, RioUnico, Savannah Sweet, Sweet Dixie, and Granex 33.The total bulb S values are within the range of thosereported for onions of broad genetic backgroundgrownunder high S fertility (Randle & Bussard, 1993a). Withno Se added, cultivars with the highest accumulationof total bulb S were Rio Blanco Grande, Dessex, RioRingo, Southern Belle, Henry’s Special, Dehydrator# 3, Granex 33, and Silver Spring, and cultivars with

Table 4. Bulb enzymatically formed pyruvic acid in �Mol perg fresh wt for short-day onion cultivars grown with 2.0 mgNa2SeO4 per litre or no Se in nutrient solutions

Cultivars Sez No Sez Significance

Primavera 4.5 f 5.6 d P = 0.08

Super X 5.3 def 6.4 bcd ns

Rio Unico 5.6 cdef 6.5 bcd ns

Rio Blanco Grande 5.6 cdef 6.5 bcd ns

T-321 5.7 cdef 6.5 bcd ns

Savannah Sweet 4.9 ef 3.8 e ns

Sweet Dixie 6.5 bc 8.2 bc P = 0.003

Dessex 6.1 cd 6.3 cd ns

Rio Ringo 5.2 def 6.0 d ns

Southern Belle 7.4 b 8.3 b P = 0.07

Henry’s Special 6.3 bcd 5.8 d ns

Dehydrator # 3 9.6 a 11.8 a P = 0.01

Granex 33 5.5 cdef 7.4 bcd P = 0.02

Spring Star 5.9 cde 6.7 bcd ns

Silver Spring 7.4 b 8.2 bc ns

Spring Sun 5.6 cdef 7.1 bcd P = 0.05

Mean 6.1 6.9 P = 0.0001

ns Nonsignificant. z Mean separation within columns by Dun-can’s multiple range test P = 0.05. Among Se differences deter-mined by t-tests.

the lowest total bulb S were Primavera, Super X, RioUnico, T-321, Savannah Sweet, Sweet Dixie, SpringStar, and Silver Spring. Significant increases in totalbulb S among 0 mg Se and 2.0 mg Na2SeO4 per litretreatments were detected for the cultivars Super X, T-321, Dessex, Silver Spring, and Spring Sun (Table 3).

Enzymatically formed pyruvic acid differed amongNa2SeO4 treatments (P = 0.0001) and cultivars (P =0.0001), but not for the interaction of Se and cultivar.Plants grown without Se were generally more pun-gent than those with supplemental Se. With 2.0 mgNa2SeO4 per litre, EPY averaged 6.1 �Mol pyruvicacid per g fresh wt (Table 4). The 0 mg Se treatmentresulted in an average EPY of 6.9 �Mol pyruvic acidper g fresh wt. Dehydrator # 3 had the highest EPY withthe 2.0 mg Na2SeO4 per litre treatment (Table 4). Thecultivars Primavera, Super X, Rio Unico, Rio BlancoGrande, T-321, Savannah Sweet, Rio Ringo, Granex33, and Spring Sun had the lowest EPY. With no addedSe Dehydrator # 3 had the highest EPY, while Primav-era, Super X, Rio Unico, Rio Blanco Grande, T-321,Dessex, Henry’s Special, Granex 33, Spring Star, andSpring Sun were the lowest. Significant decreases inEPY with Se added to the nutrient solution were seenfor Primavera, Sweet Dixie, Southern Belle, Dehy-

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Table 5. Correlation coefficients between enzymatically formed pyru-vic acid (EPY), soluble solids content (SSC), and selenium (Se) andsulfur (S) accumulation in bulb tissues for 16 onion cultivars with andwithout Na2SeO4 treatments

2.0 mg Na2SeO4 per litre

Variable EPY SSC Se S

EPY 0.75*** 0.02ns 0.26*

SSC - 0.27* 0.21ns

Se - 0.10ns

S

0 mg Na2SeO4 per litre

Variable EPY SSC Se S

EPY 0.64*** na 0.32*

SSC ns 0.31*

Se 0.37*

S

ns, *, *** Not significant and significant at P = 0.05, and 0.001,respectively. na = not applicable.

drator # 3, Granex 33, and Spring Sun. The deceasein EPY caused by Se fertility may have come fromthe inhibition of cysteine resulting from the competi-tive nature of selenide and sulfide for the enzyme cys-teine synthase (Ng & Anderson, 1978). Cysteine isan integral metabolite in the onion biosynthetic path-way. Decreases in cysteine would ultimately lead todecreases in pungency. In any event, Se lowered EPYby at least 1.0 �Mol for most cultivars, which is aperceptible difference in flavour intensity (Randle &Bussard, 1993a). The mechanism responsible for Seinduced EPY decreases warrants further investigation.

Soluble solids content (SSC) differed only amongcultivars (P = 0.0001). Selenium fertility had no signif-icant affect on SSC. Under both Se treatments, ‘Dehy-drator # 3’ had the highest SSC (data not shown).Previ-ously, differences in S fertility were shown to influenceSSC and individual onion water-soluble carbohydrates(Randle, 1992).

Significant positive correlations were foundbetween EPY and SSC (r = 0.75***) and betweenEPY and total bulb S (r = 0.26*) with the 2.0 mgNa2SeO4 per litre (Table 5). With no added Se therewas a significant, but poor positive correlation (r =0.31*) between SSC and total bulb S. Previously, poorand non-significant correlations were found betweenEPY and SSC, or between EPY and total bulb S withhigh S fertility and no additional Se (Randle & Bus-sard, 1993a). A poor but significant negative correla-tion was found between SSC and Se accumulation (r =

- 0.27*) with the 2.0 mg Na2SeO4 per litre treatmentconcentration.

Variability for bulb Se accumulation in short-dayonion cultivars has been established. At a high Se fer-tility the cultivars accumulated from 60.3 to 112.9 �gSe per g dry wt in the bulb tissues. The current studydemonstrates the ability of onions to accumulate high-er levels of Se in bulb tissues than previously reported.Selenium has been identified as an essential micronu-trient in mammalian health, with a recommended dai-ly allowance of between 60 to 200 �g depending onbody weight (Levander, 1991). Consumption of Se inhigh quantities, however can produce toxic symptoms(Combs & Combs, 1986). Health benefits attributed toSe range from immune system enhancement to can-cer prevention (Vandenbrandt et al., 1993; Schrauzer& Sacher, 1994). Assuming our cultivars averaged10% dry matter, one would need to ingest between8.8 and 16.6 g of selenized fresh onions to receive theU.S. RDA of 100 �g. Testing has shown the potentialfor vegetable Allium to deliver selenium-substitutedanalogs for cancer prevention (Ip & Lisk, 1994; Ipet al., 1992). Selenium-enriched Allium resulted inminimal accumulations of Se in body tissues, nor-mally associated with standard Se supplements suchas selenomethionine and selenite. In addition, no sig-nificant alterations in the production and function ofmammalian selenoenzyme activities were found withorganic Se (Ip & Lisk, 1994). Phenotypic variationestablished in this study provides the basis for inves-tigating the heritability of Se accumulation in onion.This information is important as consumers becomeincreasingly aware of the nutritional and health bene-fits of food.

References

Brown, T.A. & A. Shrift, 1982. Selenium: toxicity and tolerance inhigher plants. Biol Rev 57: 59–84.

Combs, G.F. Jr. & S.B. Combs, 1986. The role of selenium innutrition. Academic Press, Orlando, Fla.

Freeman, G.G. & N. Mossadeghi, 1970. Effect of sulphate nutritionon flavour components of onion (Allium cepa). J Sci Food Agr21 (2): 610–615.

Gutenmann, W.H. & D.J. Lisk, 1996. Increasing selenium in field-grown onions by planting in peat moss pots containing fly ash.Chemosphere 32 (9): 1851–1853.

Hoagland, D.R. & D.I. Arnon, 1950. The water-culture method forgrowing plants without soil. Calif Agr Expt Sta Circ 347.

Ip, C. & D.J. Lisk, 1994. Enrichment of selenium in allium vegeta-bles for cancer prevention. Carcinogenesis 15 (9): 1881–1885.

390

Ip, C., D.J. Lisk & G.S. Stoewsand, 1992. Mammary cancer preven-tion by regular garlic and selenium-enriched garlic. Nutr Cancer17 (3): 279–286.

Jones, H.A. & L.K. Mann, 1963. Onions and their allies. Botany,cultivation and utilization. Interscience Publ., NY.

Kopsell, D.A. & W.M. Randle, 1997. Selenate concentration affectsselenium and sulfur uptake and accumulation by ‘Granex 33’onions. J Am Soc Hort Sci 122: in press.

Landcaster, J.E. & M.J. Boland, 1990. Flavor biochemistry. In: H.D.Rabinowitch & J.L. Brewster (Eds.), Onions and Allied Crops,Vol. 3, pp. 33–72. CRC Press, Boca Raton, Fla.

Leggett, J.E. & E. Epstein, 1956. Kinetics of sulfate absorption bybarley roots. Plant Physiol 31: 222–226.

Levander, O.A., 1991. Scientific rationale for the 1989 recommendeddietary allowance for selenium. J Am Dietetic Assoc 91: 1572.

Mikkelsen, R.L., A.L. Page & F.T. Bingham, 1989. Factors affectingselenium accumulation by agricultural crops. In: L.W. Jacobs(Ed.), Selenium in Agriculture and the Environment, pp. 65–94.American Society of Agronomy, Inc and Soil Science Society ofAmerica, Inc, Madison, Wis.

Milchunas, D.G., W.K. Lauenroth & J.L. Dodd, 1983. The inter-action of atmospheric and soil sulfur on the sulfur and seleniumconcentration of range plants. Plant Soil 72: 117–125.

Ng, B.H. & J.W. Anderson, 1978. Synthesis of selenocysteine by cys-teine synthase from selenium accumulator and non-accumulatorplants. Phytochem 17: 2069–2074.

Ng, B.H. & J.W. Anderson, 1979. Light-dependent incorporation ofselenite and sulphite into selenocysteine and cysteine by isolatedpea chloroplast. Phytochem 18: 573–580.

Randle, W.M., 1992. Sulfur nutrition affects nonstructural water-soluble carbohydrates in onion germplasm. HortSci 27 (1): 52–55.

Randle, W.M. & M.L. Bussard, 1993a. Pungency and sugars ofshort-day onions as affected by sulfur nutrition. J Amer Soc HortSci 118 (6): 766–770.

Randle, W.M. & M.L. Bussard, 1993b. Streamlining onion pungencyanalyses. HortSci 28 (1): 60.

Randle, W.M., M.L. Bussard & D.F. Warnock, 1993. Ontogeny andsulfur fertility affect leaf sulfur in short-day onions. J Amer SocHort Sci 118 (6): 762–765.

Randle, W.M., E.B. Block, M.H. Littlejohn, D. Putman & M.L.Bussard, 1994. Onion (Allium cepa L.) thiosulfinates respond toincreasing sulfur fertility. J Agric Food Chem 42: 2085–2088.

Rennenberg, H., 1984. The fate of excess sulfur in higher plants.Ann Rev Plant Physiol 35: 121–153.

Schrauzer, G.N. & J. Sacher, 1994. Selenium in the maintenanceand therapy of HIV-infected patients. Chem Biol Interactions 91:199–205.

Shane, B.S., C.B. Littman, L.A. Essick, W.H. Gutenmann, G.J.Doss & D.J. Lisk, 1988. Uptake of selenium and mutagens byvegetables grown in fly ash containing greenhouse media. J AgrFood Chem 36: 328–333.

Vandenbrandt, P.A., R.A. Goldbohm, P. Vantveer, P. Bode, E.Dorant, R.J.J. Hermus & F. Sturmans, 1993. A prospective cohortstudy on selenium status and the risk of lung-cancer. Cancer Res53: 4860–4865.